The other day, Sarah noted that on her laundry detergent, it said “In case of accidental ingestion, give a glass of water or milk, and contact a poison control center.” She thought it odd that they would want you to drink milk. The purpose of drinking at all was clear… to dilute the detergent. Even very dangerous poisons can be handled by your body if they are diluted enough. But what was so special about milk? We had two theories: one involved the basicity of milk, and the other involved the lactose sugar in milk.

I looked into it further, and my lactose guess was right. See, milk contains a sugar called lactose that the human body is unable to digest by itself. Lactose is a disaccharide (sugar made up of two units) consisting of one glucose unit and one galactose unit connected by a beta linkage. Lactose is digested in mammals (like humans) only with the help of an enzyme called lactase which cleaves the lactose in half… allowing the individual sugars to be absorbed by the body. The problem here is that there is a limited amount of lactase produced, and as humans age, many stop producing it altogether, making it harder and harder to digest the lactose sugars in milk. Eventually, many adults become lactose intolerant, meaning that they are unable to digest a significant amount of lactose. Interestingly, lactose intolerance varies widely by ethnicities. Those of African or Asian descent are almost always lactose intolerant. Europeans and some from India and the Middle-East retain lactase at a higher rate, likely due to a micro-evolutionary adaptation resulting from cultures in which lactose-containing foods are more common.

At any rate, even someone with the ability to digest lactose only has a limited amount of the lactase enzyme with which to process the sugar. Once all available enzymes are put to work breaking up lactose, additional incoming lactose molecules are put on a waiting list. While they’re waiting for a lactase spot to open up, your incredibly acidic gastric juices start doing a number on the milk that is just sitting in your stomach. The hydrochloric acid in your stomach turns the milk into hard-to-digest curds… sort of like what milk looks like when you leave it out for a few days. These curds end up coating your stomach and your intestines, and give you a case of indigestion. And if you’ve just swallowed a poison, indigestion is exactly what you want!

So there’s the answer… by drinking milk, you not only dilute the detergent (or other poison), you overwhelm the lactase enzymes, allowing your stomach’s hydrochloric acid to curdle the milk, which coats your stomach and intestines, slowing down the rate with which your body absorbs the poison.

So why not just induce vomiting? Well, you could choke, your stomach acid will erode your esophagus, the substance you swallowed could be harmful to your lungs or esophagus, and you’ll become severely dehydrated, which could actually be worse for you than the poison you ingested. The milk just “pauses” your digestion until you can seek proper medical care.