Dark diarrhea can mean something as harmless as last night’s blueberry smoothie, or it can signal bleeding somewhere in your digestive tract. The key distinction is whether your stool is simply dark brown or truly black and tarry, which looks like wet coffee grounds or road tar and often has a distinctive foul smell. That tarry appearance, called melena, points to a medical issue that needs prompt attention. A dark brown loose stool, on the other hand, is far more common and usually has a straightforward explanation.
Dark Brown vs. Black and Tarry
Color alone doesn’t tell the whole story. Dark brown diarrhea is typically caused by something you ate, drank, or took as a supplement. It passes within a day or two once the offending substance clears your system. The texture is loose but otherwise unremarkable.
Black, tarry stool is different in several ways. It tends to be sticky, almost like tar or thick motor oil. It also carries a sharp, unusually unpleasant odor that’s distinct from normal stool. This appearance comes from blood that has been partially digested as it travels through your stomach and intestines. The digestive process breaks down the red blood cells and turns the blood dark black rather than bright red. If your diarrhea matches this description, it suggests bleeding in your upper digestive tract: the esophagus, stomach, or the first section of the small intestine.
Common Harmless Causes
Most of the time, dark diarrhea has a benign explanation. Several everyday foods, supplements, and medications can temporarily darken your stool.
- Blueberries: Eating a large amount can turn stool so dark it looks almost black.
- Black licorice: The dark dye passes through your system and stains stool.
- Beets: A red pigment called betanin can give stool a deep reddish-black appearance that mimics blood.
- Brightly colored candy: If you eat handfuls of rainbow-colored candy, the mixed dyes can combine and turn stool black.
- Iron supplements: Unabsorbed iron is a well-known cause of dark green to black stool. This is a normal side effect of oral iron tablets.
- Bismuth subsalicylate (Pepto-Bismol): The active ingredient reacts with trace amounts of sulfur in your saliva and digestive system, forming a black compound called bismuth sulfide. This can turn stool jet black for a day or two.
- Activated charcoal supplements: These are sometimes taken for gas and bloating, and the black powder simply stains stool dark as it passes through.
If you recently consumed any of these, that’s likely your answer. The color change should resolve within one to three days after you stop.
When Dark Diarrhea Signals Bleeding
If none of the harmless causes apply, dark or black diarrhea may indicate bleeding in the upper part of your digestive tract. The most common causes include peptic ulcers in the stomach or upper small intestine, inflammation or erosion of the stomach lining, and tears in the esophagus caused by severe or prolonged vomiting.
Liver disease can also be responsible. In advanced cirrhosis, blood pressure rises in the vein that carries blood from your gut to your liver. That increased pressure forces blood through small, fragile vessels lining your stomach and the lower part of your esophagus. These swollen vessels, called varices, can burst under pressure, releasing blood into your digestive tract. The result is black, tarry stool or vomiting blood. In rarer cases, cancers of the stomach, esophagus, or pancreas can cause slow or intermittent bleeding that darkens stool over time.
Pain relievers like aspirin and ibuprofen are a particularly common culprit. Regular use can irritate or erode the stomach lining enough to cause slow bleeding you might not feel.
Accompanying Symptoms That Matter
Dark diarrhea on its own, especially after eating certain foods, rarely signals danger. But when it appears alongside other symptoms, the picture changes. Pay close attention if you also experience dizziness or lightheadedness, a racing heartbeat, unusual fatigue or weakness, abdominal pain, or vomiting that looks like coffee grounds. These symptoms together suggest you may be losing enough blood to affect your circulation. Feeling faint when you stand up is an especially telling sign, because it indicates your blood volume has dropped.
How Bleeding Is Identified
When a healthcare provider suspects your dark stool comes from internal bleeding rather than diet, the first step is often a stool sample test. A fecal occult blood test (FOBT) or fecal immunochemical test (FIT) checks for hidden blood that isn’t visible to the naked eye. These tests are simple but imperfect. They can miss some bleeding, and certain foods or medications can trigger false results, so they’re a starting point rather than a definitive answer.
If blood is detected, or if your symptoms are concerning enough, the next step is typically a scope procedure to look directly at the lining of your esophagus, stomach, and upper intestine. This lets the provider find the bleeding source and, in many cases, treat it during the same procedure. Ulcers and erosions are often managed with acid-reducing medications that help the damaged tissue heal.
What to Do Right Now
Start by thinking about what you’ve eaten, drunk, or taken in the past 24 to 48 hours. Iron supplements, bismuth-based antacids, blueberries, and dark-colored foods are the most common explanations. If one of those fits, stop the food or supplement and watch whether the color returns to normal within a couple of days.
If you can’t identify a dietary cause, if the stool is sticky and tar-like with an unusually strong odor, or if you feel dizzy, weak, or nauseous alongside the dark diarrhea, treat it as urgent. Significant upper GI bleeding can escalate quickly, and early evaluation makes a meaningful difference in outcomes.