What usually turns poop black — and why it’s harmless
For most people who notice black stool, the explanation is sitting in the medicine cabinet, not the ER:
- Iron supplements — unabsorbed iron reacts with sulfur compounds in the gut, forming black iron sulfide. Stool stays normally formed, not sticky, without a strong odor [3].
- Pepto-Bismol (bismuth subsalicylate) — the same chemistry with bismuth instead of iron, often paired with a temporarily blackened tongue [4].
- Black licorice and activated charcoal — pigment passing through mostly unchanged.
In every one of these cases, the stool looks black but behaves normally: formed, not tarry, no distinctive smell.
What is melena, and why does it look different?
Melena is the black stool that actually signals a problem: it’s sticky, tar-like, and has a distinctly foul odor that benign black stool doesn’t have. It happens when blood from higher up in the GI tract — the stomach, esophagus, or upper small intestine — gets partially digested during the trip through your gut, which is what turns it black and tarry rather than red [1].
It doesn’t take much blood to cause it — roughly 100–200 mL, about half a can of soda’s worth — and melena can keep showing up for several days after the actual bleeding has already stopped [1]. So a black stool today confirms a real bleed happened; it doesn’t necessarily mean it’s still happening right now.
How do I actually tell the two apart?
Texture and smell, not just color, is the differentiator:
| Benign black stool (iron, Pepto, licorice) | Melena | |
|---|---|---|
| Texture | Normally formed | Sticky, tarry |
| Odor | Unremarkable | Distinctly foul |
| Other symptoms | None | Sometimes dizziness, weakness, abdominal pain |
| What caused it | A pill or food you can name | Bleeding higher in the GI tract |
If you’re on iron or Pepto-Bismol and the stool is otherwise normal in every other way, that’s almost certainly your answer. If it’s tarry, sticky, and smells distinctly bad — or you can’t point to iron/Pepto/licorice as the cause — treat it as melena until proven otherwise.
When to actually worry
Melena is never a “wait and see” finding — NHS guidance treats black or dark-red stool as needing same-day medical attention at minimum [5]. Escalate to emergency care right away if black, tarry stool comes with:
- Dizziness, lightheadedness, fainting, or a racing heartbeat — signs of significant blood loss [1].
- Vomiting blood, bright red or coffee-ground-colored.
- Severe abdominal pain.
Otherwise — tarry, sticky, foul black stool without those signs still needs a same-day medical evaluation, not a wait-it-out approach [1][2].
If your black stool is formed, not sticky, doesn’t smell unusual, and you can point to iron, Pepto-Bismol, or licorice as the reason — you’ve checked the thing that actually matters here. This isn’t something you need to keep searching about.
Still not sure which bucket you’re in? The PoopRx wizard walks through it in a couple minutes, free, no login. See also the full poop color chart and Blood in Stool: When to Worry for the red-blood version of this same question.