Black, tarry stool usually means one of two things: something you ate or swallowed is changing the color, or blood from your upper digestive tract is being digested on its way through. The second possibility, called melena, is a medical emergency. Telling the two apart comes down to texture, smell, and what else is going on in your body.
What Makes Stool Look Like Tar
When bleeding happens high in the digestive tract, in the stomach or the first part of the small intestine, the blood doesn’t stay red. Digestive chemicals and stomach acid break down the hemoglobin as it travels through roughly 20 feet of intestine. By the time it reaches the other end, it has turned into a black, sticky, tar-like substance with a distinctly foul smell. That smell is a reliable clue: it’s sharper and more offensive than the usual odor, because it’s the byproduct of blood being chemically digested.
True melena looks and feels different from simply dark stool. It’s not just black; it’s shiny, sticky, and clings to the toilet bowl. If your stool is dark but formed, firm, and doesn’t have that unmistakable odor, the cause is more likely something benign.
Harmless Causes of Black Stool
Several foods and medications turn stool convincingly black without any bleeding involved. Iron supplements are one of the most common culprits. Bismuth, the active ingredient in Pepto-Bismol, reacts with small amounts of sulfur naturally present in your saliva and digestive system to form bismuth sulfide, a black compound that darkens both your tongue and your stool. This effect usually fades within several days of stopping the medication.
Foods that can do the same include black licorice, blueberries, blood sausage, and activated charcoal. The key difference is that these produce dark stool without the tarry, sticky texture or the harsh smell associated with digested blood. If you recently ate any of these or started a new supplement, that’s the most likely explanation.
Peptic Ulcers: The Most Common Cause
When tarry stool does signal bleeding, a peptic ulcer is the most frequent source. These are open sores in the lining of the stomach or the upper small intestine, and they develop when the protective mucus layer breaks down and acid eats into the tissue underneath. Two things cause the vast majority of peptic ulcers: infection with a bacterium called H. pylori, and regular use of NSAIDs like ibuprofen, naproxen, or aspirin.
NSAIDs work by blocking a pathway that, among other things, maintains the stomach’s protective defenses. These defenses include the blood flow to the stomach lining, the secretion of mucus and bicarbonate, and the ability of the lining to repair itself. When you suppress all of that, the stomach becomes vulnerable to its own acid. Symptomatic or complicated ulcers occur in about 1 in 20 NSAID users overall, and in 1 in 7 older adults taking NSAIDs. Even low-dose aspirin taken daily for heart protection doubles to quadruples the risk. Enteric-coated or buffered aspirin does not appear to reduce this risk, because the damage is largely a systemic effect rather than just local irritation from the pill itself.
Combining an NSAID with aspirin raises the annual risk of a significant upper GI event to about 5.6%. Alcohol also contributes, as do bile salts and the H. pylori bacterium itself, which was first linked to stomach inflammation in 1983 and is now recognized as a central driver of ulcer disease.
Other Conditions That Cause Upper GI Bleeding
Beyond ulcers, several other conditions can bleed high enough in the digestive tract to produce tarry stool. Gastritis, an inflammation of the stomach lining, can erode tissue enough to cause slow bleeding. Esophageal varices, which are swollen veins in the esophagus often tied to liver disease, can bleed heavily and without warning. Tears in the lining where the esophagus meets the stomach (often caused by forceful vomiting) are another source. Less commonly, tumors in the stomach or upper intestine can bleed intermittently.
Warning Signs That Need Immediate Attention
Black, tarry stool on its own warrants prompt medical evaluation. But certain accompanying symptoms point to active, significant blood loss that needs emergency care. These include dizziness or fainting, a rapid pulse, cold or clammy skin, nausea or vomiting (especially vomiting blood or material that looks like coffee grounds), pale or grayish skin, confusion or unusual agitation, and producing very little urine. These are signs of shock, meaning your body is losing blood faster than it can compensate.
Other red flags that suggest a serious underlying problem include unexplained weight loss, difficulty swallowing that gets progressively worse, feeling full after eating very little, and a family history of gastrointestinal cancer. Any of these alongside tarry stool should move up your timeline for getting evaluated.
How Doctors Find the Source
The first step is usually confirming that the black color is actually from blood. A simple stool test can detect hidden blood that isn’t visible to the naked eye. If that test is positive, or if the clinical picture already points strongly to bleeding, the next step is typically an upper endoscopy. This involves a thin, flexible camera passed through the mouth and into the stomach and upper small intestine. It lets the doctor see the source of bleeding directly and, in many cases, treat it during the same procedure by cauterizing the bleeding site, applying clips, or injecting medication to stop the bleeding.
Blood tests to check for anemia (a low red blood cell count) help gauge how much blood has been lost. If you’ve been bleeding slowly over days or weeks, you may have become significantly anemic without realizing it, which explains symptoms like fatigue, lightheadedness, and shortness of breath with activity.
What Raises Your Risk
Certain factors make upper GI bleeding more likely, and knowing them helps you assess your own situation. Regular NSAID use is at the top of the list, particularly if you’re over 60, taking high doses, or combining pain relievers with aspirin. A prior history of ulcers or GI bleeding significantly raises the chance of it happening again. Alcohol use irritates and thins the stomach lining. Liver disease, especially cirrhosis, increases the risk of esophageal varices. Anticoagulant medications (blood thinners) don’t cause ulcers on their own, but they make any existing erosion bleed more freely and heal more slowly.
H. pylori infection is extremely common worldwide and often causes no symptoms at all until an ulcer develops. It can be detected with a breath test or stool test and treated with a course of antibiotics combined with acid-reducing medication.