Dark or black stool usually means one of two things: something you ate or swallowed is changing the color, or there’s bleeding somewhere in your upper digestive tract. The difference matters a lot, and it’s usually possible to figure out which category you’re in based on a few simple clues.
Harmless Causes: Food, Drinks, and Medications
The most common reason for dark stool is something you recently consumed. Iron supplements are one of the biggest culprits, turning stool black or very dark green. Bismuth subsalicylate, the active ingredient in Pepto-Bismol and Kaopectate, does the same thing. Black licorice (the real kind, not just licorice-flavored candy) can also darken stool noticeably.
Certain foods play a role too. Blueberries, blackberries, dark chocolate, beets, and foods with dark artificial dyes can all shift your stool toward brown-black. These color changes typically clear up within a day or two after you stop eating the food or taking the supplement. If you can trace the timing back to something you consumed, and you feel fine otherwise, that’s a reassuring sign.
When Dark Stool Signals Bleeding
Black, tarry stool with a distinctly foul smell has a medical name: melena. It looks different from stool that’s simply dark from food. Melena is sticky, almost like tar, and has an unusually strong odor. That appearance comes from blood that has been partially digested as it travels through your stomach and intestines. By the time it reaches the other end, the hemoglobin in the blood has broken down into dark compounds, giving the stool its characteristic black, sticky quality.
The bleeding source is almost always in the upper digestive tract: the esophagus, stomach, or the first section of the small intestine. The most common causes include:
- Peptic ulcers: Open sores in the stomach lining or upper small intestine. Some ulcers cause abdominal pain, but others are “silent” and produce no symptoms until they bleed.
- Gastritis: Inflammation of the stomach lining, often from prolonged use of anti-inflammatory painkillers or bacterial infection.
- Esophageal varices: Swollen, fragile veins in the lower esophagus. These develop when scar tissue in the liver (cirrhosis) blocks normal blood flow, forcing blood through smaller vessels that aren’t built to handle the pressure. They can leak or rupture suddenly. This is most common in people with serious liver disease from long-term alcohol use, hepatitis, or fatty liver disease.
- Tears in the esophageal lining: Sometimes called Mallory-Weiss tears, these can happen after forceful vomiting or retching.
- Tumors: Growths in the stomach or upper intestine can bleed slowly, sometimes without any pain at all.
Where you feel pain (if you feel any) can offer a clue to the source. Upper abdominal pain often points to a stomach condition. Chest pain may suggest an esophageal problem. Back pain can indicate a pancreatic issue. But the absence of pain doesn’t rule out something serious.
How to Tell the Difference
Start by reviewing what you’ve eaten or taken in the last 24 to 48 hours. If you recently took iron supplements, Pepto-Bismol, or ate a large serving of blueberries, that’s likely your answer. Stool darkened by food or medication is typically firm or normal in consistency, not tarry or sticky.
Melena from bleeding looks and smells different. It’s loose, shiny, and coats the toilet bowl. If you’re unsure, your doctor can run a simple stool test to check for hidden blood. The newer version of this test, called a fecal immunochemical test (FIT), detects colorectal cancer with about 76% to 89% sensitivity, making it significantly more reliable than older chemical-based stool tests, which catch only 39% to 59% of cancers. These tests are quick and noninvasive, and they can settle the question of whether blood is present.
Warning Signs That Need Urgent Attention
Dark stool on its own, with no other symptoms, is often benign. But certain combinations of symptoms signal active blood loss and require emergency care. Get to an emergency room if you notice black, tarry stools along with any of the following:
- Vomiting blood or vomit that looks like dark coffee grounds
- Dizziness, weakness, or lightheadedness, which can indicate a drop in blood pressure from significant blood loss
- Heart palpitations or shortness of breath
- Several consecutive days of black, tarry stools
Severe bleeding that causes a significant drop in blood pressure may require hospitalization, IV fluids, blood transfusions, or a procedure to locate and stop the bleeding source. The standard diagnostic tool in these situations is an upper endoscopy, where a small camera is passed through the mouth to visually inspect the esophagus, stomach, and upper intestine.
How Long Dark Stool Lasts
If the cause is dietary, stool color typically returns to normal within one to two days after you stop eating the food or taking the supplement. If the cause is bleeding that’s been treated, it takes longer. Blood that was already in the digestive tract before treatment still needs to pass through. Depending on how quickly your gut moves things along, melena can persist for up to five days after the actual bleeding has stopped. This is normal and doesn’t necessarily mean you’re still bleeding, but your doctor will likely monitor you to confirm.
A single episode of slightly darker stool that you can connect to something you ate is rarely cause for concern. Persistent dark stool, especially if it’s tarry or accompanied by fatigue, weight loss, or abdominal pain, warrants a medical evaluation to identify or rule out a source of bleeding.