What causes stool to appear slightly black?

Black stool—medically termed melena—is typically caused by upper gastrointestinal (GI) bleeding, where blood is digested as it passes through the stomach and small intestine. The hemoglobin in the blood undergoes chemical breakdown by gastric acid and intestinal enzymes, converting iron to ferrous sulfide, which imparts a characteristic tarry, sticky, and foul-smelling appearance to the stool. Common underlying causes include peptic ulcer disease, gastritis, esophageal varices, Mallory-Weiss tears, or erosive esophagitis. Less commonly, melena may result from tumors (e.g., gastric adenocarcinoma or GI stromal tumors), vascular malformations (e.g., angiodysplasia), or severe inflammatory conditions such as Crohn’s disease involving the upper GI tract.

It’s important to distinguish true melena from pseudomelena—stool that appears black but is not due to bleeding. Certain foods (e.g., black licorice, blueberries, blood sausage) and medications (especially iron supplements and bismuth-containing products like Pepto-Bismol) can darken stool without indicating pathology. However, because melena often signifies clinically significant bleeding—and may precede hemodynamic instability—any new-onset black, tarry stool warrants prompt medical evaluation. This typically includes history and physical examination, complete blood count, coagulation studies, and urgent upper endoscopy (esophagogastroduodenoscopy) to localize and potentially treat the source of bleeding.

If accompanied by symptoms such as dizziness, syncope, palpitations, orthostatic hypotension, or hematemesis (vomiting blood), melena becomes a gastrointestinal emergency requiring immediate hospital referral and possible resuscitation with intravenous fluids or blood transfusion. Do not delay evaluation—even if symptoms seem mild—since upper GI bleeding can progress rapidly and unpredictably.