Black stool usually means one of two things: something you ate or swallowed is changing the color, or there is bleeding somewhere in your upper digestive tract. The difference matters a lot, and fortunately, it’s usually possible to figure out which one is happening based on what you’ve recently consumed, what the stool looks and smells like, and whether you have any other symptoms.
How Blood Turns Stool Black
When bleeding occurs in the stomach or upper small intestine, the blood doesn’t stay red. Gastric acid converts the hemoglobin in red blood cells into a brown-black substance called hematin. As that altered blood continues through the intestines, further oxidation deepens the color. By the time it reaches the colon, it’s dark enough to turn the entire stool black.
The medical term for this is melena, and it has a distinct appearance. Melena is not just dark stool. It’s tarry, sticky, and has a noticeably foul smell that’s different from a normal bowel movement. If your stool is black but formed normally and doesn’t have that sticky, tar-like quality, a dietary or supplement-related cause is far more likely.
Common Harmless Causes
Several everyday foods, supplements, and medications can turn stool black without any bleeding involved:
- Iron supplements: Unabsorbed iron passing through the gut is one of the most common causes. At supplemental doses above about 20 to 25 mg per day, black stools become increasingly likely. This is harmless and stops when you stop taking the supplement.
- Bismuth medications: Products like Pepto-Bismol contain bismuth, which reacts with trace amounts of sulfur in your saliva and digestive system to form bismuth sulfide, a black compound. Your tongue can turn black too.
- Foods: Black licorice, blueberries, blood sausage, and dark-colored foods can all darken stool noticeably.
- Activated charcoal: Sometimes taken as a supplement or used in medical settings, charcoal turns stool very dark or black.
If you can trace the timing to something on this list, that’s almost certainly your answer. The color change typically resolves within a day or two of stopping the food, supplement, or medication.
Medical Causes of Black Stool
When black, tarry stool isn’t explained by diet or medication, it points to bleeding in the upper gastrointestinal tract: the esophagus, stomach, or the first section of the small intestine. The most common causes include:
Peptic ulcers are the leading cause of upper GI bleeding. These are open sores on the lining of the stomach or upper small intestine, often caused by a bacterial infection or long-term use of anti-inflammatory painkillers like ibuprofen or aspirin. They can bleed slowly enough that the only sign is gradually darkening stool.
Esophageal varices are swollen veins in the esophagus that develop in people with serious liver disease, most commonly from long-term heavy alcohol use. When these veins rupture, they can bleed heavily.
Tears in the esophagus, known as Mallory-Weiss tears, can happen after forceful vomiting or retching. These are also more common in people who drink heavily.
Gastritis and inflammation of the stomach lining can cause enough surface bleeding to produce black stool, particularly in people taking blood thinners or anti-inflammatory drugs regularly.
How to Tell the Difference
The texture and smell of the stool give you the most useful information. Melena from actual bleeding is sticky and tar-like, coats the toilet bowl, and has a strong, distinctive odor that most people describe as unusually foul. Black stool from iron pills or blueberries looks dark but has a normal consistency and smell.
One important limitation to know: the standard stool test used to screen for colon cancer (called a fecal immunochemical test, or FIT) does not detect upper GI bleeding. Hemoglobin breaks down too much during its long transit through the intestines for this test to pick it up. If your doctor suspects upper GI bleeding, they’ll use a different type of test or proceed directly to an endoscopy, where a small camera is passed down the throat to look at the esophagus, stomach, and upper intestine.
Warning Signs That Need Urgent Attention
Black, tarry stool on its own is worth getting checked. But certain accompanying symptoms suggest more significant blood loss and call for urgent or emergency care:
- Dizziness, weakness, or lightheadedness, which signal that enough blood has been lost to affect circulation
- Heart palpitations or shortness of breath, which indicate the heart is working harder to compensate for reduced blood volume
- Vomiting blood or vomit that looks like dark coffee grounds
- Several consecutive days of black, tarry stools
If you experience any of these alongside dark stool, go to an emergency room. The initial focus will be on stabilizing your blood pressure and fluid levels before any diagnostic procedures. An endoscopy is typically performed within 24 hours of arrival to locate and treat the source of bleeding.
What to Do Next
Start by reviewing what you’ve eaten or taken in the past 48 hours. Iron supplements, bismuth-based medications, and dark-colored foods are responsible for the vast majority of black stool episodes. If you identify a clear dietary cause and feel otherwise fine, you can simply monitor. The color should return to normal within a couple of days after you stop the offending item.
If you can’t identify a dietary or supplement-related cause, or if the stool is sticky and tar-like, contact your doctor promptly. Even slow upper GI bleeding can lead to significant blood loss over time, causing fatigue, weakness, and anemia. Catching it early usually means simpler, more effective treatment. Peptic ulcers, the most common culprit, are highly treatable with medications that reduce stomach acid and, when needed, antibiotics to clear the underlying infection.