Black stools may indicate that the patient may have gastrointestinal bleeding, and may also be caused by oral intake of certain medications and foods. 1. Gastrointestinal bleeding: patients with gastrointestinal bleeding often have black stools, mainly due to the fact that trivalent ferrous ions in the blood turn into divalent ferrous ions when there is bleeding in the upper gastrointestinal tract. The color of ferrous iron is mostly black, so it will show black stool. It is necessary to improve the gastroscopy, blood routine and other tests in time to clarify the cause of the disease, so as not to delay the condition. 2. Drug factors: some drugs, such as ferrous colloid and bismuth pectin, may also cause black stool. The dark color of black stool caused by this kind of drugs usually occurs on the same day or the next day after taking the drugs orally, and there is no obvious gastrointestinal discomfort, and the black stool will be relieved or disappeared immediately after stopping taking the above drugs. 3. Dietary factors: Part of the diet can also lead to dark stools, such as animal offal and animal blood, excessive consumption of green leafy vegetables such as spinach and bok choy, etc., which can lead to dark stools. Black stools can also be caused by other reasons. After eliminating dietary and medication factors, it is necessary to consult a doctor in a timely manner to complete gastroenteroscopy and other examinations to clarify the specific causes of black stools, so as to avoid delaying the condition.