Does Epsom Salt Work? The science and mythology of Epsom salt bathing for recovery from muscle pain, soreness, or injury

A cup or two of Epsom salt in a bath supposedly relieves pain and soreness from exercise and injury, mainly by “detoxifying” or delivering magnesium. Is it medicine or myth? This is a deep dive into the topic from a scientific perspective. Epsom salts are specifically thought to be a good treatment for muscle pain from over-exertion (delayed-onset muscle soreness), arthritis, myofascial pain syndrome (“trigger points”), fibromyalgia, and speeding healing1 from minor injuries such as muscle strains and tendinitis. The “exact” mechanism given is almost always detoxification, perhaps via osmosis, with no further information provided. A few proponents say that it’s good for pain because it’s a good source of magnesium. Claims and recommendations of this nature can be found by the thousands online. Bags and cartons of Epsom salts are available at any drugstore. Why, I have a package right here.2 It says: Dissolve desired amount (1–2 cups) of crystals in a hot bath to produce a mineral water treatment to aid in the relief of muscular aches and pains. All the claims are critically analyzed here. Spoiler alert: none of them are well-supported. On the other hand, there are few solid conclusions here either. The only thing I am absolutely certain of is that the “osmosis” idea is total bollocks, but biologically incoherent.

Says who? How does it work? When I went to my super-duper advanced massage therapy college, my instructors suggested Epsom salt baths as a good thing to prescribe to our clients. No scientific basis for this idea was ever presented: it was just one of those things that everybody “knew,” a folk remedy justified by the generations of wise old wives and bathers. The physiology of it certainly wasn’t explained. No one ever mentioned it being a source of magnesium, even though that is by far the most plausible explanation for any benefit it might have. Mostly my colleagues and mentors just made vague references to “detoxifying” the muscles, perhaps “by osmosis.” Nothing more exact was ever discussed because, frankly, I am sure that not one person in the building could have even named the compound “magnesium sulfate.”3 Me included. I have wondered ever since if there was anything to it. I am strongly skeptical of all health-related claims involving “toxins,” mostly because people who toss that word around never seem to know which toxins they are talking about.4 I have thoroughly studied the subject of post-exercise muscle soreness — probably the leading cause of hot baths — only to discover that it’s basically been proven that there are no known remedies for it.5 And after a long, hot Epsom salt bath of my own one night — which had no apparent effect on my unusually sore muscles, as usual — I decided it was time for a proper reality check. I started studying this topic (and I’ve never really stopped). Does an Epsom salt bath actually do anything? Does brining yourself like a turkey do any good? Can you pickle your pain away? Is there any plausible way that Epsom salts could have an effect on your sore muscle tissue, or on the healing of injuries? GO TO TOP • CONTENTS • NOTES

What is Epsom salt? And why do people always stick an S on the end? Epsom salt is magnesium sulfate heptahydrate, usually shortened just to magnesium sulfate (note also the British spelling “sulphate”). It was originally obtained by boiling down mineral waters at Epsom, England. The magnesium is considered the “active ingredient.” It is often quaintly referred to in the plural — Epsom salts instead of Epsom salt — but it is just one kind of salt, and other than tradition there’s no more reason to say “salts” than there is to say “please pass the table salts.” It’s just an odd affectation, and both the singular and plural form are fine. Other than tradition there’s no more reason to say Epsom “salts” — plural — than there is to say “please pass the table salts.” The chemical structure of Epsom salts … so that you know this is a serious article. About footnotes. There are 62 footnotes in this document. Click to make them pop up without losing your place. There are two types: more interesting extra content,1 and boring reference stuff.2 Try one! About green links. Standard blue links go to other pages on PainScience.com, or on other websites, as expected. But green links go to other sections of this page, like this link to the top of the page, or the links in the table of contents. GO TO TOP • CONTENTS • NOTES

Almost no Epsom salt science A search for scientific evidence concerning Epsom salt baths is disappointing. There are basically no studies of their effect on body pain. There weren’t when I first tackled this topic in the early 2000s, and there still aren’t any as of 2020. There may never be. Folk remedies are often neglected by researchers, but not usually so completely. There are usually at least a few experiments testing popular remedies kicking around. Why wouldn’t the use of Epsom salts for muscle soreness be similarly blessed? A crystal of magnesium sulfate heptahydrate — Epsom salt. There is plenty of research relevant to other medical uses of Epsom salts.6 For instance, on my package of Epsom salts, instructions are also given for internal usage as a laxative — which does work78 and is actually FDA approved9 and probably the most common and generally known medical usage. Other (internal) uses of magnesium sulfate include the treatment of irregular heart rhythm, low blood magnesium,10 eclampsia,11 and severe tetanus.12 There are also some incredibly bogus and crazy (dangerous) uses of Epsom salts. For instance, naturopaths may use it to try to dissolve gallstones — which doesn’t work, and can cause serious poisoning. But there appears to be simply nothing at all published about alleviating aches and pains by any means. Apparently, researchers just aren’t interested in studying the effect of Epsom salts on muscle pain , or (more likely) they simply can’t get funding for the work. It’s not just researchers who’ve neglected it. For instance, Epsom salt baths do not even rate a mention in Home Remedies: Hydrotherapy, massage, charcoal, and other simple treatments , a large and credibly referenced compendium of traditional remedies assembled by a pair of doctors. They describe five other medicated baths — alkaline (soda) baths, starch baths, oatmeal baths, peroxide baths, and sulfur baths — for conditions ranging from poison ivy rashes to diabetic gangrene (!), but they never mention Epsom salt baths. In the near perfect absence of directly relevant science, all we can do is speculate scientifically about the possible mechanisms of action. And that I will now do, at absurd length. “(With) a grain of salt,” (or “a pinch of salt”) in modern English, is an idiom which means to view something with skepticism, or to not take it literally. GO TO TOP • CONTENTS • NOTES

Part 2 Toxins & Osmosis What “toxins” do Epsom salts supposedly get rid of? Regardless of whether Epsom salts baths might work, it’s important to understand that the words “detoxification” and “osmosis” — the ideas most commonly invoked to explain the point of Epsom salts baths — are hopelessly misleading and explain nothing. A poison is literally any harmful substance, and even something safe in typical doses becomes a poison in overdose (so you can be poisoned by either lots of water or a minuscule amount of lead). Toxins are technically poisons produced by living things, like venom or metabolic wastes, but informally the word is synonymous with poison. There’s a staggering variety of poisons and toxins, but the two major categories that are probably what most regular people hope they can “detox” are pollutants and metabolic “wastes”: Pollutants are things that get into us from the outside. The best specific candidates would be the persistent organic pollutants like pesticides, flame retardants, and polychlorinated biphenyls (PCBs, now banned, but formerly ubiquitous in many plastics). Lead is also an alarmingly common environmental poison (and much in the news lately). All of these are indeed found in our environment and our bodies, where they mostly get trapped in fat and otherwise sequestered. We definitely would like to get rid these, if only we could. Metabolic “wastes” is a much murkier category, because most of them aren’t actually wastes at all. Cellular chemistry produces a lot of molecules, with many fates. Technically these are toxins because they are biologically produced and they would be harmful in abnormal concentrations… but they are normal products of biology, and so most of them are either safely excreted, or actually re-used and re-cycled. As in the rest of nature, not much in cellular chemistry is wasted. Lactic acid is the ultimate example: misunderstood for decades, even by many people who should know better, lactic acid is not a persistent waste product and you wouldn’t want to “flush” it or “suck” it out of your muscles even if you could.13 If the mythology and misconceptions about lactic acid are this bad, it’s unlikely that there’s some other metabolic waste that is a better candidate for something that is regularly poisoning us and needs to be purged.

