What does it mean to pass black stool?

Stool color is typically yellow or yellowish-brown. In daily life, some individuals may pass black stools. Black stool (melena) can arise from several causes—not all of which indicate disease. The first possible cause is dietary influence: for example, recent consumption of dark-colored foods—such as animal blood or organ meats (e.g., liver)—can darken stool color. In such cases, there is generally no cause for concern; simply adjusting one’s diet will usually restore stool to its normal color.

Another common cause is medication intake—for instance, certain traditional Chinese medicines, bismuth-containing preparations, or iron supplements—which can also lead to black stools. These causes are readily identifiable and typically benign.

A second category involves underlying medical conditions. For example, patients with constipation may retain food in the intestines for prolonged periods; additionally, consuming large amounts of green leafy vegetables may also result in darker stool. In such instances, managing constipation—possibly with appropriate laxatives—is advisable.

However, pathologic causes must also be considered: black stool may signal gastrointestinal bleeding. Upper gastrointestinal bleeding—due to peptic ulcers, acute mucosal injury, or gastric cancer—commonly presents as melena. Lower gastrointestinal bleeding—associated with conditions such as ulcerative colitis, intestinal tuberculosis, or colorectal cancer—may also cause black stools, though it more frequently manifests as bright red or maroon blood per rectum. When the cause of black stool is unclear, prompt medical evaluation at a hospital is essential.