Black stool means black stool. Black stool is normal after taking iron supplements, but some people have poor absorption of iron or incomplete absorption due to excessive doses of iron supplements, which may also show black stool, so the absorption effect of iron supplements cannot be judged from this. In addition, eating more animal offal and animal blood, taking other drugs and bleeding in the gastrointestinal tract may also show black stools, which need to be analyzed together with fecal occult blood test and imaging examination. If you want to know how iron is absorbed, you can check serum iron, serum transferrin saturation and other related indicators. After taking iron supplements orally, iron is usually not 100% absorbed and utilized, and some of it combines with each other with hydrogen sulfide in the intestine to form black iron sulfide, thus making the stool black in color, usually without an abnormal odor, which is one of the common manifestations of taking iron therapy. Consumption of animal blood or animal liver and other iron-containing foods may also make the stool black, which is a normal phenomenon, and often the black stool can gradually disappear after stopping the intake of iron or iron-containing foods. Taking activated charcoal, bismuth and other drugs can also appear black stool, peptic ulcer, gastrointestinal tumors and other disease factors cause upper gastrointestinal bleeding, hemoglobin decomposition in the gastrointestinal tract, so that the stool appears black, often accompanied by nausea, vomiting and other gastrointestinal symptoms or abnormal fishy smell of stool. For those who have previous digestive system diseases, they should be alerted to the possibility of the disease and it is recommended to go to the hospital promptly.