Black diarrhea usually comes from one of three things: something you ate or took, a medication like Pepto-Bismol or iron supplements, or bleeding in the upper digestive tract. The cause matters a lot, because some explanations are completely harmless while others need urgent medical attention. The key to telling them apart is what else is going on in your body and what the stool actually looks and smells like.
Foods and Supplements That Turn Stool Black
Several everyday foods can make your stool look surprisingly dark. Black licorice, blueberries, blood sausage, and foods with dark dyes are common culprits. Activated charcoal, sometimes taken for digestive issues or as a supplement, also turns stool jet black. In all of these cases, the color change is temporary and harmless. It typically resolves within a day or two after you stop eating the food in question.
If you’re taking iron supplements, expect your stool to turn dark or even greenish-black. Iron oxide reacts with your digestive juices and creates a dark pigment. Iron can also speed up how quickly food moves through your intestines, which sometimes gives stool a greenish tint because bile doesn’t have enough time to break down fully. Splitting your daily iron dose into two smaller doses can reduce these effects, and taking iron with vitamin C helps your body absorb more of it, meaning less unabsorbed iron passing through to color your stool.
Why Pepto-Bismol Causes Black Stool
Bismuth, the active ingredient in Pepto-Bismol, is one of the most common reasons people notice black stool, especially black diarrhea, since they often take the medication precisely because they already have loose stools. The bismuth reacts with small amounts of sulfur naturally present in your saliva and digestive system. Together they form bismuth sulfide, a black compound that slowly works its way through your gut. As it does, it can turn both your stool and your tongue black. This is completely normal and clears up after you stop taking the medication.
Bleeding in the Upper Digestive Tract
The most serious cause of black stool is bleeding somewhere in the upper part of the digestive system: the esophagus, stomach, or the first section of the small intestine. When blood is exposed to stomach acid, the red hemoglobin in it gets oxidized and converted into a brown-black substance called hematin. By the time this digested blood reaches your colon and exits your body, it produces a distinctive type of stool called melena.
Melena looks and feels different from stool that’s simply been stained by food. It’s tarry and sticky, almost like roofing tar, and it has a strong, unusually foul odor that’s hard to miss. Black stool caused by blueberries or iron supplements, by contrast, has a normal consistency and smell. That tarry texture and sharp smell are the clearest signals that blood may be involved.
Common causes of upper digestive bleeding include stomach ulcers, inflammation of the stomach lining, tears in the esophagus, and enlarged veins in the esophagus or stomach. Bleeding from the small intestine or the right side of the colon can also produce melena, though this is less common. The farther the bleeding source is from the rectum, the darker and more digested the blood appears by the time it reaches your stool.
How to Tell Harmless From Serious
Start by thinking about what you’ve eaten or taken in the last 24 to 48 hours. If you recently had blueberries, black licorice, iron pills, or Pepto-Bismol, and you otherwise feel fine, the color change is almost certainly benign. Stop the food or supplement and see if your stool returns to normal within a day or two.
The signs that something more serious is happening are hard to miss. Watch for stool that is not just dark but actively tarry, sticky, and unusually foul-smelling. Pay close attention to how you feel overall. Dizziness, weakness, lightheadedness, heart palpitations, and shortness of breath all suggest you may be losing a significant amount of blood. Vomiting blood, or vomit that looks like dark coffee grounds, is another strong indicator of upper digestive bleeding. Several days of black, tarry stool should also prompt urgent evaluation even if you feel relatively okay, because slow bleeding can add up.
What Doctors Do to Find the Source
When black stool raises suspicion of bleeding, doctors typically start with blood tests to check for anemia and to see how well your blood is clotting. A stool test can detect hidden blood that isn’t visible to the naked eye, which helps confirm or rule out bleeding as the cause.
The most common next step is an upper endoscopy, where a thin, flexible tube with a camera is passed down the throat to visually inspect the esophagus, stomach, and upper small intestine. This lets doctors see the bleeding source directly and often treat it at the same time. If the source isn’t found there, a colonoscopy examines the large intestine, or a capsule endoscopy (swallowing a tiny pill-sized camera) can capture images throughout the small intestine. Imaging tests like CT scans or angiography, where dye is injected into an artery to highlight blood vessels, are used when the bleeding is harder to locate.
Black Stool in Newborns
If you’re a new parent, seeing very dark stool in your baby’s diaper during the first couple of days is expected. Newborns pass meconium, a sticky, dark brown or greenish-black substance that accumulated in their intestines before birth. This is completely normal and transitions to lighter colors within a few days as feeding gets established. After that initial period, black stool in an infant is not typical. Stools should generally fall within the yellow, green, or brown range. A baby who passes black stool beyond those first few days should be seen by a pediatrician.