Black stool has two broad categories of causes: harmless ones (foods, supplements, certain medications) and serious ones (bleeding in the digestive tract). The most common harmless culprits are iron supplements, bismuth-based medications like Pepto-Bismol, and dark-colored foods. The most important serious cause is bleeding in the upper digestive tract, which produces a distinctive black, tarry stool with a strong odor.
Figuring out which category you fall into usually comes down to what you’ve eaten or taken recently, what the stool looks and smells like, and whether you have any other symptoms.
Foods and Supplements That Turn Stool Black
Several everyday foods can darken your stool enough to look alarming. Black licorice, blueberries, blood sausage, and dark leafy greens are the most common dietary causes. The deep pigments in these foods pass through your digestive system largely intact, tinting stool dark green to black. This is completely harmless and resolves on its own once you stop eating the food in question.
Iron supplements are one of the most frequent causes of black stool. Black stools are a normal, expected side effect of taking iron tablets regardless of the formulation. If you recently started an iron supplement and notice the change, that’s almost certainly the explanation. After you stop taking iron, stool color typically returns to normal within two to four days, though it can take up to a week if your digestion runs on the slower side.
Activated charcoal, sometimes taken for detox trends or after certain poisonings, also turns stool jet black. This is simply the charcoal passing through your system and isn’t a sign of any problem.
Bismuth Medications (Pepto-Bismol)
If you’ve recently taken Pepto-Bismol or a similar bismuth-based antacid, that’s very likely your answer. Bismuth reacts with trace amounts of sulfur naturally present in your saliva and digestive system, forming a compound called bismuth sulfide. Bismuth sulfide is black, and it can darken both your tongue and your stool. The effect is temporary and harmless, fading within a few days after your last dose.
When Black Stool Means Bleeding
Black stool caused by bleeding in the upper digestive tract, called melena, looks and feels different from the dietary kind. Melena is not just dark. It’s tarry, sticky, and has a notably foul smell that’s hard to miss. The black color comes from blood being partially digested: as blood travels from the stomach or upper intestine through the rest of the digestive tract, stomach acid breaks down the hemoglobin in red blood cells, turning it from red to dark brown or black. It takes roughly 100 to 200 milliliters of blood in the upper digestive tract to produce visibly black stool, and the darkened stool can persist for several days even after bleeding has stopped.
Common causes of upper digestive bleeding include stomach ulcers, severe gastritis (inflammation of the stomach lining), esophageal varices (swollen veins in the esophagus, often related to liver disease), and tears in the esophageal lining from forceful vomiting. Less commonly, tumors in the stomach or upper intestine can bleed slowly enough to cause melena without other obvious symptoms.
How to Tell the Difference
The simplest first step is to think back over the past 48 hours. Did you take iron, Pepto-Bismol, activated charcoal, or eat a lot of blueberries or black licorice? If yes, and you feel otherwise fine, the stool color change is almost certainly from that source. Stop the supplement or food and watch for your stool to return to normal over the next few days.
Melena from bleeding looks different on close inspection. It tends to be loose or semi-formed, visibly tarry (like roofing tar), and the smell is distinctly worse than usual. Dietary black stool is typically firm and normally formed, just darker.
Other symptoms that suggest bleeding rather than a dietary cause include lightheadedness or dizziness, unexplained fatigue or weakness, vomiting material that looks like coffee grounds, abdominal pain, and a rapid heartbeat. These indicate blood loss and should be taken seriously. If you’re also vomiting dark, granular material, that’s partially digested blood and a strong sign of active or recent upper digestive bleeding.
How Bleeding Is Investigated
If there’s any suspicion that black stool is from bleeding rather than diet, one of the first steps is a fecal occult blood test. This checks for hidden blood in a stool sample. There are different types of this test, and some require you to avoid certain foods and medications beforehand, including red meat, vitamin C supplements over 250 mg per day, and NSAIDs like ibuprofen, because these can interfere with the results.
A positive occult blood test doesn’t pinpoint the source of bleeding, so the usual follow-up is a colonoscopy or an upper endoscopy (a camera passed through the mouth into the stomach). These let a doctor directly see where the bleeding is coming from. Blood tests to check for anemia are also standard, since ongoing slow bleeding can gradually lower your red blood cell count even before you notice other symptoms.
What You Should Do Next
If you can connect your black stool to a specific food, supplement, or medication, try stopping it and see if your stool returns to its normal color within a few days. For iron supplements, expect two to four days. For bismuth, the timeline is similar.
If your stool is black and tarry with an unusually strong smell, you can’t link it to anything you’ve taken, or you have any accompanying symptoms like dizziness, weakness, stomach pain, or vomiting dark material, this warrants prompt medical attention. Upper digestive bleeding can range from mild to life-threatening, and earlier evaluation leads to better outcomes. Even if the bleeding has stopped on its own, the black tarry appearance can persist for days afterward, so it still warrants investigation to identify and treat the underlying cause.