Black stools usually mean one of two things: something you ate or swallowed changed the color, or there’s bleeding somewhere in your upper digestive tract. The difference matters a lot, and fortunately, it’s not hard to tell the two apart once you know what to look for.
Harmless Causes: Food, Supplements, and Medications
The most common reason for black stools has nothing to do with bleeding. Several everyday foods, supplements, and over-the-counter medications can turn your stool dark brown to black.
Blueberries are a frequent culprit, especially in large quantities. The deep pigment can make stool look almost black. Black licorice does the same. Even eating a lot of brightly colored candy can mix pigments together in your gut and produce a surprisingly dark result.
Iron supplements are another well-known cause. Iron reacts with digestive enzymes and bacteria in your gut to form dark compounds that color the stool black or very dark green. This is completely normal and expected when you’re taking iron.
Pepto-Bismol and similar bismuth-based stomach medications can also turn your stool (and even your tongue) black. This happens because bismuth reacts with tiny amounts of sulfur naturally present in your saliva and digestive system, forming a harmless black substance called bismuth sulfide.
In all of these cases, the color change is temporary and stops once you stop consuming whatever caused it.
How to Tell If It’s Blood
When black stool is caused by blood, the medical term is melena. It looks and smells distinctly different from stool that’s simply been stained by food or medication.
True melena is jet black with a tarry, sticky consistency, almost like roofing tar. It also has a uniquely strong, foul odor that’s noticeably worse than normal stool. That smell comes from blood being broken down and digested as it passes through your intestines. If your stool is black but has a normal texture and doesn’t carry that distinctive smell, blood is much less likely to be the cause.
It takes at least 50 milliliters of blood in the stomach (roughly 3 tablespoons) to turn stool black. The blood appears dark rather than red because it has traveled through the upper digestive tract, where stomach acid and intestinal bacteria break it down over several hours before it reaches the colon.
What Causes Bleeding in the Upper Digestive Tract
When melena is present, the bleeding source is almost always above the first bend of the small intestine, in the stomach or the tube connecting the mouth to the stomach. The most common causes are peptic ulcers, which are open sores in the lining of the stomach or the first section of the small intestine. These ulcers can erode into blood vessels and bleed slowly or heavily.
Gastritis, an inflammation of the stomach lining, can also produce enough bleeding to darken stool. This is sometimes triggered by long-term use of anti-inflammatory painkillers or by a bacterial infection in the stomach.
A more serious cause involves swollen veins in the lower part of the esophagus, which develop in people with advanced liver disease. When the liver is scarred (cirrhosis), blood pressure builds up in the veins that drain into it, forcing blood into smaller veins that aren’t designed to handle the load. These fragile, overstretched veins can rupture and bleed heavily. This type of bleeding is the third most common cause of upper digestive tract bleeding after stomach and intestinal ulcers, and it can be life-threatening. For some people with liver disease, a black, tarry stool is the first sign that these swollen veins exist.
Symptoms That Signal an Emergency
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms mean you’re losing blood fast enough to affect your circulation. Feeling lightheaded or dizzy when standing, a racing heartbeat, unusual fatigue or weakness, pale skin, or feeling faint all suggest significant blood loss. Vomiting blood, which can look bright red or like dark coffee grounds, alongside black stool points to active bleeding in the upper digestive tract. These combinations need emergency care, not a scheduled appointment.
How Doctors Confirm Blood in Stool
If you bring up black stools with your doctor and the cause isn’t obvious from your diet or medications, a stool test can check for hidden blood. These tests, called fecal occult blood tests, detect blood that isn’t visible to the naked eye. The newer versions (fecal immunochemical tests) are more accurate than older methods and specifically detect human blood, so they won’t be thrown off by what you ate. A positive result typically leads to further evaluation, such as an endoscopy, where a small camera is passed through the mouth to inspect the stomach and upper intestine directly.
Black Stool in Newborns
If you’re a new parent, a blackish-green, thick, sticky stool in the first day or two is completely normal. This is meconium, your baby’s first poop, made up of materials swallowed in the womb. Babies typically pass meconium within 24 to 48 hours after birth. Once they start feeding on breast milk or formula, stool color transitions to lighter greens and yellows. If a baby’s stool stays black beyond the first couple of days, or turns black again after it has already changed color, that’s worth flagging with a pediatrician.
A Simple Way to Think About It
Start by reviewing what you’ve eaten, drunk, or taken in the last day or two. Iron supplements, Pepto-Bismol, blueberries, and black licorice are the usual suspects. If any of those apply and your stool has a normal texture and smell, the color change is almost certainly harmless.
If you can’t explain it with diet or medications, pay attention to the texture and smell. Tarry, sticky, and unusually foul-smelling stool that persists beyond a single bowel movement points toward blood in the digestive tract and needs medical evaluation. The sooner bleeding is identified, the more straightforward it is to treat the underlying cause.