Very dark or black stool is usually caused by something you ate or a supplement you’re taking, not by something dangerous. The most common culprits are iron supplements, bismuth-based stomach medicines, and deeply pigmented foods like blueberries or black licorice. That said, truly black stool that’s also tarry and foul-smelling can signal bleeding in the digestive tract, which needs prompt attention.
Foods That Turn Stool Dark
Several everyday foods can push stool color from its normal brown all the way to near-black. Blueberries are one of the most common offenders. Eat enough of them and the deep blue pigment can make stool look almost black. Black licorice does the same, as does blood sausage. Even heavily dyed candy, when multiple colors mix in your gut, can produce surprisingly dark results.
Beets deserve a special mention because they cause a different kind of alarm. A red pigment called betanin gives beets their intense color, and it can make stool appear reddish or blood-red. This catches people off guard but is completely harmless. Once you stop eating the food in question, stool color typically returns to normal within a few days as the pigment clears your system.
Medications and Supplements
Iron supplements are one of the most frequent non-food causes of very dark stool. Iron oxidizes as it moves through your digestive tract, turning stool dark green to black. This is expected and not a sign of a problem.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar stomach remedies, is another reliable stool-darkener. When bismuth meets the small amounts of sulfur naturally present in your saliva and digestive system, the two combine to form bismuth sulfide, a black substance. As it works its way through your gut, it can turn both your stool and your tongue black. The NHS notes this is harmless and clears on its own after you stop taking the medicine. Activated charcoal, sometimes taken for digestive issues or detox purposes, also turns stool jet black.
When Dark Stool Means Bleeding
The kind of dark stool that warrants concern has a name: melena. It looks and feels distinctly different from stool that’s simply dark brown or colored by food. Classic melena is jet black with a tarry, sticky consistency, almost like roofing tar. It also has a particularly strong, offensive odor that most people describe as unlike anything they’ve smelled before. That smell comes from blood being broken down and digested as it passes through your gut.
Melena originates from bleeding in the upper digestive tract, usually the stomach or the upper portion of the small intestine. It can also come from the lower esophagus if blood was swallowed. Less commonly, bleeding from the lower small intestine or upper large intestine can produce melena if digestion is moving slowly enough for the blood to be fully broken down. Peptic ulcers, inflammation of the stomach lining, and enlarged veins in the esophagus are among the more common sources of this type of bleeding.
Dark Brown vs. Black: How to Tell the Difference
Normal stool ranges from light to dark brown, and day-to-day variation is common. Eating more meat, for example, often produces darker brown stool. The key distinction isn’t just color but texture and smell. Stool that’s simply very dark brown, holds its usual shape, and doesn’t have an unusually foul odor is almost always benign. It’s stool that is black, sticky, loose, and strikingly foul-smelling that points toward digested blood.
A quick mental checklist can help you sort things out. Think back over the past day or two: did you eat blueberries, dark leafy greens, or black licorice? Are you taking iron pills, Pepto-Bismol, or activated charcoal? If the answer to any of these is yes, that’s the most likely explanation. If none of those apply and your stool is truly black and tarry, that’s when it makes sense to get evaluated.
How Doctors Investigate Dark Stool
When the cause isn’t obvious from your diet or medications, a doctor may start with a fecal occult blood test to check whether blood is present in your stool. The most common version is the fecal immunochemical test, or FIT. You collect a small sample from a single bowel movement, and the lab checks it for hidden blood. FIT is generally preferred because it’s more sensitive and doesn’t require any dietary restrictions beforehand. An older version, the guaiac test, requires samples from three separate bowel movements smeared onto special cards.
These tests can only detect whether blood is present or absent. They don’t pinpoint where the bleeding is coming from. If the result is positive, the next step is usually a colonoscopy or an upper endoscopy (a scope passed through the mouth into the stomach) to locate the source directly. If you already have visible blood in your stool, significant abdominal pain, or feel lightheaded and weak, your doctor will likely skip the screening test and move straight to imaging or endoscopy.
What to Watch For
Dark stool on its own, when you can trace it to a food or supplement, rarely needs any follow-up beyond waiting a few days for color to normalize. Stop the suspected food or medication and monitor. If stool returns to its normal brown within two to three days, you have your answer.
The combination of symptoms matters more than color alone. Black, tarry stool paired with dizziness, fatigue, abdominal pain, vomiting (especially vomit that looks like coffee grounds), or a racing heartbeat suggests active bleeding that’s affecting your blood volume. That combination calls for urgent evaluation, not a wait-and-see approach.