The causes of diarrhea after drinking black coffee include drinking too much, allergy, and drinking on an empty stomach. 1. Drinking too much: If you drink too much black coffee or the concentration is too high, it may stimulate the gastrointestinal tract, leading to gastrointestinal tract peristalsis, resulting in diarrhea. 2. Allergy: if allergic to black coffee, drinking black coffee can lead to diarrhea and need to avoid drinking black coffee. 3. Drinking on an empty stomach: Drinking black coffee on an empty stomach has a greater stimulation of the gastrointestinal tract, promoting gastric acid secretion, leading to gastrointestinal peristalsis, resulting in diarrhea. If there is slight diarrhea after drinking black coffee, you can stop drinking black coffee and stop having diarrhea. If the diarrhea after drinking black coffee is serious, or accompanied by nausea, vomiting and other symptoms, you need to consult a doctor in time, and actively cooperate with the treatment.