Osmosis: not what most people think Why does osmosis even come up for this topic? It seems to be the layman’s notion of how Epsom salts detoxify. I guess they visualize toxins being sucked out of the body through the skin. But even if those nasty toxins are in there and need out-sucking, that is not how osmosis works. Fact-checked to death This osmosis point has been carefully checked and clarified ad nauseum over many years, because this is an absurdly contentious paragraph. It’s not wrong. (And yes, I am aware that water is only one possible solvent — that’s a deliberate simplification.14) Many people get osmosis bass-ackwards: they believe it refers to the movement of things floating in water across a membrane, but that is wrong by definition. It’s actually the water itself that moves. Osmosis refers to the movement of water only across thin membranes, towards the side that is “thicker” with dissolved particles.15 Take it from the Osmosis Cats!show cats16 You can demonstrate this clearly by soaking a potato in salty water. The water is clearly “sucked” osmotically out of the potato’s cells: it loses its plumpness and goes limp. Poor little potato. It’s the water that’s moves around. (Or cats.) And so, by definition, Epsom salts baths cannot suck the toxins out of anyone or anything (or suck magnesium ions into anyone or anything). GO TO TOP • CONTENTS • NOTES

What about sweating? And ionic attraction? A reader spelled out a couple of other commonly paired ideas about how Epsom salts baths might detoxify: I always thought epsom salt baths were supposed to detoxify by opening your pores, and then the salts (ionized) help to supplement your body’s own detox processes (sweating toxins out through pores) by drawing the toxins out through the pores. It’s not osmosis, although the scientifically uninitiated might confuse it with such. It’s just toxins coming out through sweat pores, like they’re supposed to do, only with a little boost because of an ionic attraction. The “scientifically uninitiated” might also think we sweat out toxins. This is one of the most obviously ridiculous of all ideas about detox. Sweating is for cooling, not taking out the trash — it’s not a significant excretory pathway.17 What we can “sweat out” is extremely limited, and what little we can excrete through our pores, we will excrete, with or without a bath … and if we can’t excrete it that way, a bath won’t help. It’s not like sweating is broken and a hot bath will fix it. No one has ever cured anything but stress in a steam room. A sweat lodge has never saved anyone from any kind of poisoning. And even if sweat-to-detox was a thing, you’d still have to explain how cooler, non-sweaty salt soaks are supposed to detox.18 Ionic attraction is just some icing on this quackery cake. It’s an awful idea, most widely known from “ionic foot baths,” one of the oiliest of all snake oils. Ionic bonding is electrostatic stickiness, the atomic scale equivalent of rubbing a balloon and then sticking it to your hair. It doesn’t reach out and grab things like a tractor beam, let alone across a multicellular structure like a sweat gland; it’s more like velcro, bonds forming only when ions actually touch each other. Even if sweat glands did squirt out toxins, ionic attraction can’t help out by reaching across the gland any more than a charged balloon is going to scoot all the way across a room to get to your hair. Sweat glands are a lot bigger than ions. To an ion, a gland might as well be a giant train station. There’s more,1920 but this is too silly to cover in depth. Detoxing by ionic attraction is pure marketing bafflegab, found only on websites like SacredHeartHolisticHealing.com. And now back to osmosis … GO TO TOP • CONTENTS • NOTES

Part 3 Absorption Can Epsom salts actually get through your skin in the first place?

Skin is definitely waterproof (and therefore also osmosis-proof) If Epsom salts get into the body, it’s definitely not by osmosis. Osmosis doesn’t work through the skin, because skin is almost perfectly waterproof. If it weren’t, you would dehydrate like an earthworm on a sunny sidewalk. (You do dehydrate significantly by sweating in a bath, of course21… but Epsom salts do not boost that (see last section). The top layer of the skin, the stratum corneum, consists of dead, dry cells stuffed with a kind of embalming substance, keratin, a fibrous protein. Water can’t go through or around the keratin, thanks to a microscopic “uniquely structured fatty layer” between them, which no one knew existed until 2012.22 The presence and arrangement of lipids betwixt the keratin molecules results in “exceedingly low permeability.”23 There are routes past the stratum corneum, “shunt pathways” such as hair follicles and sweat glands, and some water molecules end up there, in contact with relatively naked cells — but these are waterproof in their own right. Plus we have glands that coat the skin in waterproofing oils! So there’s that too. When those oils wash off, the dead skin cells can soak up a little water and swell a bit, like soaked beans. (Fun fact: that’s not the cause of skin pruning. Skin pruning is actually an active process for improving grip in wet conditions.24) Remember the main myth about osmosis, explained in detail above: osmosis is the movement of solvents (like water) across membranes, not the stuff dissolved in them (like ions of magnesium). What is waterproof is osmosis-proof by definition. The skin is an effective barrier to diffusion of water molecules and therefore of osmosis. This is not to say that nothing gets past the skin, just not much, and definitely not water. Skin: also a booze barrier “Human skin has unique properties of which functioning as a physicochemical barrier is one of the most apparent. The human integument is able to resist the penetration of many molecules.”25 Alcohol molecules, for instance, maybe. You can’t get drunk through your skin, alas. Contrary to the Danish myth. As amusingly shown by Danish researchers in late 2010.26 It’s funny, but it’s not a joke. Of course, the Danish experiment isn’t perfect, and may be contradicted/complicated by other studies in the future. (It would be interesting to see what happens with ethanol, for instance.) But the point is that it’s completely non-obvious to people what substances can or cannot get through the skin. Ask your friends: most of them will guess that some alcohol probably does get through the skin — maybe not enough to get drunk (or booze baths would be a more popular practice), but some. In fact, obviously either very little or none at all gets through, contrary to “common sense.” GO TO TOP • CONTENTS • NOTES

