Black stool can be caused by physiological factors and pathological factors (such as gastric ulcer and colitis). 1. Physiological factors: mostly caused by taking certain foods or drugs, such as iron, bismuth and other drugs, and pig liver, animal blood and other foods, digested through the digestive tract, can make the stool appear black. 2. Pathological factors (1) Gastric ulcer: Gastric ulcer is the damage of gastric mucous membrane, forming an ulcer, when the ulcer invades the small blood vessels, black stool may appear. (2) Colitis: It is caused by internal bleeding in the intestines. This is when there is damage to the intestinal mucosa, and there is oozing or bleeding in the intestines, so black stools will appear when defecating. If the patient appears black stool, first exclude physiological factors, if not, we recommend timely medical treatment to identify the cause of the disease after targeted treatment.