Dark green stools may be caused by dietary reasons, medications, and upper gastrointestinal bleeding. 1. Dietary reasons: eating too much green leafy vegetables will increase the burden on the gastrointestinal tract, causing indigestion, undigested and absorbed vegetables will be discharged with the stool, so that the stool is dark green and black. 2. Drugs: oral iron supplements, bismuth, can make the stools show dark green or black. Common drugs include ferrous sulfate, ferrous succinate, ferrous fumarate, bismuth potassium citrate, bismuth pectin and so on. 3. Upper gastrointestinal bleeding: mostly seen in esophageal mucosal bleeding, gastric ulcer bleeding, duodenal ulcer bleeding, etc. Because the bleeding location is located in the upper gastrointestinal tract, resulting in iron in the blood cells in the digestive tract for a long time to be oxidized, which makes the stools dark greenish-black. If the above symptoms occur, it is recommended to go to the hospital in time for early and regular treatment.