So, what does get across the skin? Because obviously some things do The skin is not a perfect barrier to all substances in all ways, which is obvious because of medicinal patches and creams, allergic reactions, and contact poisons. Some things do indeed get past that fibrous, fatty outer layer to interact with the living cells beneath, or even into the interstitial fluids and blood stream. How do they do it? Size might matter. If molecules are small enough, they can slip through the skin like a small fish through a loose net. In 2000, Bos and Meinardi argued that a teensy enough molecule, smaller than an atomic mass of 500 Daltons, can drift through the stratum corneum27 — the “500-Dalton rule.” And the magnesium ions in an Epsom salt bath are way smaller than 500 Daltons, at a barely-there molecular weight of just 24 Daltons. I’m sure we can just stop there. There’s probably no debate or uncertainty about this, probably no other obscure and technical chemistry considerations. The magnesium is small enough to get through, case closed. Ha ha, just kidding! As in sex, so too in chemistry: size is not the only thing that matters . Water molecules are also extremely tiny — just 18 Daltons — but recall from above that the skin is specifically structured to keep those teensy molecules out. And there are other ways to ban molecules. For instance, cells in the living layer of the skin take an active role in managing the passage of some substances. It’s likely that size doesn’t matter at all in some contexts. Bos and Meinardi were not the last word on the 500-Dalton rule: they knew that most contact allergens were sub-500 molecules, but they didn’t realize that most other relevant molecules are too.28 Contact allergens don’t actually have to get through the stratum corneum to cause trouble — they can get through glands and hair follicles.29 Size might still matter for drugs,30 but it does not for allergens. And still more complexity: magnesium ions have some special properties that might be highly relevant to their absorption. Bizarrely, they may swell dramatically when wet, like tapioca! In fact, this has been the conventional wisdom for some time, and one of the main reasons that many experts have dismissed the possibility of magnesium absorption.31 So … now is the case closed? Wet magnesium gets too fat for absorption? Still no! It turns out this rabbit hole goes way deeper than any rabbit would ever care to burrow. The conventional wisdom about Mg ion swelling has been challenged by some recent research. And I’m going to get into it, because whee, science is fun! But I’ve already gone far enough to make the important meta point here: no one bloody knows how this actually works, and if you think you can guess whether or not magnesium ions get through the skin, please give your head a shake. Biology and chemistry is mind-bogglingly complex and the details are truly, madly, deeply non-guessable. GO TO TOP • CONTENTS • NOTES

Testing magnesium absorption on harvested human skin samples Science: Hey, can I have that skin off your belly? You don’t need it right? Tummy Tuck Patient: Um, sure … Science: Thanks! Got to find out if magnesium ions can get through that. This section is all about one odd experiment32 that involves a number of strange rituals performed on skin samples, like gluing hair follicles shut with super glue. It almost answers the tricky scientific question of whether magnesium ions can be absorbed through the skin, but it still falls short. It does convincingly show that magnesium ions can diffuse through the stratum corneum, and that hair follicles probably facilitate that movement, but it does not establish that they do so in clinically meaningful numbers, especially in the conditions of a typical Epsom salts bath. Regardless, it’s neato science. These chemists had complicated chemistry reasons to believe that damp magnesium ions actually do not swell up too much to fit through the atomic-scale cracks in the stratum corneum, 33 so they set out to test it. They tested absorption on patches of skin harvested from patients who got tummy tucks. Don’t need that skin any more? Donate it to science! The main features of their experiment: They tested the skin as-is, but for comparison they also tested skin that was damaged by applying tape and ripping it off, 30 times!

And they tested skin with hair follicles that they actually plugged with tiny little microscopic dabs of super glue (the details are a bit technical, but basically that’s what they were up to).

And they used a fluorescent dye that binds to magnesium ions, so they could track its progress through the skin. Like I said: an odd experiment. They tested 5, 15, and 60-minute exposures of two concentrations of magnesium solution, medium and strong, corresponding to ocean water and the Dead Sea respectively. (Note that these concentrations are much greater than the concentration of salt in a typical Epsom salt bath. Which matters.) Their key findings: “topically applied magnesium permeates through human stratum corneum”

“magnesium permeability varies based on concentration and time of exposure”

“hair follicles have significant contribution towards magnesium penetration through skin” Magnesium ions diffusing through the stratum corneum. The brighter the warm-toned pixels here, the more magnesium. So, voila, absorption! But there are some major caveats, of course. It’s clear from their imagery that a lot of Mg ions remain “stuck” in the most superficial layer of the stratum corneum, like hair in a drain filter.

Even at concentrations much higher than anyone would normally put in their bath, “a significant increase was only observed at 60 minutes.” Almost nobody is taking super salty hour-long baths (except at “float spas,” more on this below). At 15 minutes, the medium concentration exposure is barely different from none at all.

Although they claim a “significant” contribution of hair follicles, this is an irritatingly poor word choice. They mean statistically significant, not “important” or “large”… and it’s right on the very margin even of statistical significance (p <0.05). I got into the details of this science not because it proves that magnesium soaks through skin, but because it’s charmingly weird, and because it proves that the whole problem is so absurdly complicated that we absolutely cannot guess the solution. Not even an extremely educated guess. Walter White couldn’t guess it. There are just too many ways the messy details of biology might surprise us. The only way to actually find out is to set aside the speculation about what’s possible in principle, and just do some proper before-after testing of magnesium absorption, in actual medicinal situations … GO TO TOP • CONTENTS • NOTES

Some evidence against absorption: Israeli soldiers smear magnesium all over themselves in high concentrations and it doesn’t get inside This quote from a book by Dr. Kenneth B. Singleton34 was submitted to me by a reader as an allegedly “authoritative” opinion on absorption: Regularly bathing in hot water to which Epsom salts have been added can help draw out toxins from the skin. Actually, that is not an authoritative opinion: it’s a vague and silly one. That anyone would mistake it for authoritative is rather depressing.35 The only thing that can determine whether magnesium heptahydrate is absorbed from a bath is careful, thorough testing — opinion is irrelevant, even from a real expert. Fortunately, not all my mail is so depressingly gullible. Hat tip to reader Bryan B. who found an interesting Israeli military study and sent it to me. (I love it when readers do that.) It’s a safety study of a lotion developed “to improve protection against chemical warfare agents.”36 Like suntan lotion, but for chemical burns. Yikes. This lotion had rather a lot of magnesium in it. And soldiers were not poisoned by the magnesium. Indeed, it didn’t appear to cross the skin at all: “there were no significant differences in magnesium levels between the placebo and the study groups in any of the applications.” The delivery system — lotion — could be quite different than soaking in water with dissolved magnesium sulfate. But it is pretty noteworthy evidence that absorption is minimal or nil when putting high concentrations of Mg on the skin. That information is not necessarily correct, but it is certainly more authoritative and worth bearing in mind, than the opinion of Dr. Singleton. GO TO TOP • CONTENTS • NOTES

Frequently cited evidence for absorption: Rosemary Waring’s little unpublished 2006 study In 2006, Rosemary Waring, a British biochemist at the University of Birmingham, did a nice science experiment with Epsom salts.37 She did more or less exactly what any curious person would do if she wanted to know whether or not Epsom salts can get past skin: she measured magnesium and sulfate in the blood and urine both before and after people bathed in Epsom salts. Dr. Rosemary

