Black stool is not always dangerous, but it can be. The most common harmless causes are foods like blueberries and black licorice, iron supplements, and over-the-counter stomach medications like Pepto-Bismol. The most concerning cause is bleeding in the upper digestive tract. The key to telling the difference is the stool’s texture and smell, not just its color.
Harmless Causes of Black Stool
Several everyday foods and supplements can turn your stool noticeably dark or fully black. The most common culprits are blueberries, black licorice, blood sausage, activated charcoal, and iron supplements. None of these are cause for concern, and the color change resolves on its own once the food or supplement passes through your system.
Iron supplements are a particularly frequent cause. In studies of people taking oral iron, anywhere from 8% to 31% developed black stools depending on the type and dose. Higher-dose formulations cause the change more often. If you recently started an iron supplement and your stool turned dark, that’s the likely explanation.
Pepto-Bismol and similar bismuth-containing medications also reliably cause black stool (and sometimes a black tongue). This happens because the bismuth reacts with trace amounts of sulfur in your digestive system, forming a black compound called bismuth sulfide. It looks alarming but is completely harmless and clears up after you stop taking the medication.
When Black Stool Signals Bleeding
The medical term for black, tarry stool caused by bleeding is melena. It takes roughly 50 milliliters of blood in the stomach, about three tablespoons, to turn stool black. The blood darkens as it travels through the digestive tract, so by the time it reaches the toilet it no longer looks red. This is why melena strongly points to bleeding in the upper part of the digestive system: the esophagus, stomach, or upper small intestine.
The conditions that most commonly cause this kind of bleeding include peptic ulcers, inflammation of the stomach lining (gastritis), tears in the esophagus, inflamed or damaged veins in the esophagus from liver disease, and in some cases, cancer of the stomach or esophagus.
How to Tell the Difference
Color alone isn’t enough to distinguish a harmless cause from a serious one. The texture and smell matter more. Melena from internal bleeding has three distinct characteristics: it is sticky and tar-like in consistency, it has a notably foul smell (much worse than a typical bowel movement), and it tends to be very dark, almost like asphalt. If your stool is simply dark but formed normally and doesn’t have an unusually strong odor, a food or supplement is the more likely explanation.
Think about what you’ve eaten or taken in the past day or two. A first black stool typically appears 4 to 20 hours after the cause enters the stomach, whether that’s blood from internal bleeding or blueberries from breakfast. If you can trace the timing back to a food, medication, or supplement, you probably have your answer.
Symptoms That Signal an Emergency
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms mean you should go to an emergency room rather than wait for an appointment:
- Vomiting blood or vomit that looks like dark coffee grounds
- Dizziness, weakness, or lightheadedness, which can indicate significant blood loss
- Heart palpitations or shortness of breath
- Several consecutive days of black, tarry stools
These signs suggest active or substantial bleeding. Blood loss from the upper digestive tract can escalate quickly, and treatment is most effective when started early.
How Doctors Check for Blood in Stool
If your doctor suspects bleeding but isn’t sure, the first step is usually a stool test that detects hidden (occult) blood. The newer version of this test, called a fecal immunochemical test or FIT, catches about 86% of colorectal cancers and is more accurate than the older guaiac-based test, which detects roughly 68%. These tests can help confirm whether the black color is actually from blood or from something harmless. If blood is confirmed, an endoscopy (a thin camera passed into the upper digestive tract) is the standard next step to find and often treat the source of bleeding.
Black Stool in Newborns
If you’re a new parent, black stool in the first couple of days is completely normal. Newborns pass a thick, dark, tar-like substance called meconium, which is made up of everything the baby ingested in the womb. Healthy full-term newborns typically pass meconium within 24 to 48 hours of birth, after which stool gradually transitions to a lighter color. If a newborn hasn’t passed meconium after 48 hours, or if black stools return later in infancy, that’s worth bringing up with a pediatrician.