Diarrhea after drinking milk in the morning is most likely due to protein allergy or lactose intolerance. Protein allergy is mainly because the body’s immune system treats the proteins in milk as foreign, and then an immune response occurs, which may result in gastrointestinal discomfort such as diarrhea. Lactose intolerance is due to nutrient absorption disorders, due to the lack of lactase, the body can not break down lactose into monosaccharides, the intestines can not be absorbed, undigested lactose will enter the large intestine directly, stimulating intestinal peristalsis to speed up, so that the intestinal accumulation of large quantities of short-chain fatty acids and hydrogen, which leads to bloating, diarrhea, or abdominal cramps and other symptoms of discomfort. Lactose intolerant people should especially pay attention to avoid drinking milk on an empty stomach. If the patient’s diarrhea symptoms are serious, or continue to see no improvement, it is recommended to go to the regular hospital in a timely manner, after the cause is clear, under the guidance of the doctor for targeted treatment.