Waring She found them to be higher after the baths! 16 out of 19 people had more magnesium and sulfate in their blood after the baths than they did before the baths.38 Dr. Waring’s results are straightforward. No therapeutic effects of Epsom salt were studied or claimed — she just studied absorption, and did not try to make any more of it, showing the restraint of a pro. What could be simpler? I was so interested in these results (although still a bit skeptical) that I contacted Dr. Waring by email. “I agree that it is a bit surprising,” she replied, “but the results are certainly there and in fact there are hints in the past literature that this could happen.” And how would it work, I asked, this crossing of the skin? Dr. Waring: I don’t have any evidence as to how magnesium sulphate crosses the skin, though I have always assumed that it simply diffuses across the stratum corneum, helped by the fact that it’s in a hot bath. Possibly not the best science ever done Dr. Waring’s study has been cited thrown in my face by countless people over the years who think I am not aware of it. Every member of this widely distributed angry mob read this article only just far enough to get angry enough about my skepticism to send an email. Many of them claimed to have read the whole thing, but apparently they missed Dr. Waring’s name in a heading, her photo, or the full section of analysis. And, shocker, none of them seemed to be aware of the potential problems with Dr. Waring’s research. And there are some problems. For starters, unfortunately fourteen years later, her tiny experiment has never actually been published,39 and that’s a major reason for caution.40 It is a basic rule of science that evidence can’t really be taken too seriously until it has been exposed to peer review and repeated by other scientists. Just because experimental results haven’t been replicated yet doesn’t mean we ignore them, but it does mean that we have to take them with a grain of salt. (That pun was simply unavoidable.) Also, Dr. Waring also has a relevant bias: she’s interested in magnesium supplementation as an autism treatment.41 I also think it’s notable that Dr. Waring’s speculation about mechanism is extremely basic. People assume because of her unpublished study that she’s a big time salt absorption expert, but obviously she wasn’t considering any of the chemical complexities discussed above … or below. She assumed, like most people, that the heat of a bath probably increases the permeability of the skin. But that’s not a safe assumption, or an expert assumption. GO TO TOP • CONTENTS • NOTES

Hot hot hot! Does heat increase skin permeability? Enough of it sure does. But probably not bath heat. Speaking of studies that get thrown in my face, someone haughtily hurled this one at me as if it was the last word, absolute proof that a hot bath boosts magnesium absorption. Let’s look … A 2008 experiment showed that brief, intense heating of the skin can dramatically increase its permeability.42 Park et al tested short bursts of heat: 5 milliseconds to 5 seconds at 100 to 315 degrees Celcius. With more heat, dramatically more molecules could cross (the duration of exposure had less effect). Skin permeability was increased by a few multiples in the low end of the range, all the way up to three orders of magnitude at the most extreme temperatures. Wow. The mechanism is fascinating: enough heat can basically burn microscopic holes in the surface of the skin, creating artificial pores . Ouch? It sounds awful, but it’s actually painless, because the application of heat is so brief. At lower temperatures, the increased permeability is due to messing with the stratum corneum lipid and keratin structures, making them a less effective barrier. Is any of this applicable to baths and the absorption of magnesium sulfate? The highest comfortable bath temperature for most humans is around just 40˚C (104˚F), which is far cooler than the coolest temperature tested in this experiment. The effect studied mostly depends on actually damaging the skin. It is conceivable that permeability starts increasing at lower temperatures with longer exposures … but sixty degrees lower? For the duration of a bath? Probably not for most substances. Skin probably evolved to be a good barrier across the range of temperatures humans are exposed to, which would certainly include 40˚C. Also, not all substances will respond the same way to heat. If you studied transdermal delivery of many different substances at 100˚, you’d probably see a wide range of effects. The only way to know if the skin is more permeable to magnesium at 100˚ or 40˚than at room temperature (as with a lotion) is to check — and no one has, to the best of my knowledge. And so this study definitely does not actually show that the heat of a bath enhances magnesium absorption … and neither does any other study I’m aware of. GO TO TOP • CONTENTS • NOTES

Maybe up your bum? Or hoo hoo? Nostrils? Absorption via mucus membranes How else could magnesium sulfate possibly get into the bloodstream? If it does, as Dr. Waring’s experiment seemed to show? Reader Adrian J. had an unusual idea: Is it possible that the salt diffuses across the epithelium in the anus if the rectum relaxes to some degree in the warm water? Wow, that’s some awesome lateral thinking! And I think it’s actually plausible, because now we’re talking about a “mucus membrane,” a completely different and much more permeable layer than skin. For what it’s worth, we know that alcohol absorbs quite handily through the rectum — rather too well, in fact, so do not try at home.43 But it has to be pretty much injected. (Live a little: click that footnote!) But I find myself uncomfortably wondering … just how much do I relax in a hot bath? That much? And how much salt could diffuse across that more permeable but much smaller membrane? It’s a tiny target! I shudder to think of the methods required to test this. Vaginal absorption? The up-yer-bum hypothesis was surprising, but you’d be even more surprised by how many readers have asked if the vagina might be an absorption route. A fair question, but this has the same problem as anal absorption: too small and too tight. After quizzing several bemused femme-friends about it, I am confident that it would be highly irregular for any respectable quantity of bath water to percolate into one’s ya-ya. And you thought an article about salt baths would be boring! No wonder this is the most popular Epsom salts analysis on the internet! How about inhalation? Another possibility for mucus membrane absorption Maybe salt can be inhaled with steam. There’s actually a therapy (“halotherapy”) based on this, but it’s a weak idea in general and impossible as a salt delivery system: when water evaporates, it leaves most solutes behind, salt in particular.44 Many substances evaporate with water and “contaminate” steam — lots of volatile compounds and assorted tiny particles basically just getting thrown around, which is why we can smell a bubbling pot of soup — but these occur only in trace amounts, mostly nowhere near enough to be plausibly medicinal. Human olfaction, despite being shabby by animal kingdom standards, can still get a nice rich scent from a mind-bogglingly small number of molecules. Water from a soup is still remarkably pure despite the odour, and definitely has no salt in it. Another related possibility is that we might inhale tiny droplets of water (aerosols of salt water) that float over the surface of a bath. Such droplets would contain dissolved salts at the same concentration as the bath, but these are nearly microscopic tiny water droplets. Remember that most people can swallow an entire magnesium pill with no obvious effect, which contains insanely more magnesium than you could ever absorb from the air over an Epsom salt bath, assuming there’s any at all, most of which would never even come close to a mucus membrane. Again, not really a plausible source of medicinal absorption. GO TO TOP • CONTENTS • NOTES

