This is due to the black stool caused by the ulceration and bleeding of the outer wall of the tumor and the reduction reaction of gastric acid, which reduces the trivalent iron ions to divalent iron ions. However, not all black stools are cancerous. Sometimes black stools may be caused by dietary habits, bleeding ulcers in the gastrointestinal tract, and medications. In general, black stools are caused by bleeding in the upper gastrointestinal tract, such as gastric ulcers and erosive gastritis, or in the lower gastrointestinal tract, where the blood is not removed in time and stays in the intestines for a long time. These digestive tract diseases are the causes of black stool; in addition, after eating some animal offal and animal blood products, it may also cause black stool. However, for some elderly patients with cardiovascular diseases, long-term use of aspirin may also lead to acute bleeding and black stools; for patients with iron deficiency anemia, they may take iron supplements for a long time, which may also cause black stools; there are other medications for stomach diseases, which may also cause black stools; as to which condition causes black stools, they can be diagnosed by fecal occult blood and gastrointestinal examination. As to which condition causes black stool, it can be diagnosed by fecal occult blood and gastroscopy. In conclusion, after finding black stool, do not be anxious and arbitrarily think that it is cancer, but seek timely medical diagnosis and analysis.