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 of them indicate disease. The first possible cause is dietary influence—for example, consuming dark-colored foods such as animal blood or organ meats (e.g., liver) may darken stool color. In such cases, there is generally no cause for concern; simply adjusting your diet will usually restore stool to its normal color.

Certain medications may also cause black stools—for instance, some traditional Chinese medicines, bismuth-containing preparations, or iron supplements.

The second category involves pathological conditions. For example, constipation may prolong intestinal transit time, allowing food residues—particularly green leafy vegetables—to undergo greater bacterial degradation, potentially resulting in darker stools.

In such cases, managing constipation—possibly with appropriate laxatives—is advisable. More seriously, black stools may result from gastrointestinal bleeding. Upper gastrointestinal bleeding—such as that caused by peptic ulcers, acute mucosal injury, or gastric cancer—often leads to melena. Lower gastrointestinal bleeding—including that associated with ulcerative colitis, intestinal tuberculosis, or colorectal cancer—can also produce black stools. If you are uncertain about the underlying cause, seek prompt medical evaluation at a hospital.