Absorption “conclusions” The Epsom Salt Council claims, without detail or nuance, that “magnesium sulfate is absorbed through your skin,” citing the opinion of the notorious Dr. Christine Northrup to support the point, without so much as a link to substantiate that this is in fact her opinion. But it probably is: Dr. Northrup is not stingy with her beliefs. She’s on the record expressing belief in chakras, astrology, angels, mysticism, feng shui, and Tarot cards, and she has earned extensive, harsh criticism for the way she “famously pushes woo in the cause of women’s health.” But the Epsom Salt Council is happy to cite her. The same page is littered with similar examples of shallow reporting and appeals to dubious “authority.” For instance, further along, they also cite Carolyn Dean on this topic … a former MD who had her Canadian license to practice medicine revoked. And so on. That’s the kind of quality scholarship The Epsom Salt Council has brought to the conversation. In this article, I’ve presented several lines of evidence, demonstrating that the absorption question is surprisingly complicated, interesting, and inconclusive. We will have to live with the mystery. Meanwhile, it is obviously reasonable to be skeptical, as many experts are. There are many reasons to suspect that absorption is trivial. A thorough 2017 scientific review of both the evidence and rationale for transdermal absorption of magnesium makes a critical point: although there may now be adequate evidence to suggest that some transdermal absorption is possible in the right conditions, that evidence is not nearly strong enough to support claims that it is superior to oral supplementation.45 Gröber et al conclude that they “cannot yet recommend the application of transdermal magnesium.” It’s not just a matter of whether Epsom salts can be absorbed … it also has to be a better way of getting magnesium into the body than simply swallowing it, which is very easy. That’s a much higher bar to clear. And then there’s the question of whether or not a little extra salt in your blood actually matters medically. And that finally brings us to the second major part of the article … GO TO TOP • CONTENTS • NOTES

Part 4 The therapeutic value of Epsom salt If magnesium and sulfate ions can get from bath to blood, what exactly do they do when they get there? If Epsom salts do get across the skin, so what? Is it any good to have some extra ions of magnesium and sulfate kicking around your bloodstream? Why did the ions cross the skin anyway? What’s the point? Magnesium deficiency may be a real problem, and it may also be related to pain, supplementation might make sense, and soaking in the stuff could be a way of getting some magnesium… but probably not as good as just eating it. Sulphate deficiency could also be thing, but to a much lesser degree, and much less clearly. It’s a much more obscure topic (very little science). There is little doubt that magnesium sulfate probably has some effects on physiology in some contexts. Several of those effects are well known, including a few common medical applications mentioned earlier. There are also unpleasant effects, like diarrhea. But, judging from the established medical uses/effects of Epsom salt, there is definitely no particular reason so far to believe that having more magnesium or sulfate in your blood is going to be much use to you — unless you have eclampsia, tetanus, or a confirmed magnesium deficiency. I’m aware of only one science experiment that seems to be directly relevant to the effect of more magnesium in the bloodsteam on pain. It was a study of the acute effects of injected magnesium sulfate, which is presumably very similar to absorbing it, just faster. The injections “did not reduce muscle pain” and caused “unpleasant side effects.”46 Yuck! Not exactly encouraging! So there’s not really any particular reason to believe anything about the therapeutic effects of Epsom salts for aches and pain. We can really only speculate. And speculating about basic biology is really difficult. It’s a great way to be wrong.

Sulfate supplementation (what little there is to say) Magnesium is a mainstream electrolyte, a famous molecule, and many people are supplementing magnesium, for better or worse. But sulfate is obscure, and sulfate supplementation is quite rare. The magnesium information below is resting on fairly firm foundations, plenty of science; sulfate is an information desert by comparison. You can find a few popular articles about sulfate deficiency… most of which actually have very little to say about sulfate and quickly veer off into magnesium. There’s almost no science at all about sulfate deficiency. I asked Dr. Waring to speculate about the therapeutic effects of Epsom salt, and she asserted that patients with rheumatoid arthritis are known to have “low sulfate levels,” but this is not really established science.47 And that’s really all I have — all anyone has — on the topic of sulfate supplementation. For the rest of the article, I’ll just be focusing on magnesium, which is on much firmer ground conceptually… GO TO TOP • CONTENTS • NOTES

Magnesium deficiency and supplementation Magnesium is one of the essential minerals — “life finds a way,” but not without a good supply of Mg it doesn’t. We use a lot of the stuff: it’s one of the big three mineral nutrients, along with sodium and potassium. If it’s missing, blood pressure rises, glucose tolerance drops, and our nerves get just a wee bit trigger happy. Interestingly, fixing a deficiency is actually sedative! It literally calms your nerves, and that could certainly be relevant to chronic pain.48 So, despite its biological importance, deficiencies may be both common and related to pain. It’s in roughly the same category as vitamin D deficiency and pain: it might matter to a lot of people, especially pain patients, and supplementing is quite safe.49 In the extremely scammy world of supplements and nutraceuticals, vitamin D and magnesium both stand out as being less lame (especially for chronic pain patients). The other big idea about magnesium is that it prevents cramps. This idea is mostly inspired by the myth that cramps are caused by dehydration and a shortage of electrolytes like Mg.50 If low Mg doesn’t cause cramps, adding Mg probably won’t prevent them.51 Availability: magnesium is easy to get Regardless of its biological effects, making a point of getting extra magnesium from pills or baths probably isn’t necessary: just eat right! Unlike with vitamin D, which is a bit tricky for a lot of people to get, it’s downright easy to get enough Mg from a reasonably healthy diet. Despite this, Mg deficiency is common even in wealthy places because grains and meat are poor sources, and the good sources are not nearly as popular (salad, basically — leafy greens and nuts are especially good). But anyone with a magnesium deficiency can suck it up and eat a nutty salad a couple times a week and Bob’s your uncle . So while some people might need magnesium, and deficiency might be related to pain, it’s easy to fix with food and even easier to fix by taking pills. Bathing in the stuff is like going a half hour out of your way to buy stale bread from a corner store when you live next to a good bakery. But I’ll get into it anyway, just to be thorough. GO TO TOP • CONTENTS • NOTES

Epsom salts baths as a source of magnesium To embrace Epsom salts baths as a helpful method of supplementing magnesium, at least three things need to be established: Magnesium deficiency has to actually be causally linked to common aches and pains — not more serious/chronic pain. While it would be a big deal if we knew that magnesium deficiency was a factor in chronic or serious pain, it wouldn’t have anything to do with the legend of Epsom salts, which are supposedly good for minor acute pain. Magnesium supplementation has to actually be an effective treatment (regardless of the source). Even if a lack of the stuff is a problem, simply oversupplying the system will not necessarily help. Not all deficiencies are caused by an inadequate supply. The classic example is malabsorption of iron as a cause of iron deficiency: it doesn’t matter how much you consume if you can’t absorb it. An Epsom salts bath specifically has to be actually better than dietary sources or oral supplementation — otherwise, just eat the stuff! None of these things has actually been established, and the absence of any one of them is a deal-breaker for bathing in magnesium. The last one is the most obviously flawed: it’s extremely unlikely that bathing is the best way to supplement magnesium, especially when there are such obviously viable, convenient alternatives. The increased levels of magnesium ions shown by Dr. Waring’s experiment — the most absorption-friendly evidence available — were quite small (and there was no increase at all in some subjects). Consistency in dosing would also be impossible by bath, since we simply haven’t got a clue how much gets absorbed, if any, never mind its chemical fate after that. While it of course possible that we can supplement magnesium by soaking in it, I can’t really see why anyone would actually prefer that option to just eating some extra nutty salads. GO TO TOP • CONTENTS • NOTES

