Stool color is primarily determined by the amount of bile, or digestive fluid, it contains. Bile starts out as a yellow-green but tends to turn brown as it’s broken down by the enzymes in your digestive tract.
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Diet, medications, and health conditions can turn stool tan, red, yellow, black, and green.
Here are a few potential reasons your stool may be black.
1. You Eat a Lot of Dark-Colored Foods
Dark foods can turn your poop nearly black, especially if you’ve eaten them in great quantities (much more than typical serving size). Common culprits include black licorice, blueberries, beets, and blood sausage.
The natural compounds that give these foods their dark color can stain your stool, Dr. Johnson explains. Blueberries, for example, contain pigmented antioxidants called anthocyanins, which can give poop a darker brown shade.
2. You’re Taking a Supplement, Like Iron
Black poop is a potential side effect of certain dietary supplements, particularly iron supplements, says Janese Laster, MD, a board-certified gastroenterologist, internist, obesity medicine specialist, and nutritionist in private practice in Washington, DC.
If iron is the culprit, your stool will likely be a greenish or grayish shade of black.
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Dr. Laster says this is a normal side effect caused by the way iron pills are broken down in the gut.
Supplements that contain activated charcoal — which are sometimes touted as a remedy for gas and bloating — can also darken your poop.
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Activated charcoal is a black powder, and just like dark-colored foods, its midnight hue can stain your poop black.
3. You’re Taking Anti-Diarrheals
Black stool is a common side effect of over-the-counter anti-diarrheal medicines, which are used to treat digestive issues like heartburn, indigestion, diarrhea, and nausea.
The offender is bismuth subsalicylate, the active ingredient in many anti-diarrheal medications.
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“Bismuth subsalicylate can react with naturally occurring substances in your gastrointestinal tract and result in black or dark poop,” Johnson explains. You might notice your tongue appears darker, too.
These are typical side effects that disappear after you stop taking the medication.
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4. You Have a Gastrointestinal Bleed
In more serious cases, black stool can signal bleeding in the digestive tract. Doctors refer to this kind of black poop as melena, Johnson says, and it can signal bleeding in the stomach, esophagus, or small intestine caused by ulcers (open sores in the lining of the gut), cancer, or a tear in the lining of the gastrointestinal (GI) tract.
Johnson explains that melena is black instead of red because the blood has passed through the colon, where bacteria break down its hemoglobin (the protein that lends blood its scarlet hue). For this reason, it also usually has a tar-like texture.
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Some people are more prone to this kind of internal bleeding, including those who take blood-thinning medications like warfarin or use nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, or naproxen sodium, Laster says. Blood thinners make it harder for your blood to clot, so a small tear or sore in the intestines that would normally heal just fine could lead to worrisome bleeding, Laster says. NSAIDs, on the other hand, can damage the inner lining of the GI tract, leading to ulcers in this lining that cause bleeding.
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People who had gastrointestinal surgery or have mechanical heart valves (often used to replace diseased or damaged heart valves) are also at greater risk of melena, as are those with the following conditions:
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Melena requires immediate medical attention.
