What Causes Black Stool?

Typically, when upper gastrointestinal (GI) bleeding occurs, blood passes through the intestines. Under the action of intestinal flora, iron in hemoglobin is converted into ferrous sulfide. Ferrous sulfide stimulates the intestinal mucosa to secrete mucus, resulting in black, tarry stools with a glossy, asphalt-like appearance. Black stool—also known as melena—is a common symptom of upper GI bleeding. So, what causes black stool? The following explains this question.

Causes of Black Stool

1. Pathological Factors

Malignant tumors such as rectal cancer or colon cancer may cause black stool. Therefore, patients experiencing black stools should promptly undergo gastroscopy or colonoscopy at a hospital. Additionally, hematologic disorders—including leukemia, hemophilia, thrombocytopenia, and aplastic anemia—can also lead to black stools. Patients are advised to seek timely medical evaluation and treatment to prevent disease progression.

2. Constipation-Related Factors

Constipation can also cause black stools, as prolonged intestinal transit time allows fecal material to remain in the gut longer, potentially darkening stool color. This may be accompanied by difficulty defecating and hard, dry stools. Patients are advised to consume more fiber-rich, bowel-lubricating foods to alleviate symptoms; pharmacologic intervention may be necessary if required. Moreover, dietary factors may contribute to black stools—for example, ingestion of animal blood, beef, pork liver, mulberries, or waxberries may darken stool color.

3. Medication-Related Factors

Certain medications—including iron supplements for anemia, traditional Chinese medicines, gastric protectants (e.g., sucralfate), and bismuth-containing compounds (e.g., colloidal bismuth subcitrate)—may cause black stools. Furthermore, black stool may result from GI diseases that induce GI bleeding, thereby producing melena. GI conditions associated with black stools include gastric ulcers, duodenal ulcers, intussusception, gastric ptosis, and acute gastritis.

The above outlines the potential causes of black stool. We hope this information is helpful to you.