If you are tired of working and your stool is black, it may be caused by dietary reasons, medication reasons, gastrointestinal bleeding and other reasons. 1. Dietary reasons: if you eat some animal offal before work, you can have black stools. 2. Drug reasons: often eat some iron supplements iron, or eat some gastric mucosa protector, such as bismuth potassium citrate and other drugs, can also appear in the phenomenon of black stools. 3. Gastrointestinal bleeding: besides, bleeding in the gastrointestinal tract can also cause the phenomenon of black stools, and there are many diseases that cause bleeding in the gastrointestinal tract, such as gastritis, gastric ulcer, gastric cancer, intestinal inflammation, intestinal polyps, intestinal cancer and other diseases, so it is necessary to be further judged by gastrointestinal examination. It is recommended to go to the hospital for consultation in time.