Melena refers to black, tarry, foul-smelling stool that develops when blood from the digestive tract gets broken down by bacteria and digestive enzymes. That breakdown takes hours, so melena almost always points to an upper GI bleed, specifically bleeding that started above the ligament of Treitz, the landmark where the duodenum meets the jejunum. Classic sources include peptic ulcer disease, gastritis, and esophageal varices, with chronic NSAID use acting as a frequent trigger.
In roughly 90% of cases it signals an upper source.
Inside, we walk through why melena is almost always an upper GI clue, how its digestion timeline sets it apart from hematochezia and hematemesis, and what clinicians do to pinpoint the actual bleeding source.
Melena as a Clinical Marker of Gastrointestinal Bleeding
Melena is a dark, sticky stool with a smell that’s hard to mistake, and its appearance tells a clear story about what’s going on inside your gut. The black color comes from hematin, a dark pigment formed when colonic bacteria break down hemoglobin from red blood cells. The tarry consistency comes from blood mixing with digestive secretions and intestinal mucus.
That transformation is slow. Blood needs hours of contact with gastric acid, bile, and bacterial enzymes before it turns from bright red into the sticky black stool you see in the toilet bowl. When melena shows up, the bleeding has been happening well above the rectum, giving the gut time to digest and chemically alter the blood.
Three features define melena:
- Color: Jet black, sometimes described as similar to coffee grounds.
- Consistency: Tarry, sticky, and loose, with a tendency to cling to the toilet bowl.
- Odor: A strong, distinctive smell caused by indole and skatole, two compounds produced when intestinal bacteria digest blood proteins.
Clinicians treat melena as a sign of active or recent bleeding that requires prompt investigation. Even a single episode can signal meaningful blood loss, because the gut typically needs roughly 50 to 100 mL of blood before melena becomes visible.
Why Melena Typically Signals an Upper GI Source
Because blood must travel through several feet of intestine to develop that black, tarry appearance, melena points to bleeding that started high up. Any source proximal to the ligament of Treitz can produce melena when bleeding is slow enough for digestion to occur.
Common Upper GI Culprits
A peptic ulcer, either a gastric ulcer in the stomach lining or a duodenal ulcer in the first part of the small intestine, is the most frequent cause of melena in clinical practice. These open sores can ooze steadily, allowing blood to mix with digestive juices before it reaches the colon.
Gastric and duodenal ulcers account for a large share of upper GI hemorrhage cases, and chronic NSAID use remains a leading preventable trigger. Esophageal varices, which are enlarged veins in the esophagus that often develop in people with advanced liver disease, can also bleed massively and produce melena. Gastritis, an inflammation of the stomach lining, and erosions caused by heavy NSAID or aspirin use round out the usual suspects.
Rare Lower GI Exceptions
Right-sided colon bleeding occasionally mimics an upper source. When blood from a lesion in the ascending colon (the first section of the large intestine, on the right side of the abdomen) moves through the bowel slowly, it can darken and produce a stool that looks similar to melena. Sources include angiodysplasia, a condition where fragile, dilated blood vessels in the colon wall can leak, and slow right-sided tumors.
The key word here is slow. Fast-moving lower GI bleeding typically produces bright red or maroon stool instead, which is why hematochezia almost always signals a lower source.
Melena Volume Threshold and the Digestion Timeline
Melena doesn’t appear from a trickle. Clinicians generally estimate that at least 50 to 100 mL of blood must enter the GI tract to produce one episode of melena. Smaller amounts may go unnoticed or show up only as a positive fecal occult blood test, a lab check that looks for hidden blood in stool.
After blood enters the upper digestive tract, transit time shapes the stool’s appearance. Blood from the stomach or duodenum typically takes 14 to 48 hours to travel through the small intestine and colon. The slower the transit, the more thoroughly bacteria act on the hemoglobin, and the darker and stickier the stool becomes.
When transit speeds up, even right-sided colon bleeding may produce a darker, looser stool rather than bright red blood. Stress, infection, and certain medications can all accelerate gut motility. That variability is one reason clinicians never diagnose the bleeding source based on stool appearance alone.
The foul smell of melena isn’t just unpleasant. It comes from indole and skatole, two bacterial byproducts, and it actually helps clinicians distinguish true digested blood from food-related stool discoloration.
Distinguishing Melena From Hematochezia and Hematemesis
Three terms describe how blood exits the GI tract, and each tells a different story. Understanding melena vs hematochezia vs hematemesis helps you and your care team communicate clearly about what you’re seeing.
| Feature | Melena | Hematochezia | Hematemesis |
|---|---|---|---|
| Appearance | Black, tarry stool | Bright red or maroon blood per rectum | Vomited blood, red or coffee-ground-like |
| Typical source | Upper GI (above ligament of Treitz) | Lower GI (below ligament of Treitz) | Upper GI (esophagus, stomach, duodenum) |
| Transit involved | Slow; blood digested | Rapid; blood passes quickly | None; blood exits via vomiting |
| Approximate blood volume | ≥50–100 mL | Varies, often large | Varies, often large |
| Odor | Distinctive, foul | Normal stool odor | Often metallic or coffee-like |
Hematochezia signals rapid transit. It describes bright red or maroon blood passed through the rectum, usually with normal stool odor, and typically points to a lower GI source such as hemorrhoids, diverticulosis (small pouches in the colon wall that can bleed), angiodysplasia, or colorectal cancer.
