Black poop

Black poop is often caused by something you took or ate: iron supplements, bismuth medicines like Pepto-Bismol, activated charcoal, black licorice, blueberries or blood sausage. Black stool that is also tarry, sticky and unusually foul-smelling is different. That is called melena, it can mean bleeding in the upper digestive tract, and it needs same-day medical care, even if you take iron.

What a scan can and cannot do here: Stoolio's Color Assessment includes a very dark shade and tracks it over time, alongside your Bristol type and 12 visual readings. A photo cannot tell iron from bleeding, though, so black and tarry stool is a call for a clinician, not an app.

Why can poop be black?

Either something dark passed through you, or blood did. Blood from the esophagus, stomach or the first part of the small intestine is digested on its way down, which turns it from red to black and gives stool a tarry, sticky texture and a strong smell. Iron, bismuth and dark foods also darken stool, but without the tarry texture or the distinctive odor.

Common explanations

• Iron supplements. The NHS says darker-than-usual poo is common on iron and usually harmless, and Cleveland Clinic notes it can look dark green or even blackish.
• Bismuth subsalicylate (Pepto-Bismol, Kaopectate). Bismuth reacts with sulfur compounds in the gut to form bismuth sulfide, which colors stool black but is harmless. It can darken the tongue too, and MedlinePlus notes the darkening usually goes away a few days after stopping.
• Activated charcoal, black licorice, blueberries, blood sausage and large amounts of other dark foods (MedlinePlus, Cleveland Clinic).
• Bleeding in the upper digestive tract. Peptic ulcers are the most common cause of sudden upper GI bleeding; gastritis, inflammation of the esophagus and other causes are also possible (MedlinePlus, NIDDK).

Foods that can turn poop black

Black licorice, blueberries, blood sausage, and large amounts of dark foods such as dark leafy greens, grape juice or dark chocolate cookies. A food-driven darkening should fade within a day or two once the food has passed.

Dark from iron, or tarry from bleeding?

Cleveland Clinic describes classic melena as jet black with a tarry, sticky consistency and a particularly strong, offensive odor, which stained-black stool from iron or bismuth does not have. There are caveats: melena can be wetter when bleeding also causes diarrhea, and a small amount of bleeding can look more dark brown than black. Only a stool test can confirm whether blood is present.

That is why the NHS, even while calling darker poo on iron common, still advises urgent help if poo is black and tar-like. Being on iron does not rule out bleeding.

When to check in with a clinician

• Black, tarry or sticky stool: get medical care the same day. NIDDK advises seeking help right away for symptoms of GI bleeding.
• Get emergency care if black stool comes with vomiting blood or material that looks like coffee grounds, dizziness or fainting, weakness, a racing heart or shortness of breath.
• Black stool you cannot explain with iron, bismuth or food: contact a healthcare provider promptly (MedlinePlus).

In newborns, the first few stools (meconium) are normally greenish-black and sticky. After those first days, black stool in a baby needs a call to the pediatrician.

What Stoolio looks at visually

Very dark is one of the six shades in Stoolio's Color Assessment, alongside pale, light brown, brown, dark brown and green. For someone taking iron, tracking that shade shows what their normal looks like, so a real change stands out. But a scan cannot see the difference between iron-dark and bleeding, and it cannot test for blood. If your stool is black and tarry, skip the scan and call a clinician. See the full poop color chart.

Sources

1. MedlinePlus. Black or tarry stools.
2. Cleveland Clinic. Melena (Black Stool).
3. NHS. Side effects of ferrous sulfate.
4. Cleveland Clinic Health Essentials. Can Food Change the Color of Your Poop? 2025.
5. MedlinePlus. Bismuth Subsalicylate.
6. Thazhath SS, et al. Oral bismuth for chronic intractable diarrheal conditions? Clin Exp Gastroenterol. 2013.
7. NIDDK. Symptoms & Causes of GI Bleeding.
8. DiGregorio AM, Alvey H. Gastrointestinal Bleeding. StatPearls.
9. Mayo Clinic. Stool color. 2024.
10. NHS. Bleeding from the bottom (rectal bleeding).
11. AAP HealthyChildren. The Many Colors of Baby Poop.