Black stools range from harmless to serious depending on their cause, and the key differences come down to texture, consistency, and smell. A stool that’s black from food or medication typically looks dark but otherwise normal in shape and firmness. A stool that’s black from internal bleeding looks and feels distinctly different: it’s tarry, sticky, and has an unusually foul odor that’s hard to miss.
Black Stools From Bleeding vs. Harmless Causes
The medical term for black, tarry stool caused by bleeding is melena. It has a very specific appearance. The color is deep black, often compared to roofing tar or thick motor oil. The texture is sticky and shiny, and it tends to cling to the toilet bowl rather than breaking apart in water. It’s also loose or semi-formed rather than a solid, well-shaped stool.
The smell is the most distinctive feature. When blood travels through your digestive tract, stomach acid and intestinal bacteria break it down over several hours. That chemical process produces a sharp, intensely foul odor that’s noticeably different from the smell of a normal bowel movement. If you’ve had black stool and weren’t sure whether to worry, the smell is often what separates something serious from something harmless. Stool that’s been stained black by food or medication won’t have that distinctive odor.
By contrast, black stool from a harmless cause looks dark but keeps its normal shape, texture, and firmness. It may be very dark brown or true black, but it won’t be sticky, tarry, or unusually foul-smelling.
Common Harmless Causes
Several everyday foods and medications turn stool black without any cause for concern:
- Bismuth medications (like Pepto-Bismol) are one of the most common culprits. The active ingredient reacts with small amounts of sulfur naturally present in your saliva and digestive tract, forming a black compound called bismuth sulfide. This can turn both your stool and your tongue black. The color gradually fades once you stop taking it.
- Iron supplements frequently cause dark green to black stools. This is a normal side effect of unabsorbed iron passing through.
- Activated charcoal turns stool jet black, which makes sense given the color of charcoal itself.
- Certain foods like black licorice, blueberries, and blood sausage can darken stool noticeably. The effect typically lasts a day or two after you stop eating the food.
With all of these causes, your stool will be dark but otherwise normal. It won’t be tarry, sticky, or unusually smelly.
What Causes the Tarry, Sticky Kind
Melena happens when you’re bleeding somewhere in the upper part of your digestive system, typically the stomach or the first section of the small intestine. As blood sits in stomach acid, the acid oxidizes the red hemoglobin in your blood and converts it into a dark brown-black compound. The longer the blood spends traveling through your intestines, the darker and more tar-like the stool becomes.
This kind of bleeding can come from stomach ulcers, inflammation of the stomach lining, or other conditions affecting the upper digestive tract. It’s a sign that a meaningful amount of blood has entered the system. Upper gastrointestinal bleeding carries a 10% hospital mortality rate, which is why tarry black stools are treated as an urgent finding.
How Doctors Tell the Difference
If there’s any uncertainty about whether black stool contains blood, a simple stool test can settle the question. The most common version is the fecal immunochemical test, or FIT, which uses an antibody that attaches specifically to human hemoglobin in stool. It’s considered more accurate than the older guaiac-based test, which uses a chemical reaction to detect blood but can be thrown off by certain foods. Red meat, for example, contains enough trace blood to trigger a false positive on guaiac tests.
These tests detect blood that’s invisible to the naked eye, so they’re useful even when stool color seems ambiguous. Your doctor can run one from a small stool sample.
When Black Stool Needs Urgent Attention
If your stool is black, tarry, and sticky with an unusually strong smell, that combination points toward active or recent bleeding. This is especially urgent if you also feel lightheaded, dizzy, weak, or notice your heart racing. Those symptoms suggest enough blood loss to affect your circulation.
If you can connect the color to something you recently ate or a medication you’re taking, and the stool looks normal aside from being dark, it’s almost certainly harmless. The color should return to normal within a couple of days after you stop the food or medication. But if the tarry texture or foul smell is present, or if you’re unsure, getting it checked quickly matters. The visual and textural differences between harmless black stool and melena are distinct enough that once you know what to look for, the comparison is usually clear.