What does black stool indicate?

Black stool may result from dietary factors, medication use, or gastrointestinal bleeding.

1. Dietary Factors

Consumption of dark-colored foods—such as duck blood, chicken liver, or pork blood—can cause black stool, which appears upon defecation.

2. Medication-Related Factors

Long-term use of bismuth-containing medications—commonly gastric mucosal protectants such as colloidal bismuth pectin capsules or compound bismuth aluminate capsules—or long-term use of anticoagulants—including aspirin and rivaroxaban—may also lead to black stool.

3. Gastrointestinal Bleeding

Gastrointestinal bleeding caused by conditions such as peptic ulcer disease, erosive gastritis, or small intestinal vascular malformations allows blood to enter the intestinal tract. Hemoglobin-derived iron in the blood is metabolized by intestinal bacteria into iron sulfide. Iron sulfide irritates the intestinal mucosa, prompting increased mucus secretion, which may result in black stool (melena).