What causes black stool?

Black stool—medically termed melena—typically indicates the presence of digested blood in the gastrointestinal (GI) tract. When blood from the upper GI tract (e.g., esophagus, stomach, or duodenum) passes through the digestive system, hemoglobin is broken down by gastric acid and intestinal enzymes, converting iron into ferrous sulfide—a compound that imparts a characteristic tarry, black, and often foul-smelling appearance to the stool.

Common causes of melena include peptic ulcer disease, gastritis, esophageal varices (often related to liver cirrhosis), Mallory-Weiss tears (mucosal lacerations at the gastroesophageal junction), and erosive esophagitis. Less common but serious etiologies include gastric or esophageal malignancies, Dieulafoy lesions, or vascular malformations. Importantly, melena usually signifies bleeding proximal to the ligament of Treitz; bleeding from more distal sites (e.g., colon) typically results in maroon or bright red blood per rectum—not black stool.

It’s essential to distinguish true melena from pseudomelena—black discoloration caused by non-hemorrhagic factors. These include ingestion of iron supplements, bismuth-containing medications (e.g., Pepto-Bismol), activated charcoal, or foods rich in dark pigments (e.g., black licorice, blueberries, or large amounts of spinach). A careful history—including recent medications, diet, and associated symptoms (e.g., abdominal pain, hematemesis, dizziness, syncope, or fatigue)—is critical for accurate assessment.

Melena is a clinical red flag requiring prompt medical evaluation. Patients should seek urgent care if melena is accompanied by signs of hemodynamic instability (e.g., tachycardia, hypotension, pallor, or altered mental status), persistent vomiting, or evidence of overt bleeding. Diagnostic workup commonly includes complete blood count, coagulation studies, fecal occult blood testing (though often unnecessary if melena is visually apparent), and urgent upper endoscopy to localize and potentially treat the source of bleeding.