Dark stool usually means one of two things: something you ate or swallowed changed the color, or blood from somewhere in your upper digestive tract has mixed into the stool. The first is harmless. The second can be serious. Telling them apart is straightforward once you know what to look for.
Harmless Causes of Dark Stool
Several common foods, supplements, and over-the-counter medications can turn your stool dark brown or even jet black. Iron supplements are one of the most frequent culprits. Bismuth-based medications like Pepto-Bismol do the same thing. Activated charcoal, black licorice, blueberries, and blood sausage can all darken stool noticeably.
If something you consumed is responsible, the color change is temporary and stops within a day or two after you stop taking the supplement or eating the food. Critically, the stool will look dark but otherwise normal in texture and smell. That distinction matters a lot.
When Dark Stool Signals Bleeding
Dark stool caused by bleeding in the upper digestive tract, called melena, looks and behaves differently from food-stained stool. Classic melena is jet black with a tarry, sticky consistency. It also has a strong, distinctly foul odor, which is a byproduct of blood being broken down as it travels through your digestive system. The longer the blood has traveled, the darker and more pungent the stool becomes. A smaller amount of bleeding may produce stool that looks more dark brown than black.
If your stool was simply stained by iron pills or bismuth, you won’t notice that distinctive smell. That odor is one of the most reliable ways to tell the difference at home.
What Causes Upper Digestive Bleeding
The most common source of melena is a peptic ulcer, a sore in the lining of the stomach or the first part of the small intestine. Use of NSAIDs (ibuprofen, aspirin, naproxen) and infection with a bacterium called H. pylori together account for roughly 80% of peptic ulcer disease and ulcer-related bleeding.
Other causes include gastritis (inflammation of the stomach lining), esophagitis (inflammation of the esophagus), variceal bleeding from swollen veins in the esophagus (often related to liver disease), Mallory-Weiss tears from forceful vomiting, and, less commonly, cancers of the upper digestive tract. Erosive conditions like gastritis and esophagitis tend to have an excellent prognosis when treated.
Symptoms That Accompany Serious Bleeding
Dark, tarry stool on its own warrants attention, but certain accompanying symptoms make the situation more urgent. Lightheadedness, dizziness, or fainting suggest you’re losing enough blood to affect circulation. Vomiting blood or material that looks like coffee grounds points to active upper digestive bleeding. Abdominal pain is common with ulcers and gastritis.
Life-threatening hemorrhage can cause a sharp drop in blood pressure, rapid heart rate, and significant weakness. If you’re experiencing tarry black stool alongside any of these symptoms, that combination needs emergency evaluation.
How Doctors Check for Hidden Blood
When stool color is ambiguous, a fecal occult blood test can detect tiny amounts of blood invisible to the naked eye. There are two main versions. A fecal immunochemical test (FIT) requires a single stool sample collected with a stick or device from the test kit. A guaiac-based test typically requires samples from three separate bowel movements, smeared onto cards. Some guaiac tests use flushable pads that change color in the toilet bowl when blood is present.
These tests only confirm whether blood is there or not. They can’t identify where the bleeding is coming from. If the result is positive, the next step is usually a colonoscopy or an upper endoscopy to examine the digestive tract directly and locate the source.
Dark Stool in Babies and Children
Newborns pass a thick, dark, tar-like first stool called meconium, which is made up of everything they swallowed in the womb: amniotic fluid, mucus, bile, and shed cells. This is completely normal and clears within the first few days of life.
After those first few days, black tarry stool in an infant is no longer expected. In a baby older than three days, it may signal blood in the gastrointestinal tract and should be evaluated. Dark green stool in babies is a different story and is almost always caused by iron-fortified formula or an iron supplement. Hard, dark stools paired with straining typically indicate constipation rather than bleeding.
A Quick Way to Sort It Out
Start with the simplest explanation. Think about what you’ve eaten or taken in the last 24 to 48 hours. Iron supplements, bismuth medications, blueberries, and black licorice are the usual suspects. If the stool is dark but has a normal texture and no unusual smell, one of these is almost certainly the cause.
If the stool is black, sticky, tar-like, and has a strong offensive odor, that pattern points toward digested blood. Combine that with symptoms like dizziness, stomach pain, or vomiting, and the picture becomes clearer. Stopping the suspected food or supplement for a couple of days is a reasonable first step when you have no other symptoms. If the dark color persists after that, or if you feel unwell, getting checked is the right move.