Dark black poop is usually caused by something you ate, drank, or took as a supplement, but it can also signal bleeding in the upper digestive tract. The key difference comes down to texture and smell: harmless black stool from food or supplements looks normal aside from the color, while black stool caused by bleeding is sticky, tarry, and has a distinctly foul odor.
Harmless Causes of Black Stool
Several everyday foods, supplements, and over-the-counter medications can turn your stool dark black. The most common culprits include iron supplements, bismuth-based stomach medicines like Pepto-Bismol, activated charcoal, black licorice, blueberries, and blood sausage.
Iron supplements are one of the most frequent causes. Dark or black stool is a well-known side effect of oral iron tablets, and it happens because your body doesn’t absorb all the iron you swallow. The excess passes through and darkens the stool. This is completely normal and not a reason to stop taking your supplement.
Bismuth, the active ingredient in several popular antacids and anti-diarrhea products, works a little differently. When bismuth reacts with sulfur-producing bacteria in your digestive tract, it forms a dark compound called bismuth sulfide. This can stain both your tongue and your stool black. It looks alarming but is harmless.
If a food or supplement is the cause, your stool should return to its normal brown color within a few days of stopping whatever caused it. The stool will look dark but otherwise have a normal texture and no unusual smell.
When Black Stool Means Bleeding
Black, tarry stool caused by bleeding in the digestive tract has a medical name: melena. It happens when blood from the esophagus, stomach, or upper small intestine is broken down by digestive enzymes and bacteria as it travels through the gut. That process turns the blood from red to jet black. Roughly 100 to 200 milliliters of blood (about half to one cup) in the upper digestive tract is enough to produce melena, and the stool can remain black for several days even after the bleeding has stopped.
While upper GI bleeding is the most common source, bleeding from the small bowel or the right side of the colon can also produce black stool. The further the blood travels through the digestive tract, the darker and more digested it becomes.
Common causes of upper GI bleeding include stomach ulcers, inflammation of the stomach lining, tears in the esophagus, and in more serious cases, tumors or enlarged veins in the esophagus related to liver disease.
How to Tell the Difference
The distinction between harmless dark stool and melena is usually obvious once you know what to look for. Melena is jet black with a sticky, tarry consistency, almost like roofing tar. It also has a particularly strong, offensive odor that’s hard to miss. This smell comes from blood being broken down and partially digested as it moves through the intestines.
Stool that’s been darkened by food, iron, or bismuth looks dark but keeps its normal texture and doesn’t have that characteristic smell. A small amount of bleeding may also look more dark brown than truly black, so color alone isn’t always a reliable guide. If you’re unsure, the texture and smell are more telling.
Black Stool in Newborns
If you’re a new parent noticing very dark stool in your baby’s diaper, there’s likely no reason to worry. A newborn’s first stool, called meconium, is blackish-green, thick, and sticky, resembling tar or sludge. This is made up of materials the baby ingested in the womb. Babies typically pass meconium within 24 to 48 hours after birth, and the stool gradually transitions to a lighter color over the following days as the baby begins feeding.
How Doctors Check for Blood
If there’s any question about whether black stool contains blood, a simple stool test can provide answers. The most common options are a guaiac-based test (gFOBT) and a newer immunochemical test (FIT). Both detect hidden blood that isn’t visible to the naked eye, but they differ in accuracy. FIT tests catch about 76% of colorectal cancers compared to roughly 39% for the older guaiac test, making FIT the preferred screening tool in most settings. Both tests are highly specific, meaning a positive result is rarely a false alarm.
If a stool test comes back positive, further evaluation typically involves an endoscopy, where a camera is passed into the upper digestive tract to locate the source of bleeding.
What You Should Do
Start by thinking about what you’ve eaten or taken in the last day or two. If you’re on iron supplements, taking Pepto-Bismol, or recently ate a large serving of blueberries or black licorice, that’s very likely the explanation. Stop the suspected cause and watch for the stool to return to brown within a few days.
If you can’t link the color to anything you’ve eaten or taken, or if the stool is sticky and tarry with a strong smell, that warrants prompt medical attention. The same applies if you notice other symptoms alongside black stool, such as dizziness, lightheadedness, vomiting (especially if the vomit looks like coffee grounds), abdominal pain, or feeling unusually weak. These can indicate active bleeding that needs evaluation quickly.