Allergy, lactose intolerance and indigestion should be considered when drinking milk. 1. Allergy: milk as a kind of heterogeneous protein may trigger allergic reactions, diarrhea, abdominal pain and other symptoms. 2. Lactose intolerance: This is usually caused by a relative or absolute deficiency of lactase, which results in the lactose in milk not being completely broken down, thus triggering diarrhea symptoms. 3. Indigestion: It can be seen in gastritis, enteritis, peptic ulcer, functional gastrointestinal disease and other diseases. As the gastrointestinal tract digestion and absorption function is reduced, can not fully digest and absorb the nutrients in the milk, and cause diarrhea symptoms. For the case of diarrhea after drinking milk, it is recommended to consult a doctor to clarify the specific causes.