Welcome to Dear Julia, a column where readers submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? Julia Belluz sifts through the research and consults with experts in the field to figure out how science can help us live happier and healthier lives.

Dear Julia: Which dietary supplements can I trust?

Let me be clear: I’m not a fan of supplements. After writing about them for the past several years, I have come think they should be avoided unless absolutely necessary.

Study after study has demonstrated that our beloved multivitamins don't actually effect most of the health outcomes they claim to — from staving off cognitive decline to preventing cardiovascular disease and cancer. The health benefits of probiotics have been wildly exaggerated (there’s good evidence that they can reduce the risk of diarrhea caused by antibiotics — but that’s about it). And taking antioxidant supplements like beta carotene and vitamin E might even harm you.

Supplement makers don't need to prove their products are effective or even safe before putting them on store shelves — and problems with quality and adulteration appear to be distressingly common. A Vox review of government databases, court documents, and scientific studies uncovered more than 850 products that contained illegal and/or hidden ingredients — including banned drugs, pharmaceuticals like antidepressants, and other synthetic chemicals that have never been tested on humans.

Because of lax regulation, there's no way to know if the contents of your pill bottle will work as promised, or even if it contains what’s listed on the label.

Related The hidden drugs in your favorite supplements

Having said all that, there are times when you might actually need to take a supplement. Maybe you’ve been diagnosed with a medical condition — like an iron deficiency. Or you’re planning on becoming pregnant and your doctor suggests folic acid tablets, which have been shown to prevent neural tube defects.

In these cases — when a real live health professional (and not a gym trainer or celebrity doctor) suggests a supplement — you’ll want to find as trustworthy a product as possible. And there are little tricks you can use to tell which products are higher quality and carry less risk. (They also happen to be the tricks athletes, like Olympians, employ to make sure they're using clean products.) Here’s a quick guide.

Look for third-party certification seals on the label

To fill in some of the regulatory gaps around supplements, third-party verification systems have popped up.

One such organization is the United States Pharmacopeial Convention, an independent group of scientists that sets quality and purity standards for medicines, food ingredients, and dietary supplements. Any product with a USP mark has gone through the group’s testing and auditing program. Among other things, it checks that what’s on the label is in the bottle (in the declared amounts) and that the product is free of contaminants and was made in a clean facility.

NSF International, formerly known as the National Sanitation Foundation, is another public health standards organization that audits supplements. The NSF mark also signals that a product’s ingredients and dosing have been independently vetted. NSF has an added perk: It also tests for more than 200 substances — such as narcotics, steroids, and stimulants that have been banned in sports competitions. This is why professional athletes tend to favor NSF-verified products.

But these systems, while helpful, aren’t perfect. Note that they don’t evaluate the health claims — like creating stronger bones, shiny hair, or strong nails — so often listed on product labels.

They also can’t entirely guarantee safety and purity. "Even if it’s third-party certified, that’s a risk reduction step — not a guarantee," warned Amy Eichner, a special adviser on drugs and supplements at the US Anti-Doping Agency. She spends her days consulting athletes and their teams on how to find safe supplements, and always reminds them that no third-party certification system, no matter how good, can identify every possible contaminant or prohibited substance. Supplements can also vary batch to batch.

"All programs have ways to help manage the risk," she said. "But because of the way products are manufactured, you can never be 100 percent sure."

You’ll also want to cross-check USP or NSF marks on labels with the organizations' websites, since some supplement makers slap bogus certification claims on their products.

Avoid weight loss, muscle-building, and sexual enhancement supplements

Independent researchers’ biggest concerns tend to be around the herbs and botanicals that promise to help you lose weight or increase your sexual or athletic prowess. (Vitamin and mineral supplements — like iron, folic acid, or vitamin D — are generally viewed as less troublesome.) Our supplement database uncovered much of the same: Weight loss products seemed to contain hidden drugs most frequently, followed by sexual enhancement supplements and muscle builders.

"When the supplement industry starts to dabble in all kinds of crazy, novel ingredients that have no established nutrition value, and they advertise those novel ingredients as performance-enhancing, that’s when you start to enter risky territory," Eichner said.

USADA categorically advises athletes against any sports products that promise to build muscle, or boost testosterone or energy.

Pieter Cohen, a Harvard supplement researcher, agreed. "Completely avoid pre-workout and muscle-building supplements," he said. "These [product] groups are very often spiked and can contain dangerous ingredients with serious health effects."

From a safety perspective, protein powders and amino acids haven’t drawn much concern. But, Cohen added, "It’s unlikely that an average athlete needs these powders made in factories — it’s always best to obtain your protein from food. There’s also been a lot of controversy about whether the protein powders even contain the real thing." (Sometimes companies use inexpensive amino acids, which aren’t actually adequate protein substitutes — which may be why meta-analyses have found these powders don’t generally help build muscles.)

Appetite suppressants are notoriously dangerous, too. Our supplement database found 240 products that contained sibutramine, a drug the government removed from the market in 2010 for safety reasons. It can increase blood pressure and pulse rate, causing coronary artery disease, heart failure, arrhythmias, stroke, and even death.

Same goes for sexual enhancers. In our search, we found 159 supplements that contained sildenafil, the active pharmaceutical ingredient in the prescription drug Viagra. As a 2013 commentary in JAMA Internal Medicine summed up, "The quantity of adulterated sex supplements sold in the United States is staggering." For example, the researchers pointed to a single manufacturer in Utah, which, as of 2011, produced a million capsules per month of sexual enhancement supplements that were tainted with prescription-strength drugs.

Do your research before you go to the store

While weight loss, muscle-building, and sexual enhancement supplements may be the riskiest, vitamins and minerals do not have a completely clean track record.

Consider the case of Purity First B-50, which was sold as a vitamin B dietary supplement. The Food and Drug Administration found anabolic steroids in the product, which caused liver and thyroid problems, fatigue, and muscle cramping. Women who took the product experienced unusual hair growth and missed menstruation. In men, the alleged vitamin D caused impotence.

Again, the lax regulatory environment truly means you never know what’s lurking in your supplement bottle, no matter how benign it may seem.

So your best safeguard, Eichner said, is to do your homework before hitting the supplement aisle.

USADA has a really useful red flag checklist and a supplement safety information site. The organization advises buyers to cross-check the supplements they want to buy on USADA’s list of high-risk products, FDA’s Tainted Supplements page, and the World Anti-Doping Agency’s prohibited list. You can also find out whether a company has received a warning letter or has been the subject of government enforcement actions by searching the product and company name at FDA.gov or FTC.gov.

"Don’t impulse buy. And don’t be impressed by promises on the label," Eichner added. "There’s a lot you can do before picking your product that can really help protect you."

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