What are the causes of dark tar‑colored stools (melena)?

Causes of Dark Tar Stools (Melena)

Dark tar-colored stools (melena) result from digested blood in the gastrointestinal tract and most commonly originate from peptic ulcers, gastroduodenal erosions, esophagitis, varices, and Mallory-Weiss tears in the upper GI tract. 1

Primary Upper GI Causes

Most Common Sources

• Peptic ulcer disease is the leading non-variceal cause, typically associated with Helicobacter pylori infection or chronic NSAID use 1
• Gastroduodenal erosions frequently occur in patients taking NSAIDs or experiencing stress-related mucosal injury 1
• Esophagitis from acid reflux or other inflammatory processes 1
• Esophageal or gastric varices in patients with cirrhosis or portal hypertension, carrying approximately 30% mortality risk 1
• Mallory-Weiss tears from forceful vomiting or retching 1

Frequently Overlooked Upper GI Sources

• Cameron's erosions in large hiatal hernias are commonly missed, particularly in patients with upper GI symptoms 1
• Gastric antral vascular ectasia (watermelon stomach), especially in patients with chronic kidney disease or cirrhosis 1
• Dieulafoy's lesion, a rare but important cause characterized by a large caliber artery protruding through the gastric wall 1
• Angiodysplasia accounts for up to 80% of obscure bleeding, particularly in patients over 40 years old 1

Rare but Life-Threatening Upper GI Causes

• Aortoenteric fistula must be suspected in any patient with prior abdominal aortic aneurysm repair presenting with melena 1
• Hemobilia and hemosuccus pancreaticus together account for roughly 1 in 500 cases of upper GI bleeding 1

Lower GI Sources (When Upper Endoscopy Is Negative)

Approximately 10–15% of patients presenting with melena have a lower GI source, requiring colonoscopy when EGD is unrevealing 1:

• Diverticulosis is the most common lower GI source in older adults, accounting for 20–41% of cases, with incidence rising over 200-fold from the third to eighth decade 1
• Angiodysplasia contributes 3–40% of lower GI bleeding 1
• Ischemic colitis represents 10–21% of episodes, occurring more frequently in elderly patients with cardiovascular disease 1
• Colorectal cancer or adenomatous polyps account for 6–27% of cases 1
• Slow bleeding from the right colon can occasionally produce dark stools, though typically not true melena 1

Small Bowel Sources (Obscure Bleeding)

When both upper endoscopy and colonoscopy are negative 1:

• Small bowel tumors are the most common cause in patients under 50 years old 1
• Angiodysplasia remains a leading cause, especially in patients over 40 1
• NSAID-induced ulcers throughout the small intestine 1
• Crohn's disease, particularly in younger patients with inflammatory bowel disease history 1
• The presence of melena doubles the odds of finding a bleeding site within the proximal small intestine (OR 1.97) 2

Medication-Related Causes

• Direct oral anticoagulants (DOACs) including apixaban markedly increase GI bleeding risk from any pre-existing lesion 1
• NSAIDs cause both gastroduodenal erosions and small bowel ulceration 1
• Anticoagulation therapy (warfarin, DOACs) unmasks underlying GI pathology rather than creating new lesions 1

Non-Bleeding Causes (Important Pitfall)

Black stools lacking the sticky, tar-like consistency and characteristic odor of melena typically do not represent digested blood and generally do not require urgent GI evaluation unless other concerning features are present 1:

• Dietary iron supplements 1
• Bismuth-containing medications (Pepto-Bismol) 1
• Certain foods (black licorice, blueberries) 1

Age-Specific Considerations

Elderly Patients (>65 years)

• Vascular lesions (angiodysplasia) account for up to 40% of causes in patients over 40 years 1
• Diverticulosis incidence increases dramatically with age 1
• Mortality reaches up to 30% in patients older than 90 years 1
• Comorbidities (cardiovascular, renal, liver disease) are the strongest predictors of death 1

Younger Patients (<50 years)

• Small bowel tumors are the most common cause and require aggressive investigation 1
• Crohn's disease should be considered 1

Critical Clinical Pearls

• Melena indicates blood has been exposed to gastric acid and digested, suggesting a source proximal to the ligament of Treitz 1
• Massive upper GI bleeding can present as bright red blood per rectum (hematochezia) rather than melena if transit time is rapid enough 1
• The combination of heart rate >100 bpm, systolic BP <100 mmHg, and hemoglobin <100 g/L carries approximately 80% risk of ongoing bleeding or death 1
• Hemodynamic instability despite resuscitation is a high-risk feature requiring aggressive management 1