Black stool can range from a dark, greenish-black color to a deep jet black, depending on the cause. The key details to pay attention to aren’t just color but also texture, consistency, and smell, because these help distinguish a harmless color change from one that signals bleeding in the digestive tract.
What Melena Looks Like
Melena is the medical term for black stool caused by digested blood, and it has a very distinct appearance. Classic melena is jet black with a tarry, sticky consistency, similar to roofing tar or thick molasses. It tends to cling to the toilet bowl and can be difficult to flush. A smaller amount of bleeding may produce stool that looks more dark brown than truly black, which can make it harder to identify.
The smell is often the most telling feature. Melena has a particularly strong, offensive odor that’s noticeably different from the usual unpleasantness of a bowel movement. That smell comes from blood being broken down by stomach acid and digestive enzymes as it travels through the intestines. The longer blood spends in the digestive tract, the darker and more foul-smelling it becomes. If your stool is black from food or medication, it won’t have this distinctive odor.
Why Digested Blood Turns Black
When bleeding occurs high in the digestive tract, in the stomach or the first part of the small intestine, the blood doesn’t stay red. Stomach acid converts hemoglobin (the oxygen-carrying protein in red blood cells) into a brown-black compound. As the blood continues moving through the intestines, further oxidation turns it progressively darker. By the time it reaches the stool, it’s jet black.
It takes a meaningful amount of blood to produce visible melena. Roughly 100 to 200 milliliters, about half a cup to just under a cup, needs to be present in the upper digestive tract for the stool to turn noticeably black. Melena can also persist for several days after the bleeding has actually stopped, because residual blood continues working its way through the system.
Harmless Causes of Black Stool
Plenty of things turn stool black without any bleeding involved. The most common culprits are:
- Bismuth-based medications (like Pepto-Bismol): The active ingredient reacts with small amounts of sulfur naturally present in your saliva and digestive system, forming a black compound called bismuth sulfide. This darkens your stool as it passes through.
- Iron supplements: Unabsorbed iron oxidizes in the gut and commonly turns stool dark green to black.
- Activated charcoal: Produces a very dark or black stool simply because of its color.
- Certain foods: Black licorice, blueberries, and blood sausage can all darken stool noticeably.
Stool that’s black from these sources typically looks different from melena. It’s usually firmer, with a more normal consistency. It won’t be sticky or tar-like, and it won’t have the strong, distinctive smell that digested blood produces.
How to Tell the Difference
Three features separate concerning black stool from harmless black stool: texture, stickiness, and odor. If the stool is formed and relatively solid, and you’ve recently taken iron, bismuth, or eaten dark-colored foods, the color change is almost certainly benign. If it’s loose, sticky, coats the bowl like tar, and smells unusually foul, that pattern points toward digested blood.
Think about timing, too. If you started a new iron supplement two days ago or took Pepto-Bismol yesterday, the connection is straightforward. But if black, tarry stool appears without an obvious dietary or medication explanation, that’s worth taking seriously. The same applies if you notice other symptoms alongside it, like feeling lightheaded, unusually fatigued, or short of breath, all of which can indicate blood loss.
How Blood in Stool Is Confirmed
When there’s uncertainty about whether stool contains blood, a simple stool test can settle the question. The most common version is called a fecal immunochemical test (FIT), which specifically detects human blood. FIT picks up colorectal bleeding with about 87% sensitivity and comparable specificity, making it a reliable screening tool. An older version, the guaiac-based test, is less sensitive (around 50 to 70%) but still useful in certain contexts.
These tests matter because visual appearance alone isn’t always definitive. Dark brown stool from minor bleeding can look a lot like iron-stained stool. A quick lab test removes the guesswork. If blood is confirmed, the next step is usually an endoscopy, a camera exam of the upper digestive tract, to find and address the source of bleeding.
What Causes Bleeding That Leads to Melena
Melena specifically points to bleeding in the upper digestive tract, meaning the stomach or the upper portion of the small intestine. The most common causes include stomach ulcers, erosion of the stomach lining (often from prolonged use of anti-inflammatory painkillers), and enlarged veins in the esophagus. Less commonly, tumors or tears in the lining of the esophagus can be responsible.
Lower in the digestive tract, bleeding tends to produce red or maroon-colored stool rather than black, because the blood doesn’t spend enough time exposed to stomach acid and digestive enzymes to undergo that chemical transformation. The dividing line is a structure in the upper small intestine. Bleeding above it generally produces melena; bleeding below it generally produces brighter red blood.