Black Stool: Causes and When to Worry
Black stool can be alarming, but it has two very different explanations — one usually harmless, one that needs prompt attention. Knowing which is which mostly comes down to what you've been taking and what the stool looks and feels like.
Harmless causes of black stool
- Iron supplements — a very common cause of dark or black stool.
- Bismuth subsalicylate — found in some anti-diarrhea and upset-stomach products; can blacken both stool and the tongue.
- Very dark foods — large amounts of black licorice, blueberries, or blood sausage can darken stool.
When black stool comes from these, it's usually a normal dark color rather than sticky and tarry, and it clears once you stop the supplement, medicine, or food.
When black stool is a warning sign
Black, sticky, tar-like stool with a strong odor — known medically as melena — can be a sign of bleeding in the upper digestive tract (such as the stomach or small intestine). Blood that's been partly digested on its way through the gut turns black. This is different from bright red blood, which usually comes from lower down. Melena that isn't explained by iron or bismuth needs prompt medical evaluation.
How to tell the difference
- Are you taking iron or bismuth? If yes, that's the likely cause.
- Texture: Tarry and sticky leans toward bleeding; ordinary dark and formed leans toward diet or supplements.
- Other symptoms: Dizziness, weakness, stomach pain, or vomiting (especially anything that looks like coffee grounds) alongside black tarry stool is a reason to seek urgent care.
When to see a doctor
Seek prompt medical care for black, tarry stool when you're not taking iron or bismuth, and seek urgent care if it comes with dizziness, weakness, abdominal pain, or vomiting blood. When in doubt, it's safer to get it checked.
Track it with PoopID
If you spot an unusual color, a record helps your doctor. PoopID logs stool color alongside your Bristol type from a photo, so changes aren't left to memory. For the full picture, see our stool color chart.
- Sources:
- NIDDK — Gastrointestinal bleeding. niddk.nih.gov
- NHS — Black or bloody stool. nhs.uk
Frequently asked questions
Does iron really turn stool black?
Yes. Iron supplements are one of the most common causes of dark or black stool, and it's harmless. It typically looks dark and formed rather than sticky and tarry.
What is melena?
Melena is black, sticky, tar-like stool with a strong odor, caused by partly digested blood from bleeding higher in the digestive tract. It needs prompt medical evaluation.
How do I know if black stool is dangerous?
If you're not taking iron or bismuth, and the stool is tarry and sticky — especially with dizziness, weakness, pain, or vomiting — treat it as urgent and get medical care.
Can food make my poop black?
Large amounts of very dark foods like black licorice, blueberries, or blood sausage can darken stool. It clears once the food passes.
This article is for general information only and is not medical advice. Consult a qualified healthcare professional about any symptom that concerns you.