Black stool has two main causes: something you ate or swallowed, or bleeding somewhere in your upper digestive tract. The harmless version is far more common, but telling the difference matters because upper GI bleeding can be serious. The key distinction is whether the black color comes with a tarry, sticky texture and a notably foul smell, which points toward digested blood.
Foods and Medications That Turn Stool Black
Several everyday items can darken your stool enough to cause alarm. Black licorice, blueberries, blood sausage, and dark-colored foods are the most common dietary culprits. On the medication side, iron supplements are a frequent cause, along with bismuth subsalicylate (the active ingredient in Pepto-Bismol) and activated charcoal. These substances change the color of stool through chemical reactions in your gut, not because of bleeding.
The stool in these cases is typically dark but firm, not sticky or tar-like. It also won’t smell dramatically different from normal. If you recently started an iron supplement or took an antacid containing bismuth, that’s very likely your answer. The color change usually resolves within a few days of stopping the food or medication. If there’s any doubt, a simple chemical test at your doctor’s office can confirm whether blood is present in the stool.
When Black Stool Means Bleeding
Black, tarry stool that looks like roofing tar and has an unusually strong, foul odor is called melena. It happens when blood from the upper digestive tract (your esophagus, stomach, or the first part of your small intestine) is broken down by stomach acid and digestive enzymes as it passes through. This chemical breakdown turns the blood dark black rather than the bright red you’d see from bleeding lower in the intestines. It takes roughly 100 to 200 mL of blood in the upper GI tract to produce melena, roughly half a cup to a full cup.
The most common cause is a peptic ulcer, an open sore in the lining of the stomach or upper small intestine. Other sources include inflammation of the stomach lining (gastritis), tears in the esophagus, and enlarged veins in the esophagus that can develop in people with liver disease. Cancers of the stomach or esophagus are a less common but important possibility.
Melena can persist for several days after bleeding has actually stopped, because blood already in the digestive tract continues to be processed. So even if you see black tarry stool only once or twice, it may reflect a bleed that occurred days earlier.
Why Pain Relievers Raise Your Risk
Regular use of common over-the-counter pain relievers like ibuprofen, naproxen, and aspirin significantly increases the risk of upper GI bleeding. People who take these anti-inflammatory drugs regularly are roughly 4 to 5 times more likely to develop bleeding in the stomach or upper intestine compared to people who don’t use them. Higher doses carry even greater risk, with high-dose users facing about 7 to 9 times the normal risk.
The single strongest predictor of a GI bleed is having had one before. People with a history of upper GI bleeding are about 13 times more likely to experience another episode. If you take these pain relievers regularly and notice black stool, that combination deserves prompt attention. Importantly, doctors have found that a positive blood-in-stool test should never be dismissed just because someone takes aspirin or a blood thinner. The medication may increase the chance of bleeding, but the bleeding itself still needs to be investigated.
Warning Signs That Need Urgent Attention
Black tarry stool on its own warrants a call to your doctor, but certain accompanying symptoms signal a more serious or active bleed. These include:
- Lightheadedness or dizziness, especially when standing up
- Fainting or near-fainting
- Vomiting blood or material that looks like coffee grounds
- Abdominal pain
- Rapid heartbeat or feeling your heart race
- Pale skin, unusual fatigue, or weakness
A rapid heartbeat and lightheadedness together can indicate significant blood loss. If you’re experiencing black tarry stool along with any of these symptoms, seek emergency care rather than waiting for a scheduled appointment.
How Doctors Figure Out the Cause
The first step is usually a stool test that checks for the chemical signature of blood. This is a quick office test that distinguishes harmless color changes from actual bleeding. If blood is detected, the standard next step is an upper endoscopy, a procedure where a thin flexible camera is passed through your mouth to examine your esophagus, stomach, and upper small intestine directly. A colonoscopy may also be recommended to rule out lower sources of bleeding.
Between these two procedures, doctors find the source of bleeding in about 48 to 71 percent of cases. If those initial scopes come back normal but symptoms continue, a capsule endoscopy (swallowing a tiny camera in pill form that photographs the entire small intestine) finds the source in 63 to 74 percent of remaining cases. For people whose first round of testing is negative, a second look can still reveal missed causes about 35 percent of the time.
How to Tell the Difference at Home
Before you worry, think through what you’ve eaten and taken in the last 48 to 72 hours. Iron supplements, Pepto-Bismol, and large servings of blueberries or black licorice are the most common harmless explanations. The stool will be dark but generally maintains a normal consistency.
Melena from bleeding looks and feels different. It’s notably sticky or tar-like, coats the toilet bowl, and has a sharp, distinctive odor that’s hard to miss. If your stool matches that description, or if the black color persists for more than a couple of days after you’ve stopped any suspect foods or supplements, that’s worth getting checked. The stool test to confirm or rule out blood is simple, fast, and takes the guesswork out of it.