Black poop has two broad explanations: something you ate or swallowed, or bleeding somewhere in your upper digestive tract. The harmless causes are far more common, but telling them apart matters because upper GI bleeding needs prompt medical attention. The color, texture, and smell of the stool, along with any other symptoms you’re experiencing, can help you figure out which category you’re in.
What Makes Stool Turn Black
When blood enters the stomach or upper small intestine, it has a long journey through the rest of the digestive tract before it exits. During that trip, stomach acid and digestive enzymes break down the hemoglobin in red blood cells, turning it from red to jet black. The result is a distinctive stool that doctors call melena: intensely black, sticky or tarry in texture, and noticeably foul-smelling. It looks and feels different from a normal bowel movement that just happens to be dark.
The dividing line between “upper” and “lower” GI bleeding sits at a small ligament near the end of the duodenum, the first section of your small intestine. Bleeding above that point, from the esophagus, stomach, or duodenum, produces black tarry stool. Bleeding below it, in the lower small intestine or colon, typically shows up as red or maroon blood instead.
Common Harmless Causes
Before assuming the worst, consider what you’ve been eating, drinking, or taking over the last day or two. Several everyday substances can turn stool convincingly black.
- Iron supplements. These are one of the most frequent culprits. In studies of pregnant women taking different iron formulations, black stools appeared in anywhere from 8% to 31% of users, depending on the type and dose. The higher the dose, the more likely the color change. Dark green to black is the typical range.
- Pepto-Bismol and similar antacids. The active ingredient, bismuth, reacts with trace amounts of sulfur in your saliva and digestive system to form bismuth sulfide, a harmless black compound. It can turn both your stool and your tongue black, which catches people off guard but resolves once you stop taking it.
- Blueberries. Eating a large amount can tint stool so dark it looks almost black, especially in combination with other deeply pigmented foods.
- Black licorice and dark-colored candy. The dyes in these foods can mix in the gut and produce a black or very dark stool.
Food and supplement-related black stool is typically well-formed, not sticky or tarry, and doesn’t carry the unusually strong odor associated with digested blood. If you recently started iron pills or downed a bottle of Pepto-Bismol, that’s very likely your answer. The color change stops within a couple of days once you stop consuming the substance.
Medical Causes Worth Knowing
When food and medications don’t explain the color, the concern shifts to bleeding in the upper digestive tract. Peptic ulcers, which are open sores in the lining of the stomach or duodenum, are the most common source. These can develop from long-term use of anti-inflammatory painkillers like ibuprofen or naproxen, or from a bacterial infection called H. pylori.
Other possible sources of upper GI bleeding include gastritis (inflammation of the stomach lining), tears in the esophagus from forceful vomiting, and enlarged veins in the esophagus or stomach that can develop in people with liver disease. Less commonly, tumors in the stomach or upper intestine can bleed slowly enough to produce black stool without obvious pain.
The volume of blood needed to produce melena is surprisingly small. Even a relatively minor bleed, if it originates high enough in the digestive tract, will darken the stool noticeably.
How to Tell the Difference at Home
The most reliable clues are texture, smell, and context. Melena from actual bleeding is tarry and sticky, often described as resembling roofing tar. It clings to the toilet bowl and is difficult to flush. The smell is distinctly worse than a normal bowel movement. Stool that’s black from food or supplements looks dark but keeps its usual consistency and doesn’t have that sticky, tar-like quality.
Think about timing. If the black color appeared within 24 to 48 hours of starting iron supplements, eating a pint of blueberries, or taking bismuth-based antacids, the connection is straightforward. If you can’t identify any dietary or supplement explanation, or if the stool has that unmistakable tarry texture, treat it as potentially serious.
Symptoms That Need Urgent Attention
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms mean you should head to urgent care or an emergency room rather than waiting for an appointment:
- Vomiting blood or vomit that looks like dark coffee grounds
- Dizziness, weakness, or lightheadedness, which can signal significant blood loss
- Heart palpitations or shortness of breath
- Several consecutive days of black, tarry stool
These signs suggest active bleeding that your body is struggling to compensate for. Even without those red flags, unexplained black stool that lasts more than a couple of bowel movements deserves medical evaluation. Doctors can test a stool sample for the presence of blood, which quickly confirms or rules out GI bleeding as the cause.
What to Do Right Now
Start by reviewing everything you’ve consumed in the last 48 hours: supplements, antacids, dark-colored foods, and any new medications. If you find an obvious match and you feel perfectly fine otherwise, the explanation is almost certainly benign. You can confirm by stopping the suspected cause and watching for the color to return to normal over the next one to three days.
If nothing in your diet explains the change, pay close attention to the stool’s texture and smell, and note any other symptoms. Even mild dizziness or unusual fatigue paired with black stool shifts the situation from “probably nothing” to “get checked today.” A simple stool test at your doctor’s office takes minutes and gives a clear answer about whether blood is present.