Black diarrhea is loose stool that appears dark black or tarry, and it often signals that blood has been digested somewhere in your upper gastrointestinal tract. The medical term for this is melena. While certain foods and supplements can turn stool black without any danger, truly tarry black diarrhea with a distinct foul smell is typically a sign of bleeding from the stomach, esophagus, or upper small intestine. The difference matters, and understanding what causes each type helps you figure out how urgently you need to act.
Why Black Stool Looks and Smells Different
When bleeding occurs high in the digestive tract, the blood travels through the stomach and intestines before reaching your colon. Along the way, digestive enzymes break down hemoglobin, the protein that makes blood red. By the time it exits, the blood has turned dark black and sticky. This is why melena has a tar-like consistency and a particularly strong, unpleasant odor that’s noticeably different from normal stool.
This is distinct from bright red blood in stool, which usually comes from a source much lower in the digestive tract, like the colon or rectum. The color tells you roughly where the bleeding started: black means upper, red means lower.
Harmless Causes of Black Stool
Not all black stool means bleeding. Several everyday substances can darken your stool dramatically without any medical concern. Iron supplements are one of the most common culprits. Bismuth-based medications like Pepto-Bismol also turn stool black. Activated charcoal, blueberries, black licorice, and blood sausage can do the same.
The key difference is texture and smell. Stool darkened by food or supplements tends to look dark but otherwise normal in consistency. It won’t have the sticky, tar-like quality or the strong odor of melena. If you recently started an iron supplement or ate a large amount of blueberries, that’s likely the explanation. But if you can’t connect the color to something you consumed, take it seriously.
Upper GI Bleeding: The Main Medical Cause
The most concerning cause of black diarrhea is upper gastrointestinal bleeding, which accounts for roughly 85% of all GI bleeding cases. The most common sources include peptic ulcers (open sores in the stomach or upper intestine lining), gastritis (inflammation of the stomach lining), and tears in the esophagus.
Peptic ulcers are the single most frequent cause. They develop when the protective mucus layer of the stomach breaks down, often from long-term use of anti-inflammatory painkillers or infection with a specific type of stomach bacteria. Gastritis follows a similar pattern, where chronic irritation leads to bleeding from the stomach wall. Heavy alcohol use can contribute to both conditions.
Upper GI bleeding is not rare, and it carries real risk. Despite modern treatments, mortality rates for acute upper GI bleeding still sit around 8 to 15%. That statistic isn’t meant to alarm you, but it underscores why black tarry stool isn’t something to wait out.
Liver Disease and Swollen Veins
In people with liver cirrhosis, a specific and dangerous cause of black stool involves swollen veins in the esophagus called varices. When scar tissue blocks normal blood flow through the liver, pressure builds in the large vein feeding it. Blood gets rerouted through smaller, thinner-walled veins in the lower esophagus that aren’t built to handle that volume. These veins balloon outward and can rupture, releasing blood that travels down through the digestive tract and produces black, tarry stool.
Bleeding from esophageal varices can be severe and life-threatening. If you have known liver disease and notice black stool, this is an emergency.
Warning Signs That Need Immediate Attention
Black diarrhea on its own warrants a call to your doctor, but certain accompanying symptoms mean you should get to an emergency room. Feeling lightheaded, dizzy, or faint suggests enough blood loss to affect your circulation. Vomiting blood or material that looks like coffee grounds points to active upper GI bleeding.
Signs of shock are the most urgent. These include a rapid heart rate, pale skin, cold hands and feet, sweating, confusion, or loss of consciousness. Shock from GI bleeding means blood loss has become severe enough to compromise your organs. This is a life-threatening situation.
How Doctors Identify the Source
When you present with black stool, the first step is usually a stool test to confirm whether blood is actually present. This is called a fecal occult blood test, and it detects hidden blood that isn’t visible to the naked eye. It’s worth noting that non-GI sources of blood, like swallowed blood from a nosebleed, can also produce a positive result.
If blood is confirmed, doctors typically perform an upper endoscopy, where a thin, flexible camera is passed through the mouth into the esophagus, stomach, and upper intestine. This lets them see the bleeding source directly and often treat it during the same procedure. If the upper endoscopy doesn’t find the source, a colonoscopy or small bowel evaluation may follow.
What Treatment Looks Like
Treatment depends entirely on the cause. For peptic ulcers and gastritis, the goal is to stop the bleeding and reduce stomach acid so the tissue can heal. Acid-reducing medications are the cornerstone of treatment. These work by keeping the stomach’s pH high enough to protect blood clots that form over the wound, preventing them from breaking down and rebleeding. You’ll likely stay on these medications for weeks to months while the ulcer or inflammation heals.
During an endoscopy, doctors can also treat bleeding directly by applying clips, heat, or injections to seal off a bleeding vessel. For esophageal varices, the approach involves banding the swollen veins to stop or prevent rupture. People with liver disease who experience variceal bleeding typically need ongoing monitoring because the underlying pressure problem persists.
If significant blood has been lost, you may need fluids or blood transfusions to stabilize before the source is even addressed. Hospital stays for acute upper GI bleeding vary but often last several days while doctors confirm the bleeding has stopped.
Black Stool in Babies
Newborns pass a greenish-black, tar-like stool called meconium during the first few days of life. This is completely normal. It’s the baby clearing out everything swallowed in the womb: amniotic fluid, mucus, bile, and shed cells. It’s notoriously sticky.
After the first three days, however, black tarry stool in an infant is not normal. It may indicate that blood has entered the baby’s gastrointestinal tract, and it requires a call to your pediatrician. The same applies to white, clay-colored stool, or visible blood or mucus in a baby’s diaper.