Black stool usually indicates one of two things: something you ate or swallowed, or bleeding somewhere in your upper digestive tract. The harmless causes are far more common, but telling the difference matters because upper GI bleeding requires prompt medical attention. The key distinction comes down to appearance, texture, and what else is going on in your body.
Harmless Causes: Foods, Supplements, and Medications
The most frequent reason for black stool is something you recently consumed. Black licorice, blueberries, blood sausage, and dark-colored foods can all temporarily darken your stool. These changes are cosmetic and pass within a day or two once the food moves through your system.
Iron supplements are one of the most common culprits. If you’ve recently started taking iron tablets or switched to an iron-fortified formula for a baby, expect noticeably darker stool. After you stop taking iron, stool color typically returns to normal brown within two to four days, though it can take up to a week if your digestion runs on the slower side.
Bismuth, the active ingredient in Pepto-Bismol, also turns stool black through a simple chemical reaction. When bismuth contacts the small amounts of sulfur naturally present in your saliva and digestive system, they combine to form bismuth sulfide, a black substance. This is completely harmless and clears up once you stop taking the medication.
Activated charcoal, sometimes used for digestive complaints or detox products, will do the same thing.
When Black Stool Signals Bleeding
Black stool caused by bleeding in the upper digestive tract, called melena, looks and feels different from food-related color changes. It is jet black with a tarry, sticky consistency, almost like roofing tar. It also has a distinctly strong, foul odor that’s noticeably worse than normal stool. That smell comes from blood being broken down by digestive enzymes and bacteria as it travels through the intestines.
The bleeding source is typically somewhere in the esophagus, stomach, or the first part of the small intestine. Blood turns black because it’s partially digested during the journey through the GI tract. It takes roughly 100 to 200 milliliters of blood (about half a cup to just under a cup) in the upper digestive tract to produce melena, and the black color can persist for several days even after the bleeding has stopped.
Common causes of upper GI bleeding include stomach ulcers, duodenal ulcers, inflammation of the esophagus, and abnormal blood vessels in the stomach lining. Less commonly, it can indicate stomach cancer or large polyps. Bleeding from the right side of the colon or the small bowel can also occasionally produce melena rather than the bright red blood you might expect from a lower GI source.
How to Tell the Difference
Start with the simplest question: have you recently eaten blueberries, black licorice, or blood sausage? Are you taking iron supplements, Pepto-Bismol, or activated charcoal? If the answer is yes and you feel fine otherwise, the color change is almost certainly from what you consumed.
Melena from bleeding has distinct characteristics that set it apart. The stool is sticky and tar-like rather than formed in a normal shape. The odor is unusually pungent. And crucially, bleeding significant enough to turn stool black often comes with other symptoms: lightheadedness, fatigue, weakness, pale skin, or a racing heartbeat. If you’re experiencing any of these alongside black stool, that combination points toward blood loss rather than blueberries.
If you’ve stopped a suspect food or supplement and your stool is still black after five to seven days, that’s another reason to get it checked out.
What Doctors Do to Find the Source
When black stool raises concern for bleeding, the first step is often a fecal occult blood test, which detects hidden blood in a stool sample. If that test is positive, the standard next step is direct visualization of the digestive tract with a scope.
For suspected upper GI bleeding, doctors use an upper endoscopy, a thin flexible camera passed through the mouth to examine the esophagus, stomach, and upper small intestine. A colonoscopy may also be performed to check the colon. Together, these two procedures find the bleeding source in roughly 48 to 71 percent of patients on the first attempt. When initial scoping comes back negative but symptoms continue, repeat procedures can catch previously missed lesions about 35 percent of the time.
During these procedures, doctors can often treat the problem on the spot, using the scope to cauterize a bleeding vessel, remove a polyp, or take biopsies of suspicious tissue.
Black Stool in Babies
Newborns pass a thick, black, tar-like substance called meconium during the first few days of life. This is entirely normal. It’s made up of everything the baby swallowed in the womb: amniotic fluid, mucus, bile, and shed cells. Most babies pass meconium within 48 hours of birth, and stool color then transitions to green and eventually to the yellow or brown range.
After the first three days of life, black tarry stool in a baby is no longer expected and could indicate blood entering the gastrointestinal tract. The most common benign explanation in older infants is iron-fortified formula or an iron supplement, which darkens stool the same way it does in adults. But if your baby hasn’t started iron and you’re seeing black stool beyond the newborn period, that warrants a call to your pediatrician.
Medications That Need Extra Attention
If you take aspirin, blood thinners, or anti-inflammatory pain relievers regularly, black stool deserves extra scrutiny. These medications can irritate the stomach lining or promote bleeding from existing ulcers. A positive occult blood test in someone on these medications should not simply be attributed to the medication itself without further investigation, because the drugs may be unmasking a real bleeding source that needs treatment.