No matter what magnesium can do, it can’t do everything This is a classic problem with all kinds of supposedly amazing pain cures: pain has too many different causes for one medicine to be really effective. There are many types of body pain that have little or nothing at all in common with each other physiologically. To name just a few examples of pain causes that are biologically distinctive from each other: drugs side effects

vitamin D deficiency and pain — good luck treating that by soaking in magnesium

traumatic injury pain (acute inflammation)

repetitive strain injury (subtle, low-grade inflammation and tissue degeneration)

idiopathic neuropathy

genetic hypermobility

sensitization

analgesic rebound

muscle cramping (just one of many kinds of muscle pain)52

delayed-onset muscle soreness All of these work in different ways, and so no one thing can possibly treat them all effectively, or even a few of them. (And it’s not even a complete list! Here’s a complete list.) For instance, an anti-inflammatory medication would fail with almost all of them except the acute inflammation of a recent injury. Even the subtle, low-grade inflammation of repetitive strain injury seems to be too different from acute inflammation for those medications to work. While it’s certainly possible that increasing levels of magnesium ions in the bloodstream could help with one or two kinds of pain problems, it’s extremely unlikely that it would help enough different sorts of pain to be generally “good for” pain, especially the kind of acute soreness that we all occasionally suffer from. Similarly, Epsom salts probably cannot simultaneously perform both of the two tricks most often touted: “relieve pain” and “speed healing.” Those are completely different things, and it’s unlikely that one substance can help with both goals. Those goals might even be mutually exclusive. For instance, the primary source of injury pain is inflammation — a complex and painful physiological process intended to … wait for it … speed healing. Indeed, the only known mechanism by which you can in principle recover faster from an injury would be to increase inflammation. If bathing in Epsom salts did that, it would make you hurt more, not less. Of course, there could be other ways to speed up healing — in an “anything’s possible” kind of way — but it’s still pretty far-fetched that a single molecule could pull off both that miracle and reduce pain at the same time. The point here is just that the conventional wisdom about epsom salts is pretty murky and non-specific about exactly what and how and it is supposed to help. Good for “brain troubles” Salt has been used for well, just about everything. Like these effervescent brain salts. Cory Doctorow: “The best thing about effervescent brain salt is that it’s not immediately clear whether it’s salt to make effervescent brains even more delicious, or salt to give you an effervescent brain, or effervescent salt for brains. Also, it appears to come in a Tabasco bottle & everything that comes in a Tabasco bottle is always awesome.” GO TO TOP • CONTENTS • NOTES

What’s a calcium channel, how do Epsom salts block it, and who cares? Once in a blue moon, you’ll see Epsom salts (or magnesium in particular) described as a “calcium channel blocker” with the implication that this is obviously good for pain. Unsurprisingly, this is another misleading oversimplification. Although it’s more specific and impressive sounding to talk about calcium channel blocking, it’s not a heck of a lot more meaningful than talking about “detoxification.” Calcium channels are itsy bitsy holes — molecular scale holes53 — in cell walls that let calcium in and out as a trigger for a bunch of biochemical business. They exist primarily in muscle tissue, blood vessels, and neurons. There are a number of druggy ways to interfere with them, including magnesium. Calcium channel blocking is fairly well understood physiology, and the main clinical application is to decrease blood pressure (which it does by reducing the strength of muscle contraction in the heart and blood vessels). Although other effects undoubtedly exist, there is no particular reason to believe that they have any potent effect on any flavour of pain. In fact, there is an excellent reason to believe that they don’t: because no one is even claiming it. Calcium blockers aren’t rare drugs; lots of people are walking around with these chemicals in their blood. But I don’t think that people on calcium channel blockers are walking around feeling no pain, like a superpower. All noteworthy effects of drugs quickly become well-known, even when they have nothing to do with the original idea of the drug; there are countless such “off-label” uses, many of them of trivial or even imaginary, but exploited anyway. It’s completely implausible that calcium channel blocking has a powerful pain-killing effect that no one’s noticed. Yes, it is possible that magnesium absorbed through the skin does something different, something good, for certain kinds of pain. After all, different calcium blocker drugs have different effects! But there’s not a shred of good, direct evidence of it. So it really boggles the mind that anyone would toss this idea around with any confidence. Seriously, they’re pretty much making it up as they go — wild speculation. GO TO TOP • CONTENTS • NOTES

Magnesium as a pain-killer after surgery There is some limited evidence that magnesium may reduce pain, perhaps because it is a “calcium channel blocker and N-methyl-D-aspartate antagonist,” as in a 2009 experiment.54 Several other studies have been done, with conflicting results, and generally poor quality (of course). Most of these were reviewed in 2007:55 four showed a minor but technically positive effect, seven showed no effect greater than a placebo, and in one experiment the subjects actually experienced more pain (ouch). And so, although “the biological basis for [magnesium’s] potential antinociceptive effect is promising,” the authors actually concluded that no pain-killing effect could be found. These conclusions were basically repeated in a 2018 review,56 despite the inclusion of some positive studies (like the 2009 one cited above). So much for the miracle of calcium channel blockage: the data taken as a whole doesn’t just fail the “impress me” test, it is actually convincing in the other direction. Despite the discouraging state of the evidence, this mystery is not solved yet, and there actually is a plausible mechanism for magnesium ions reducing pain. While it is clearly neither well understood nor reliable, it does exist, and it might shine in the right circumstances. But I doubt those circumstances are an Epsom salt bath. Could magnesium fail to relieve pain in all those ideal testing situations, and yet somehow succeed when absorbed from Epsom salts baths? File under “nothing’s impossible,” but the idea is definitely a reach. GO TO TOP • CONTENTS • NOTES

