Taking iron supplements can cause black stools, which is a normal phenomenon, and the color of stools will return to normal after stopping the use of iron supplements. Oral iron will discharge black stool is mainly due to iron in the human digestive tract, need to carry out a series of complex biochemical reactions, and ultimately in the form of divalent iron is absorbed by the gastrointestinal mucosa, in the process, iron in the gastrointestinal tract and hydrogen sulfide combination, then the formation of iron sulfide, resulting in black stool. In addition to iron supplements, eating foods such as animal blood and offal, or taking medications such as bismuth pectin and bismuth potassium citrate, can also cause black stools. However, it should be noted that if the black color of stool still excludes the above factors, we should be alert to the possibility of gastrointestinal bleeding, and recommend timely medical treatment as soon as possible.