Black stool usually comes from something you ate, drank, or took as a supplement or medication. In most cases, it’s harmless and clears up within a few days. But black stool can also signal bleeding in your upper digestive tract, which needs prompt medical attention. The key is figuring out which category you fall into.
Common Harmless Causes
The most frequent reasons for black stool are dietary. Iron supplements are one of the biggest culprits, and the color change is a well-known, expected side effect. Bismuth-based medications like Pepto-Bismol also reliably turn stool black. This happens because bismuth reacts with small amounts of sulfur naturally present in your saliva and digestive system, forming a compound called bismuth sulfide, which is jet black. The same reaction can temporarily darken your tongue.
Certain foods do it too. Black licorice, blueberries, blood sausage, and activated charcoal can all darken stool enough to look alarming. If you recently consumed any of these, that’s almost certainly your answer.
How Soon Stool Returns to Normal
Once you stop taking the medication or supplement causing the color change, your stool should return to its normal brown within a couple of days. With Pepto-Bismol specifically, both the darkened stool and any tongue discoloration clear up in that same short window. If the color persists more than a few days after you’ve stopped, something else is going on.
When Black Stool Means Bleeding
Black stool caused by bleeding in the upper digestive tract, known medically as melena, looks and feels different from diet-related dark stool. Melena is typically tarry, sticky, and has a distinctly foul smell that’s hard to miss. The black color comes from blood that has been partially digested: as blood travels from the stomach or upper intestine downward, stomach acid converts the hemoglobin in red blood cells into a dark brown substance. By the time it exits, it looks like tar rather than normal stool.
The most common causes of this type of bleeding include:
- Peptic ulcers: open sores in the stomach lining or the first part of the small intestine, often linked to long-term use of anti-inflammatory painkillers or a bacterial infection
- Severe inflammation: irritation of the stomach lining (gastritis) or the esophagus (esophagitis), sometimes from acid reflux or alcohol use
- Ruptured veins: swollen veins in the esophagus or stomach, most often associated with liver disease
How to Tell the Difference
Start with the simplest question: did you recently take iron, Pepto-Bismol, activated charcoal, or eat any of the foods listed above? If yes, and you feel fine otherwise, the explanation is almost certainly dietary. Stop taking the substance and watch whether your stool returns to normal within two to three days.
Melena from internal bleeding tends to come with other symptoms. You might feel lightheaded or dizzy, notice unusual fatigue, have abdominal pain, or feel your heart racing. Some people vomit material that looks like coffee grounds, which is another sign of upper digestive bleeding. The stool itself is a clue too: tarry, unusually sticky consistency with a strong odor is different from stool that’s simply dark in color but otherwise normal in texture.
If you can’t trace the color to something you consumed, or if you have any of those accompanying symptoms, get medical attention quickly. Upper GI bleeding can range from slow and mild to rapid and dangerous, and it’s not something to wait out.
How Doctors Test for Hidden Blood
When the cause isn’t obvious, doctors can test your stool for hidden (occult) blood. The newer version of this test, called FIT, is more reliable than the older method because it isn’t affected by foods you’ve eaten. The older test could give false positives from red meat, broccoli, cauliflower, carrots, and other foods with naturally occurring compounds that mimic the chemical signal of blood. If your doctor uses the older test, you may be asked to avoid certain foods for two to seven days beforehand.
A positive result on either test typically leads to further investigation, usually an endoscopy, where a camera is used to look directly at the lining of your esophagus, stomach, and upper intestine to find the source of bleeding.
What to Pay Attention To
One episode of black stool after taking Pepto-Bismol or eating a bowl of blueberries is nothing to worry about. But if black, tarry stool appears without an obvious dietary explanation, recurs over several days, or comes alongside dizziness, weakness, stomach pain, or vomiting, those are signs of active bleeding that needs evaluation. The difference between a harmless color change and a medical emergency often comes down to texture, smell, and how you feel overall.