What does it mean to pass black stool?

Stool color is typically yellow or yellowish-brown. However, some individuals may occasionally pass black stools—a condition known as melena. Several factors can cause black stools, and not all are indicative of 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), which 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, all of which may turn stools black. These causes are readily identifiable and typically benign.

The second category involves pathological conditions. For example, patients with constipation may retain food in the intestines for prolonged periods; similarly, consuming large amounts of green leafy vegetables may also lead to darker stool color.

In such cases, managing constipation—e.g., by using appropriate laxatives—is advisable. However, black stools may also signal a more serious underlying condition, such as gastrointestinal bleeding. Upper gastrointestinal bleeding—caused by peptic ulcers, acute mucosal injury, or gastric cancer—often results in melena. Lower gastrointestinal bleeding—due to conditions such as ulcerative colitis, intestinal tuberculosis, or colorectal cancer—can likewise produce black stools. When the cause remains unclear or when symptoms persist, prompt medical evaluation at a hospital is essential.