Drinking iron supplements can affect the color of the stool. Generally, iron supplements may cause the stool to be black or gray or tarry in color, and the stool will turn yellow after stopping the medication. Drinking iron supplements affects the color of the stool mainly because of the chemical reaction between iron ions and intestinal bacteria in the intestine, which then takes on a black or gray color. The black color of the stool should be distinguished from bleeding in the digestive tract. If a patient with ulcer disease has black stools after taking iron supplements, but at the same time has upper abdominal pain or blood loss. It is necessary to go to the hospital for a stool routine plus occult blood test to distinguish whether the black stool is caused by iron or bleeding.