It is possible to have diarrhea after drinking black coffee, which is mainly related to the irritation of black coffee, the patient’s allergy to black coffee, the patient himself suffers from basic gastrointestinal diseases or the coffee is contaminated and so on. 1. Irritation of black coffee: the main component of black coffee, caffeine, has a certain degree of irritation to the gastrointestinal tract, which can increase the secretion of gastric acid, gastrointestinal peristalsis leading to gastrointestinal dysfunction, causing diarrhea. Drinking coffee on an empty stomach or drinking too much at once will more easily lead to diarrhea. 2. Allergy: Some patients are allergic to caffeine, which induces an allergic reaction after drinking black coffee, leading to abdominal pain and diarrhea and other symptoms. This group of people should try to avoid eating caffeine-containing food or drinks. 3. Basic gastrointestinal diseases: If the patient himself suffers from gastrointestinal diseases such as gastroenteritis, irritable bowel syndrome, peptic ulcer, etc., the patient’s gastrointestinal sensitivity to black coffee is higher than that of a normal person, and it is more likely to have diarrhea symptoms after drinking it. 4. Coffee is contaminated: If black coffee is placed for a long time, it may harbor bacteria, fungi and other pathogenic microorganisms, and after drinking it, it may cause acute gastroenteritis and lead to diarrhea. If patients have long-term or recurrent symptoms of diarrhea after drinking black coffee, it is recommended to consult a doctor in time to find out the cause of the disease and actively receive treatment to avoid delaying the condition.