Black stool has two broad categories of causes: harmless ones like foods, supplements, and certain medications, and serious ones involving bleeding in the upper digestive tract. The key distinction isn’t just the color but the texture and smell. Understanding the difference can help you figure out whether what you’re seeing is a normal side effect or something that needs medical attention.
Harmless Causes: Foods, Supplements, and Medications
The most common reason for black stool is something you swallowed. Iron supplements are a frequent culprit, and the effect is dose-dependent. In one study of pregnant women taking different iron formulations, 31% of those on 50 mg of ferrous sulfate developed black stools, compared to 22% on 40 mg of ferrous fumarate and just 8% on 25 mg of ferrous bisglycinate. If you recently started an iron supplement and your stool turned dark green or black, that’s almost certainly the explanation.
Pepto-Bismol and similar bismuth-based stomach medications can turn stool jet black. The active ingredient, bismuth, reacts with trace amounts of sulfur in your saliva and digestive system to form bismuth sulfide, a harmless black compound. This can also temporarily darken your tongue.
Certain foods do it too. Blueberries in large quantities can produce stool dark enough to look almost black. Black licorice is another common trigger. Even eating a mix of brightly colored candies can combine to darken stool significantly. In all of these cases, the color change resolves on its own within a few days of stopping the food, supplement, or medication.
How to Tell Harmless Black Stool From Dangerous Black Stool
When black stool is caused by bleeding in the upper digestive tract (the esophagus, stomach, or upper small intestine), it has a specific medical name: melena. Melena looks and feels distinctly different from stool that’s simply been stained dark by food or iron pills.
Classic melena is jet black with a tarry, sticky consistency. It also has a particularly strong, offensive odor, which is a byproduct of blood being broken down and digested as it moves through your intestines. Stool that’s black from food or supplements won’t have that distinctive smell, and its texture will be closer to normal. If your stool is dark but formed normally and doesn’t smell unusually foul, a harmless cause is far more likely.
Bleeding Causes: What’s Happening Inside
When black, tarry stool does indicate bleeding, the source is almost always in the upper part of the digestive tract. Blood that originates lower down, in the colon or rectum, typically appears red or maroon rather than black because it hasn’t had time to be broken down by stomach acid and digestive enzymes.
Peptic ulcer disease is the single most common cause, accounting for roughly 32% to 36% of upper gastrointestinal bleeding cases. These ulcers develop when the stomach or duodenal lining breaks down from a combination of acid, digestive enzymes, infection (particularly H. pylori), medications like NSAIDs, or smoking. Inflammation of the esophagus (esophagitis) accounts for about 24% of cases, gastritis for 18% to 22%, duodenitis for 13%, and variceal bleeding for about 11%.
Variceal bleeding deserves special mention because it can be severe. Varices are swollen, fragile blood vessels that form when blood pressure in the portal vein (which carries blood from the intestines to the liver) gets too high. This most commonly happens in people with liver cirrhosis. When these vessels rupture, they can bleed heavily.
What Doctors Look For
If you report black stool to your doctor, one of the first steps is determining whether blood is actually present. A fecal occult blood test can detect hidden blood in stool that isn’t visible to the naked eye. If that test is positive, the standard next step is a colonoscopy, which allows direct visualization of the intestinal lining and the ability to biopsy or treat any lesions found on the spot.
One important point: a positive blood test should never be dismissed just because you’re taking aspirin or blood thinners. In one study, 15 of 16 patients on anticoagulants who tested positive for blood in their stool had significant underlying lesions, and 20% of those turned out to be malignant. Blood thinners may make bleeding easier, but they don’t cause lesions. The source still needs to be identified.
Black Stool in Newborns
If you’re a new parent, seeing very dark stool in the first day or two of life is completely normal. Meconium, a newborn’s first stool, is naturally dark green to black and sticky. It’s made up of materials the baby ingested in the womb and passes within the first 48 hours or so.
Black or tarry stool that appears after the meconium stage, or stool with visible blood, is a different situation. In newborns, this can sometimes result from swallowed maternal blood during delivery, which is benign. But it can also signal more serious conditions. If a newborn passes bloody or unusually dark stool beyond the first couple of days, that warrants prompt evaluation. A simple lab test called the Apt test can determine whether the blood in a baby’s stool is maternal (swallowed during birth) or the baby’s own.
Warning Signs That Need Immediate Attention
Black, tarry stool on its own is worth reporting to a doctor. But certain accompanying symptoms signal a more urgent situation: vomiting blood or material that looks like coffee grounds, feeling dizzy or lightheaded, or noticing a rapid heartbeat. These suggest significant blood loss and a drop in blood pressure. Severe bleeding cases can require hospitalization or surgery to stop the source.
If you can trace the color change to iron pills, Pepto-Bismol, or a large serving of blueberries, and your stool has a normal texture without an unusual odor, you can reasonably expect the color to return to normal within a few days of stopping the trigger. But if you’re unsure of the cause, or if the stool is sticky, tarry, and foul-smelling, treating it as a potential bleeding issue is the safer approach.