What Causes Black Stool?

In healthy adults, stool is typically well-formed and yellowish or brown in color. The fundamental pathophysiological mechanism underlying abnormal stool is impaired intestinal motility—caused by various factors—that disrupts normal colonic transit.

If black stool (melena) or black, tarry stool mixed with mucus is observed, upper gastrointestinal bleeding is highly likely—indicating pathology in the stomach, duodenum, or other proximal digestive structures, such as duodenal ulcer, gastritis, or even gastrointestinal malignancy. In such cases, upper gastrointestinal endoscopy is required to identify the precise source of bleeding and guide appropriate treatment.

Certain medications and foods can also cause black stool. For instance, patients with anemia taking iron supplements, traditional Chinese herbal preparations for blood replenishment, or animal-blood-based formulations may develop black stools. Similarly, consumption of animal blood, beef, pork liver, mulberries, or waxberries may lead to transient black discoloration of stool.

Additionally, chronic constipation may result in black-appearing stool due to prolonged colonic retention and subsequent oxidation of hemoglobin in the stool. Certain hematologic disorders—including leukemia, hemophilia, thrombocytopenia, aplastic anemia—can also manifest with melena. Likewise, Henoch-Schönlein purpura may present with black stool.