Melena is predominantly a sign of upper GI bleeding, meaning the bleeding source is located above the junction where the small intestine begins, typically in the esophagus, stomach, or duodenum. It takes as little as 50 ml of blood in the stomach to turn stools black and tarry. However, melena can occasionally originate from the right side of the colon or small intestine when gut transit is slow enough for blood to be fully digested.
Why Melena Points to an Upper GI Source
The black, tarry appearance and distinctive foul smell of melena come from blood being broken down as it travels through the digestive tract. When blood pools in the stomach or upper intestine, digestive enzymes and gut bacteria convert hemoglobin (the oxygen-carrying protein in red blood cells) into a dark pigment. This chemical transformation requires time and contact with acid and bacteria, which is why the farther the bleeding source is from the rectum, the darker and more altered the stool appears.
Bright red blood in stool (hematochezia) typically signals a lower GI source because the blood passes through quickly without being digested. Melena is essentially the opposite: blood that has had a long enough journey to be completely transformed.
When Lower GI Bleeding Causes Melena
While upper GI bleeding accounts for the majority of melena cases, it’s not the only possibility. Research has shown that instilling blood directly into the cecum (the first part of the large intestine, on the right side) can produce melena. This means bleeding from the right colon, including conditions like abnormal blood vessel clusters, colitis, tumors, and diverticulosis, can sometimes cause black tarry stools instead of the bright red bleeding you’d typically expect from a colon source.
The key factor is transit time. If stool moves slowly through the colon, as happens with conditions like paralytic ileus (temporary paralysis of gut movement), even a left-sided colon bleed could theoretically produce melena. In practice, though, lower GI sources of melena are uncommon and almost always involve the right colon, where stool still has a long path ahead before reaching the rectum.
Most Common Causes
Peptic ulcers are the single most common cause of acute upper GI bleeding and, by extension, the most frequent reason someone develops melena. These are open sores on the lining of the stomach or duodenum, often caused by a bacterial infection or long-term use of anti-inflammatory painkillers like ibuprofen.
Other common causes include:
- Gastritis: inflammation of the stomach lining, which can ooze blood slowly
- Esophageal or stomach varices: swollen, fragile veins usually caused by liver cirrhosis that can bleed heavily
- Mallory-Weiss tear: a tear in the esophageal lining from forceful vomiting
- Abnormal blood vessels: malformations in the esophagus, stomach, or duodenum that bleed intermittently
What Isn’t Actually Melena
Several common substances can turn your stool dark black without any bleeding at all. Iron supplements are a frequent culprit. Bismuth-based medications like Pepto-Bismol do the same. Black licorice, blueberries, blood sausage, and activated charcoal can also produce stools dark enough to raise concern.
The key difference is texture and smell. True melena is sticky, tarry, and has a strong, distinctive odor that’s hard to miss. Dark stool from food or supplements is typically firm and lacks that characteristic smell. If you’re unsure, a simple stool test at your doctor’s office can detect hidden blood and settle the question quickly.
How Doctors Identify the Source
When melena is confirmed, the first diagnostic step is an upper endoscopy, where a thin, flexible camera is passed through the mouth into the esophagus, stomach, and upper small intestine. This remains the gold standard for both identifying and treating the bleeding source. For higher-risk patients or those with ongoing bleeding, this procedure is typically performed within 24 hours. If variceal bleeding is suspected (common in people with known liver disease), the timeline tightens to within 12 hours.
One blood test that helps point doctors in the right direction before endoscopy is the ratio of blood urea nitrogen (BUN) to creatinine. When blood is digested in the upper GI tract, the protein breakdown products get absorbed and raise BUN levels disproportionately. A ratio of 30 to 1 or higher has about 84% accuracy for predicting an upper GI source, which helps clinicians decide whether to scope from above first.
If upper endoscopy comes back clean, a colonoscopy follows to check for right-sided colon sources. Studies have found that right-sided abnormal blood vessels, colitis, tumors, and diverticulosis are all more common in patients with melena than in the general population, confirming that these lower GI conditions do occasionally cause black tarry stools. Large right-sided polyps, interestingly, are rarely the culprit.