The green color of the stool is probably because the iron supplement is not completely absorbed and the residual iron will make the stool green, usually after three days of taking the iron supplement. If the stool is green before three days of taking the medication, or if the stool is still green after stopping the medication for more than three days, it is necessary to exclude the possibility of bleeding in the gastrointestinal tract and to take the stool for an occult blood test. A positive occult blood test indicates that there is bleeding in the gastrointestinal tract, and the patient should be asked if there is abdominal pain or discomfort in the abdomen, and then targeted electrogastroscopy or e-colonoscopy should be done to clarify.