Black stools may be associated with physiologic causes such as eating large amounts of iron-containing foods and medications, or with gastrointestinal bleeding such as gastric ulcers and ulcerative colitis. 1. Physiological reasons. When people consume a lot of iron-rich food, animal blood, etc., it can cause black stools, which is a normal physiological phenomenon; in addition, taking iron, bismuth and other drugs, such as ferrous succinate, bismuth potassium citrate, etc., can also cause black stools. 2. Pathological causes. The most common reason for black stool is gastrointestinal bleeding, such as esophageal ulcer, gastric ulcer, hookworm, ulcerative colitis, colon cancer and so on may cause gastrointestinal bleeding, resulting in black stool. When the stool is black for unknown reasons, or accompanied by abdominal pain, bloating and other abnormal symptoms, you should consult a doctor in time, under the guidance of the doctor to clarify the cause, so as to actively treat.