What Causes Black, Watery Stool?

Black, tarry stools—medically termed melena—are a clinically significant finding that typically indicate upper gastrointestinal (GI) bleeding. The dark, sticky, foul-smelling appearance results from the digestion of hemoglobin as blood passes through the small intestine and colon. Unlike bright red blood per rectum—which usually signals lower GI or anorectal sources—melena suggests bleeding proximal to the ligament of Treitz, most commonly from the esophagus, stomach, or duodenum.

Common causes include peptic ulcer disease, erosive gastritis, esophageal varices (particularly in patients with chronic liver disease), Mallory-Weiss tears, and gastric or esophageal malignancies. Less frequent but important etiologies include Dieulafoy lesions, angiodysplasia, and ingestion of certain medications or substances—such as iron supplements, bismuth-containing preparations (e.g., Pepto-Bismol), or large quantities of blue-black foods like black licorice or blueberries—which can mimic melena but do not represent true GI hemorrhage.

Crucially, melena often reflects substantial blood loss—typically at least 50–100 mL—and may be accompanied by signs of hypovolemia, including dizziness, orthostatic hypotension, tachycardia, or syncope. Patients presenting with melena require prompt clinical evaluation, including history, physical examination, laboratory testing (complete blood count, coagulation profile, liver function tests), and urgent upper endoscopy to localize and potentially treat the source of bleeding.

Self-management is inappropriate. Any episode of black, tarry stool warrants immediate medical attention—not only to rule out life-threatening conditions but also because early intervention significantly improves outcomes, especially in cases of active or recurrent bleeding.