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Black Stool: Benign Causes vs. GI Bleeding – When to Worry

Black stool (melena) can be alarming, but not all dark stools indicate bleeding. The key distinction: melena (tarry, foul-smelling, sticky) vs. pseudomelena (non-tarry, diet/medication-related).

Benign Causes (Pseudomelena)

Substance	Mechanism	Duration
Iron supplements	Unabsorbed iron oxidizes to black ferric sulfide	Resolves 1–2 days after stopping
Bismuth subsalicylate (Pepto-Bismol)	Bismuth sulfide formation	Resolves 1–3 days after stopping
Black licorice	Glycyrrhizin + food coloring	Temporary
Dark foods	Blueberries, blackberries, beets, dark chocolate, spinach	1–2 bowel movements
Activated charcoal	Adsorbent; used for overdose	While taking

GI Bleeding Causes (True Melena)

Melena indicates upper GI bleeding (proximal to ligament of Treitz). Blood is digested by gastric acid and gut bacteria → black, tarry, foul odor.

Source	Common Causes	Key Features
Esophagus	Varices (cirrhosis), Mallory-Weiss tear, esophagitis	Hematemesis often precedes melena
Stomach	Peptic ulcer (H. pylori, NSAIDs), gastritis, GAVE, cancer	Epigastric pain, anemia, H. pylori risk factors
Duodenum	Ulcer, Dieulafoy lesion, aortoenteric fistula	Post-bulbar ulcers may not cause pain
Small bowel	Angioectasia, tumor, Meckel's diverticulum, Crohn's	Occult or overt bleeding; video capsule endoscopy

Lower GI Bleeding (Hematochezia)

Bright red or maroon stool = lower GI source (colon, rectum). Can appear dark if transit slow (right colon). Causes: diverticular bleeding, ischemic colitis, IBD, hemorrhoids, cancer, angioectasia.

When to Seek Immediate Care

Black, tarry, foul-smelling stool + any of: dizziness, palpitations, shortness of breath, chest pain, confusion, syncope
Hemodynamic instability (hypotension, tachycardia)
Known cirrhosis + black stool (variceal bleed risk)
On anticoagulants/antiplatelets + melena

Diagnostic Approach

History: Medications (NSAIDs, anticoagulants, iron, bismuth), liver disease, prior GI bleeding
Exam: Orthostatic vitals, abdominal, rectal, stigmata of liver disease
Labs: CBC, coagulation, LFTs, BUN/Cr, type & screen
EGD (esophagogastroduodenoscopy): First-line for melena; within 24 hrs if active bleed
Colonoscopy: If EGD negative and hematochezia or risk factors
Video capsule/CT angiography: Obscure GI bleeding

Bottom line: If you haven't taken iron, bismuth, or eaten dark foods—and especially if you have risk factors or systemic symptoms—assume melena until proven otherwise. Seek urgent evaluation.