Another possible biological effect: antibacterial? Osmosis and all the wee beasties Reader Dorrie B. pointed out something interesting: Epsom salts might be an effective treatment for topical skin infections, as salt is certainly inhospitable to many microganisms. It can suck them dry, like the potato in the classic osmosis demonstration. This is probably the explanation for some rare prescribing practices I’ve heard about, such as recommending Epsom salts baths for anal fissures. Yes, my email inbox has anal fissures in it. An Epsom salt bath definitely cannot disinfect a puncture wound (as one of my readers was told). A strong salt solution is anti-bacterial, but the problem with rusty nails is the risk of deep injection of Clostridium tetani — far beyond the reach of any soak.57 But this is also a great example of how complex these medical puzzles can be, because salt bathing might also damage populations of other bacteria on the skin, and/or make the skin more habitable for bacteria (moister), resulting in more vulnerability to infection rather than killing one.58 Do people who bathe or swim in salt water regularly suffer any ill effects? Are they more susceptible to new infections? They might well be: even a 10% or 20% difference would not be obvious to the victims, but would nevertheless be clinically significant and biologically interesting. That research hasn’t been done, but my point is just that it’s always surprisingly difficult to say whether or not a given biological effect is good. Usually “it’s complicated” — the official slogan for this article. Infection-fighting anecdote: A good friend of mine had a substantially infected hangnail. It seemed like a good opportunity to recommend soaking an infection in Epsom salts. He gave it a try … and the infection got dramatically worse over the next 48 hours, until he had to go to the doctor to get some antibiotics. The doctor he saw suggested that the soaking probably just made the hangnail nicer for the bacteria: they do love their moist nooks and crannies! GO TO TOP • CONTENTS • NOTES

Mixing up the effects of salty and non-salty baths Obviously non-salty baths have some benefits of their own. People like baths “because reasons.”59 What’s funny is that Epsom salts routinely get the credit for these benefits. Here’s how this happens in six easy steps: Patient has a chronic ache or pain and tries to treat it with non-salty hot baths or soaking. However, because it’s just a bath and expectations are low, this effort is never particularly diligent. This is key to the setup: the patient has never really given non-salty soaking a good try. Patient gets the idea to try Epsom salts! This seems much more promising. Now there’s medicine in the bath. Thus inspired, the patient proceeds to soak quite diligently — much more diligently than ever before. When some benefit is then observed, the patient attributes this to the salt — of course. Maybe it is, but maybe it’s just the unusual regularity of the nice hot soaking. Relaxation alone is definitely a potentially relevant effect. But the point is that we obviously can’t know … but the patient is now officially biased in favour of salt. If the benefits are at all notable, this person will usually start proclaiming to anyone who will listen that they “know” that Epsom salts work. When challenged (“It might be just the hot bath, eh?”), they will almost certainly object and claim (correctly!) that they have tried simple hot soaking without results. They have indeed. But it was never actually tried well enough to really know. Tricksy, the human mind is. GO TO TOP • CONTENTS • NOTES

Floatation therapy Is there any other reason to put Epsom salts in your bath? Well, Epsom salts dissolved in your bath does make the water feel nice. 😉 No research is required to prove that: just try it! Most people agree that the water feels smoother, slicker, silkier. And then there’s the bouyancy, which is actually the basis of an entire industry. Salt makes you floatier! Infinitesimally floatier. High concentrations of Epsom salt in your bath will increase the water’s specific gravity (density) to the point where you will start to float — just like in the Dead Sea, or Utah’s Great Salt Lake — because the body is, on average, much less dense than salty water. The concentrations of salt required for floatation therapy are much higher than Epsom salt packaging recommends, by the way.60 However, any salt in your bath — Epsom or otherwise — is going to make you at least a little bit lighter in the water. Most people don't bathe in high concentration of Epsom salts for long periods — probably 99.9% of all Epsom salts baths are home baths with very low concentrations. But that’s at home! You can also pay to bathe in much, much higher concentrations. Floatation baths and tanks are quite popular, and I have tried them and I’ve enjoyed them. Although the floating industry is not new — sensory deprivation tanks have been around for decades — it has definitely been surging. There are several new float spas here in Vancouver in the late 2010s. In some places, the floatation industry is selling frequent and long salt baths, up to two hours of soaking and floating at a time. That much time is a luxury very few people can afford! Shorter floats are much more common. It’s plausible that bathing in such high concentrations has different effects than home Epsom salt bathing, but no one is studying that. The main purpose of floatation therapy is to reap the benefits of deep relaxation, which are noteworthy.61 It’s a lovely experience, but it’s also irrelevant to the relief of muscle aches and pains except via the straightforward (and perfectly legit) mechanism of relaxation. For more detailed discussion of floatation and immersion therapies, see Get in the Pool for Pain: Aquatic therapy, aquajogging, water yoga, floating and other water-based treatment and injury rehab options. GO TO TOP • CONTENTS • NOTES

Part 5 Wrapping it up

Consider the source 98% of the time, “osmosis” and “detoxification” are the concepts presented as the justification for bathing in Epsom salts. Don’t trust advice that is obviously simplistic. It involves some huge leaps of logic and detail avoidance … all made by people who are usually trying to sell the stuff. Consider the source. Epsom salt bath prescriptions are often embellished with some really strange claims of healing powers. For instance, I found one website that recommended taking Epsom salts internally as well as bathing in them: Researchers in nutrition, through controlled experimentation, have found that Magnesium sulfate accelerates the body’s healing time by 30%. As an example, if an injury required three weeks to heal under normal or standard conditions, it would only require two weeks to heal if Magnesium sulfate was added to the diet as a nutrition [sic].62 That’s really ludicrous. Accelerated healing time is a comic book concept — something Wolverine does — not an even remotely legitimate medical concept. And imagine the unpleasant surprise of the hapless reader who takes this advice when they discover the laxative effects of ingesting Epsom salts! Naturally, no source for this alleged experiment was given. Epsom salts bathing is often recommended carelessly and overconfidently, without any genuine knowledge of the physiology or science (or lack thereof). Those who claim to “know” that Epsom salts work cannot seem to demonstrate that they also “know” much about physiology or science. While it certainly remains possible that there is a therapeutic effect, it’s pretty clear that we shouldn’t take their word for it. GO TO TOP • CONTENTS • NOTES

