Why does drinking milk cause diarrhea?

As we know, the vast majority of people experience no discomfort after drinking milk, and it actually enhances our nutrition and supports overall health. However, some individuals develop diarrhea immediately after consuming milk. This is commonly due to lactose intolerance—a condition in which the body lacks sufficient lactase, the enzyme required to digest lactose, the natural sugar found in milk. When lactose-intolerant individuals consume milk, undigested lactose ferments in the gut, leading to symptoms such as diarrhea. Importantly, lactase production is not fixed; it can be induced or increased through gradual exposure.

If you initially experience diarrhea upon drinking milk, you can gradually improve your tolerance by inducing higher lactase levels. Here’s how: Begin with a very small amount—about 20–30 mL—and drink it daily for three to five days. If no diarrhea occurs, slowly increase the amount over the next week to ten days until you reach a full serving (e.g., one standard carton, ~250 mL). By then, lactase production should have been sufficiently induced, allowing you to drink a full serving daily without diarrhea—provided you maintain regular consumption.

However, if you stop drinking milk for about one to two weeks and then resume, you’ll need to restart the gradual induction process—from a small dose—to re-stimulate lactase production. Consuming a large amount abruptly after such a break may again trigger diarrhea. Therefore, for individuals with lactose intolerance, consistent, stepwise increases in milk intake effectively prevent diarrhea.