There are many causes of black stool, such as food factor, drug factor, upper gastrointestinal bleeding, lower gastrointestinal bleeding and so on. 1. Food factor: mainly affected by dietary structure, eating too much pig’s blood, pig’s liver and other iron-rich food, the iron ions in the food can not be fully absorbed, which will easily lead to black stools. 2. Drug factors: it may be due to taking bismuth (such as bismuth potassium citrate, bismuth pectin) or taking iron (such as ferrous succinate), these drugs may lead to black stools. 3. Upper gastrointestinal bleeding: for example, esophageal bleeding, gastroduodenal bleeding, when the amount of bleeding is relatively small, it is easy to lead to black stools, and if the amount of bleeding is relatively large, it can be manifested as bloody stools. 4. Lower gastrointestinal bleeding: for example, intestinal tuberculosis, ulcerative colitis, bacillary dysentery, etc., may lead to intestinal mucosal breakage, which may cause bleeding, resulting in black stools. Black stool may also be caused by other reasons, such as hemophilia, leukemia, etc. If the discharge is caused by food and drugs, you should consult a doctor in time to find out the cause and take treatment.