Selected questions and answers Some questions too common to ignore… some questions too funny not to try to answer… and one or two that are just good questions. Q. Why would the FDA allow studies to be published that show Epsom salts are more effective than their muscle relaxers and pain pills? For $1/lb of epsom salt this would kill their market and profits. A. Industry protectionism for baths? Preposterous! The FDA has literally nothing to do magnesium sulfate research. Any Big Pharma/FDA conspiracy against Epsom salt is clearly failing, because the stuff is available literally everywhere, and the FDA actually approves it for use as a laxative and a variety of external uses. Approval seems like kind of a funny way of implementing an anti-salt agenda. In short, this question is just knee-jerk anti-mainstream medicine paranoia based on major misunderstandings of what the FDA is and how it works. Nevertheless, it is a concern I’ve often see expressed, so I decided it was time to address it here. Ironically, if there is any relevant commercial bias, it is one in favour of Epsom salts. For instance, the Epsom Salt Council exists to promote the industry and is “eager to let everyone know the benefits of our product and … spread the word about the wonder that is Epsom salt.” They prominently publish uncritical and unequivocal claims of medical benefit on their website. Q. So you don’t believe that Epsom salt will do anything in a bath. How about sea salt? You believe it’s the same? Useless for muscle aches and stuff like that? A. Yes, I believe it is “the same,” at least insofar as it is probably “useless muscle aches and stuff like that.” Of course there is much more chemistry going on in sea water than Epsom salt baths, but not in any way that seems to make any practical difference. In fact, it’s pretty clear that people who swim in the ocean a lot are not enjoying impressive pain-killing benefits the rest of us are missing out on — which is yet another example of how the skin is a pretty effective barrier. Sea water isn’t medicine, and no one who matters thinks it is. Do not try at home Oddly, there are a lot of electric baths in medical history. Electricity was a wonderful way to make health ideas based on vitalism seem more real & science-y. Q. Maybe there’s a “bio-electric” function to Epsom salts in water. There’s more and more being discovered about small electric/magnetic fields and how we are affected by them. A. Er, no, I think not. It’s not inconceivable, but it is pretty far-fetched. It’s generally true that biology ingeniously exploits most properties of nature to get things done, including electromagnetism, and we likely still have things to learn about that (that’s what the book The Body Electric was about, and despite its age and flaws it’s a darned interesting read). But whatever those systems might be, it’s super unlikely that they have any meaningful interaction with a slightly salty bath, let alone one that’s relevant to aches and pains. It’s even less likely that any such effect wouldn’t be much more obvious in, say, sea water (see last question). Even if salty baths just bestowed a vague feeling of well-being and vitality, like mountain air, that would be biologically remarkable … but also still well short of a useful medical effect. And in fact salty baths do not have an obvious mountain-air like goodness. Q. Why does the bag of epsom salts (and other random places online) warn that diabetics should not take baths with epsom salts? A. That’s a bit of a stumper. As far as I can tell, it’s just about the fact that diabetic feet are vulnerable, and soaking increases the risks of infection that they might miss due to impaired sensation. The foot has a very thick and often damaged superficial layer of the skin (stratum corneum), which might make it a little more vulnerable to damage from soaking, but that’s any kind of foot soaking: I see no obvious reason for singling out Epsom salt soaks, except that it’s popular. Maybe diabetics, with their many foot problems, are somewhat more likely to want to soak their feet. Ironically, one of the few good medical benefits of Epsom salts may actually be infection control. It might be bit of a topical antibiotic… but it could also help microorganisms, a caveat we explored earlier. Q. Is Epsom salt good for a hangover? Something to do with electrolytes … A. Electrolytes are ions (like magnesium) in an electricity-conducting solution (which includes blood), and they have countless roles in our physiology. Getting drunk can deplete and/or disturb electrolytes (“electrolyte chaos” is a thing with alcoholics). So that’s the kernel of truth here, but there are many problems on the way from the kernal to the idea that Epsom salt actually being “good for a hangover.” Let’s start with the fact that it’s fairly unlikely that soaking in Epsom salts is actually an effective way to get some magnesium into you in the first place. Meanwhile, food is hands-down the best form of electrolyte supplementation, and water is the best overall treatment for hangover because most of the symptoms are due to dehydration, not electrolyte depletion/chaos. And then there’s the fact that magnesium is only one of several important electrolytes. And then there’s the biggest problem: with your garden variety hangover, there is some electrolyte imbalance due to the dehydration, but not major shortages of electrolytes. The mechanism for significant electrolyte depletion is a lot of vomitting, diarrhea, and sweating, which is only going to be applicable to the very worst of all hangovers. That’s a lot of problems! But Epsom salt is mainly reckoned to be good for a hangover for the same simplistic reasons it’s touted for everything else: detoxification and relief from aches and pains. A hangover hurts, right? And you’ve basically poisoned yourself! Since the thinking about Epsom salts never even remotely attempts to get specific about what kinds of toxins or aches and pains, really any of them are candidates. It doesn’t work. Believe me, I’ve tried. This is the kind of “medical” advice you’re going to get only from celebrities and amateurs. Q. I am currently pregnant and looking into ways to treat fluid retention. Many websites say that epsom salt baths will “draw out” the excess fluids but I can’t find any information about this being studied at all. I will probably try it tonight anyway, but as a scientist I would love to know if there is any basis to it. A. This idea about Epsom salts bathing is relatively rare, but it is also much more about the idea of osmosis than the more common detoxification claim. Any talk about the “drawing out” of water is definitely about osmosis (whether the speaker knows it or not). There’s a classic science classroom demonstration of osmosis, which involves sucking the fluids out of a potato by soaking it in salt water. That’s a perfect analogy for this claim. If a potato could be “bloated,” it could take an Epsom salts bath to solve the problem, and osmosis would be the mechanism. Unfortunately, we are not like potatoes, and this simply will not work for bloated humans, because the skin is totally impermeable to water, as discussed thoroughly in the first third of this article. Nor is there any plausible pathway for osmosis across mucus membranes — unless you’re taking a very weird bath, there simply isn’t enough contact between any mucus membrane and bathwater. GO TO TOP • CONTENTS • NOTES

Final thoughts I can do no better in defense of Epsom salt bathing for aches and pains than “anything is possible.” There is no good or specific reason to believe that bathing in dissolved Epsom salts will have the slightest effect on muscle soreness or injury recovery. Although this folk wisdom may someday prove to have a sound rationale, clearly there is none that its advocates have thought of — or even tried to think of, it seems. There’s even decent evidence that Epsom salts can’t get past the skin barrier — Israeli soldiers can smear on magnesium rich cream without the slightest effect on their blood levels of magnesium. That’s pretty damning. On the other side of the evidence, thanks to Dr. Waring, we know that it’s still possible that we are a living experiment in absorbing magnesium sulfate ions every time we bathe in dissolved Epsom salts! And maybe, just maybe, they do something worthwhile once they get past the skin. And it’s very cheap, and almost certainly safe — just as no one is obviously getting any miracle cures out of Epsom salt bathing, they aren’t suffering any obvious ill effects either. So, why not? At the very least, they’ll make your bath feel silkier! And at most? Who knows — maybe those magnesium and sulfate ions do have some healing powers. It’s not impossible. Just don’t buy into all the crap about osmosis and detoxification. As the old Scottish proverb says, “Always keep your mind open — but not so open that your brains fall out!” GO TO TOP • CONTENTS • NOTES

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About Paul Ingraham I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of ScienceBasedMedicine.org for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter.

Part 6 Appendices

Still need help with myofascial pain? Now that you probably won’t be counting on Epsom salts …

If you think this article is detailed, you should see my tutorial about muscle pain and myofascial pain syndrome! This kind of exhaustively researched writing about Epsom salts is only possible because I sell some of the other articles on this website. No writer can afford to create truly good, detailed content and then just give it all away: we have to make a living somehow. Please reward my efforts by taking a look at my tutorials. Although Epsom salts seem unlikely to be a significant source of relief, there are plenty of other options for self-treatment of muscle pain. PainScience.com publishes an extremely thorough tutorial about myofascial trigger points (muscle knots):