Why is my stool black?
Black stool can look scary, but the cause is often completely benign. The key is distinguishing between harmless dark stool from diet or supplements and melena — black, tarry stool that signals bleeding in the upper digestive tract.
Harmless causes of black stool
These are the most common reasons and are nothing to worry about:
- Iron supplements — The most frequent culprit. Iron oxidizes in the gut and turns stool very dark green or black.
- Bismuth subsalicylate — Active ingredient in Pepto-Bismol. Reacts with sulfur in your gut to create black stool (and sometimes a black tongue).
- Dark foods — Black licorice, blueberries, dark chocolate, beets, and black beans can all darken stool.
- Activated charcoal — Used in some supplements and detox products.
If your black stool started after taking any of these, it’s almost certainly the cause. Stop the supplement or food for 2–3 days and see if the color returns to normal.
What is melena?
Melena is black, tarry, sticky stool with a distinctly foul smell. It indicates digested blood from bleeding in the upper gastrointestinal (GI) tract — typically the esophagus, stomach, or duodenum.
Blood turns black as it’s digested by stomach acid and enzymes during its journey through the GI tract. This is different from bright red blood (hematochezia), which usually comes from the lower GI tract.
Common causes of melena
- Peptic ulcers — Sores in the stomach or duodenal lining, often caused by H. pylori bacteria or NSAIDs (ibuprofen, aspirin).
- Gastritis — Inflammation of the stomach lining.
- Esophageal varices — Enlarged veins in the esophagus, often related to liver disease.
- Mallory-Weiss tears — Small tears at the junction of the esophagus and stomach, sometimes from severe vomiting.
- NSAIDs and blood thinners — Long-term use of aspirin, ibuprofen, or anticoagulants increases bleeding risk.
How to tell the difference
| Feature | Diet/Supplement | Melena (GI Bleeding) |
|---|---|---|
| Color | Dark green-black | Jet black, tarry |
| Texture | Normal consistency | Sticky, tar-like |
| Smell | Normal | Distinctly foul |
| Recent iron/Pepto | Yes | No |
| Other symptoms | None | Fatigue, dizziness, pain |
When to see a doctor immediately
Seek medical attention if your black stool is:
- Tarry and sticky in texture
- Accompanied by dizziness, lightheadedness, or fainting
- Paired with abdominal pain or vomiting (especially vomiting blood)
- Persistent after stopping iron supplements or dark foods
- Occurring with fatigue or unexplained weakness
Upper GI bleeding can be serious. Don’t wait — call your doctor or go to the emergency room.
Track your stool color
Logging stool color daily helps you spot changes early and provide useful context when talking to your doctor. With Flushy, you can track color, Bristol type, and dietary tags — all privately on your phone.
This article is for informational purposes only and does not constitute medical advice. Consult a healthcare professional for personalized guidance.