Hematemesis describes blood that comes up through vomiting. Fresh red blood suggests active bleeding, while a coffee-ground appearance means the blood has already been partially digested by stomach acid. Either way, hematemesis always points to the upper GI tract.
Medications and Foods That Mimic Melena
Not every dark stool is melena. Iron supplementation and bismuth-containing medications such as bismuth subsalicylate can turn stool black and even give it a slightly different odor, though the texture usually isn’t tarry. Activated charcoal, blueberries, and dark food colorings can also darken stool without indicating bleeding.
A simple stool occult blood test or stool guaiac test, a lab method that detects hidden blood, can sort out whether the color change is from blood or from something you ate or took.
Diagnosing the Bleeding Source Behind Melena
When melena appears, the diagnostic priority is finding where the blood is coming from and stopping it. Most guidelines, including those from the American College of Gastroenterology, recommend upper endoscopy as the first-line investigation.
Endoscopy and Beyond
Upper endoscopy involves passing a thin, flexible camera through the mouth to inspect the esophagus, stomach, and duodenum directly. The procedure lets the clinician locate the source of bleeding and, in the same session, apply clips, cauterize lesions, or inject medications to stop active hemorrhage.
If upper endoscopy finds nothing, the workup usually moves to colonoscopy to rule out a lower GI source. When both scopes come back clean, additional tools come into play:
- Angiography: A radiographic technique that uses injected contrast dye to locate active bleeding by mapping blood vessels.
- CT angiography: A faster, less invasive scan to identify active bleeding when endoscopy isn’t available.
- Capsule endoscopy: A swallowed camera pill that images the small intestine, useful for lesions hidden between the upper and lower scopes.
Nasogastric Lavage and Lab Markers
A nasogastric (NG) tube, passed down the nose into the stomach, can sometimes confirm an upper GI source. Aspirating blood or coffee-ground material through the tube supports an upper-source diagnosis, though a clear aspirate doesn’t fully rule one out.
Blood tests add another layer. A falling hemoglobin level, elevated blood urea nitrogen (BUN), and low hematocrit, the proportion of red blood cells in your blood, all point to active blood loss. Clinicians often combine these markers into validated tools like the Glasgow-Blatchford score or the Rockall score to estimate bleeding severity and triage decisions.
When Melena Warrants Immediate Medical Attention
Melena always deserves medical evaluation, and several warning signs make emergency care essential.
Red-Flag Symptoms
Melena accompanied by dizziness, fainting, rapid heartbeat, pale skin, shortness of breath, or cold, clammy extremities signals significant blood loss and possible shock. Vomiting blood alongside melena means bleeding is active and ongoing.
Higher-Risk Populations
Certain groups face elevated risk of severe outcomes from GI bleeding. Older adults have less physiological reserve to tolerate blood loss. People taking anticoagulants (blood thinners) or antiplatelet drugs, including aspirin, clopidogrel, and warfarin, can bleed more heavily once a source starts. Patients with liver disease face a double threat: variceal bleeding from portal hypertension and impaired clotting.
Watchful waiting is never the right move with melena. Even one black, tarry stool signals enough blood loss to warrant evaluation, ideally the same day it appears.
Bottom Line on Melena and Its GI Source
Melena almost always originates from an upper GI lesion, but the digestive timeline that creates its distinctive appearance is what makes it such a useful clinical clue. The answer sits above the duodenojejunal junction in roughly 9 out of 10 cases, with peptic ulcer disease and gastritis leading the list. Recognizing melena early and seeking prompt evaluation can mean the difference between a straightforward fix and a life-threatening hemorrhage.
FAQ
Is melena a sign of upper or lower GI bleeding?
Roughly 90% of melena cases point to bleeding that originates above the lower gastrointestinal tract. The black, tarry appearance comes from hours of bacterial and enzymatic digestion, which only occurs when bleeding starts above the ligament of Treitz in the upper digestive tract.
Does melena always indicate an upper GI bleed?
Melena almost always points to an upper GI source, typically above the ligament of Treitz. Right-sided colon sources can occasionally produce melena if bleeding is slow enough for bacterial digestion to occur, but this remains the exception rather than the rule.
Can melena come from a lower GI source?
Rarely, yes. Slow bleeding from the right colon, particularly from angiodysplasia or right-sided tumors, can darken during prolonged transit and resemble an upper-source bleed. Most lower GI bleeding produces hematochezia instead.
What does melena stool look like?
Melena stool is jet black, sticky, tarry, and has a strong, distinctive odor. It often clings to the toilet bowl and may look similar to coffee grounds in texture.
How is melena distinguished from hematochezia?
Melena is black and tarry because blood has been digested during slow intestinal transit. Hematochezia is bright red or maroon blood that passes quickly, usually signaling a lower GI source.
Why does melena indicate upper GI bleeding?
Prolonged exposure to gastric acid, bile, and colonic bacteria is what produces melena’s distinctive black, tarry color and sticky texture. That digestive process requires hours, and only blood starting above the ligament of Treitz spends enough time in transit to turn black and tarry.
