Black stool has two broad explanations: something you ate or swallowed, or bleeding somewhere in your upper digestive tract. The harmless causes are common, but because internal bleeding can look identical at first glance, it helps to know exactly what to look for and when the color change matters.
Harmless Causes: Food, Supplements, and Medications
The most common reason for black stool is something you recently put in your mouth. Iron supplements are a frequent culprit. Your body absorbs only a portion of the iron in each pill, and the unabsorbed iron darkens stool as it passes through. This is a normal, expected side effect and not a sign of bleeding.
Bismuth, the active ingredient in Pepto-Bismol and similar antacids, also turns stool black (and sometimes your tongue, too). This happens because bismuth reacts with tiny amounts of sulfur naturally present in your saliva and digestive system, forming a black compound called bismuth sulfide. The discoloration is harmless and clears up once you stop taking the medication.
Certain foods can do the same thing. Black licorice, blueberries, blood sausage, and activated charcoal all produce dark or black-looking stool. If your stool turned black within a day or two of eating one of these foods or starting a new supplement, that’s likely your answer. The key difference from a medical cause is texture: food-related black stool looks normal in consistency, not sticky or tar-like.
When Black Stool Signals Bleeding
Black, tarry stool with a distinctly sticky texture and an unusually strong, foul smell is called melena, and it typically points to bleeding in your upper digestive tract: your esophagus, stomach, or the first section of your small intestine. The blood appears black rather than red because digestive enzymes break down hemoglobin as it travels through the gut, changing its color and consistency along the way. By the time it reaches the toilet, it looks like tar.
This is different from bright red blood in or on your stool, which usually signals bleeding lower in the digestive tract, such as the colon or rectum. The farther the bleeding source is from the exit, the darker the blood appears.
Common Causes of Upper GI Bleeding
Peptic ulcer disease, meaning open sores in the lining of the stomach or upper small intestine, is the most common cause of upper gastrointestinal bleeding. In a review of more than 10,000 patients with upper GI bleeds, peptic ulcers accounted for 27% to 40% of all episodes. These ulcers are often driven by a bacterial infection (H. pylori) or by regular use of anti-inflammatory painkillers.
NSAIDs like ibuprofen, naproxen, and aspirin deserve special attention here. Current use of any NSAID raises the risk of upper GI bleeding roughly 4.7 times compared to non-use, and higher doses push that risk even further, to about 7 times. Among individual drugs, ibuprofen carries the lowest relative risk (about double), while piroxicam and some older prescription NSAIDs carry risks 13 to 18 times higher than baseline. If you take NSAIDs regularly and notice tarry black stool, that combination is worth taking seriously.
Other sources of upper GI bleeding include inflammation of the stomach lining (gastritis), tears in the esophagus, and enlarged veins in the esophagus or stomach that can develop in people with liver disease.
How to Tell the Difference
A few practical clues help separate a harmless cause from a worrisome one:
- Texture: Melena is sticky and tar-like. Stool darkened by iron pills or food keeps its normal form.
- Smell: Digested blood has an unusually pungent odor that’s distinct from typical stool.
- Timing: If the color change started right after you began iron supplements, ate blueberries, or took Pepto-Bismol, and you feel fine otherwise, a benign cause is likely.
- Other symptoms: Melena from active bleeding often comes with lightheadedness, fatigue, weakness, or feeling faint, especially when standing up. These suggest you’re losing enough blood to affect circulation.
If you stop the suspect food or supplement and the black color disappears within a couple of days, you have your answer. If it persists or you never took anything that would explain it, the stool itself needs to be evaluated.
Warning Signs That Need Immediate Attention
Black, tarry stool combined with any of the following suggests significant blood loss and should be treated as an emergency:
- Fainting or near-fainting when you stand up
- Rapid heartbeat you can feel at rest
- Pale, clammy skin or heavy sweating
- Vomiting blood or material that looks like coffee grounds
- Feeling unusually weak or confused
A drop in blood pressure when standing, paired with a climbing heart rate, often develops after losing more than two units of blood internally. Even without dramatic symptoms, some people with slower bleeds notice only mild fatigue or a faster-than-usual pulse. Persistent tarry stool over multiple bowel movements, even without other symptoms, still warrants medical evaluation because slow, steady bleeding can cause significant anemia over time.
What Happens During Evaluation
When a doctor suspects melena, the first step is usually a stool test that detects hidden blood. This helps confirm whether the dark color is actually from blood rather than food or medication. If the test is positive, or if the clinical picture strongly suggests bleeding, an upper endoscopy is the standard next step. This involves passing a thin, flexible camera through the mouth into the esophagus, stomach, and upper small intestine to find the bleeding source. In many cases, the bleeding can be treated during the same procedure.
Blood tests check for anemia and give a sense of how much blood has been lost. If bleeding is significant, you may need IV fluids or a blood transfusion to stabilize before the source is addressed.
Reducing Your Risk
If you take NSAIDs regularly, using the lowest effective dose for the shortest time reduces your risk of stomach bleeding substantially. Taking them with food doesn’t eliminate the risk, since the damage is partly systemic rather than just from direct contact with the stomach lining. People who need long-term NSAID therapy are sometimes prescribed an acid-reducing medication alongside to protect the stomach lining.
Alcohol, smoking, and the combination of multiple blood-thinning medications all increase the chance of upper GI bleeding. If you’ve had a bleeding ulcer before, recurrence is common enough that ongoing prevention matters. Treating H. pylori infection, when present, significantly lowers the odds of a repeat episode.