Really dark stool is usually caused by something you ate, drank, or took as a supplement or medication. In most cases, it’s harmless and clears up within a day or two. But truly black, tarry, sticky stool can be a sign of bleeding in your upper digestive tract, which needs prompt medical attention. The key is knowing how to tell the difference.
Harmless vs. Concerning: How to Tell
Normal stool ranges from light to medium brown. Eating certain foods or taking certain supplements can push that color toward very dark brown or even black, and that’s not dangerous. What you’re looking for is a specific combination: stool that’s not just dark but jet black, sticky or tarry in texture, and has a noticeably foul smell that’s different from your usual bowel movements. That type of stool has a medical name (melena) and indicates that digested blood is passing through your system.
If your stool is dark but still has a normal consistency and doesn’t look like tar, the cause is almost certainly dietary or related to a supplement.
Foods and Supplements That Darken Stool
Several everyday items can turn your stool surprisingly dark:
- Blueberries: Eating a large amount can produce stool so dark it looks nearly black.
- Black licorice: The dark pigments pass straight through your digestive system.
- Blood sausage and other dark-colored foods: These contain pigments or iron that darken stool.
- Rainbow-colored candy: In large quantities, the mixed dyes can combine and turn stool black.
- Iron supplements: Oral iron tablets commonly cause dark green to black stool. This is one of the most frequent causes people notice.
- Activated charcoal: Produces very dark or black stool as the charcoal passes through.
If any of these are the cause, your stool color should return to normal within a couple of days after you stop eating the food or taking the supplement. There’s no need to stop taking iron supplements just because they change your stool color, but it’s worth knowing the connection so you don’t worry unnecessarily.
Medications That Turn Stool Black
Pepto-Bismol and similar stomach remedies containing bismuth are notorious for turning stool jet black. The active ingredient reacts with tiny amounts of sulfur naturally present in your saliva and digestive system, forming a black compound called bismuth sulfide. This can make your stool look alarming, but it’s completely harmless and resolves once you stop taking the medication. It can also temporarily darken your tongue for the same reason.
When Dark Stool Signals Bleeding
Black, tarry stool that you can’t trace back to food, supplements, or medication is a different situation. It typically means blood is entering your digestive tract somewhere between your esophagus and the upper part of your small intestine. By the time that blood travels through your system and exits, digestive enzymes have broken it down, giving it that characteristic dark, sticky, tar-like appearance.
The most common causes of this kind of upper digestive bleeding include:
- Peptic ulcers: Open sores in the lining of your stomach or the first section of your small intestine. These are the most frequent cause.
- Severe inflammation: Inflammation of the stomach lining (gastritis) or the esophagus can erode tissue enough to cause bleeding.
- Swollen veins in the esophagus or stomach: These are more common in people with liver disease and can rupture and bleed.
- Tears in the esophagus: Sometimes caused by violent or prolonged vomiting.
- Erosion of the stomach lining: Certain medications, particularly anti-inflammatory painkillers used long-term, can wear away the protective lining.
- Cancer: Less commonly, tumors in the stomach, esophagus, or pancreas can cause bleeding.
Pay attention to accompanying symptoms. Abdominal pain, dizziness, lightheadedness, weakness, or vomiting (especially vomit that looks like coffee grounds) alongside black stool are signs that bleeding may be significant. This combination warrants urgent medical evaluation.
How Doctors Check for Hidden Blood
If your doctor suspects bleeding but isn’t sure, they may start with a stool test that detects tiny amounts of blood invisible to the naked eye. This test can confirm whether blood is present, but it can’t pinpoint where the bleeding is coming from. A positive result could indicate anything from a stomach ulcer to a polyp to something as minor as swallowed blood from a nosebleed.
If the test comes back positive, the next step is usually a colonoscopy or an upper endoscopy, where a small camera examines the inside of your digestive tract to locate the source. These procedures let doctors see the problem directly and, in many cases, treat it during the same visit.
A Simple Way to Sort It Out
Before you worry, think about what you’ve eaten or taken in the past 24 to 48 hours. If you recently had blueberries, took Pepto-Bismol, started iron supplements, or ate black licorice, that’s almost certainly your answer. Stop the food or supplement and see if your stool returns to its normal color within a day or two.
If you can’t connect the color change to anything you consumed, or if the stool is truly tar-like and sticky, or if you’re experiencing pain, dizziness, or fatigue alongside the color change, that’s a situation that calls for medical evaluation sooner rather than later. Upper digestive bleeding can range from slow and subtle to sudden and serious, and catching it early makes treatment straightforward in most cases.