Dark or black stool is usually caused by something you ate, drank, or took as a supplement. Iron tablets, bismuth-based stomach medicines, and deeply pigmented foods like blueberries or black licorice are the most common culprits. Less often, very dark stool signals bleeding somewhere in the upper digestive tract, which needs prompt attention. The key distinction is whether you can trace the color change to something harmless or whether it arrived unexpectedly alongside other symptoms.
Foods That Turn Stool Dark
Deeply colored foods can tint your stool so intensely that it looks nearly black. Blueberries are one of the most common offenders. Eat enough of them and the dark pigments pass through your digestive system largely intact, producing stool that ranges from deep blue-green to almost black. Black licorice does the same thing, along with dark chocolate, blackberries, and foods made with dark food dyes. If you eat handfuls of brightly colored candy, the mixed pigments can combine in your gut and come out looking black.
Beets deserve a special mention because they contain a red pigment called betanin that can make stool look blood-red, which people sometimes mistake for a sign of bleeding. Carrots in large quantities can turn stool orange, though this tends to happen more in people who drink carrot juice regularly rather than just eating a few with dinner. Leafy greens like spinach and kale, when eaten in large amounts, produce a very dark green stool that can also be mistaken for black.
If food is the cause, your stool should return to its normal brown color within a few days of cutting that food from your diet. That short timeline is one of the easiest ways to rule out a dietary cause on your own.
Iron Supplements and Medications
Iron supplements are one of the most reliable producers of dark stool. Black stools are considered a normal and expected side effect of iron tablets. The unabsorbed iron oxidizes as it moves through your intestines, turning your stool dark green to black. This is harmless, and it actually signals that the supplement is making its way through your system. The color change typically begins within the first day or two of starting iron and continues for as long as you take it.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar stomach remedies, also turns stool black through a specific chemical reaction. Bismuth combines with small amounts of sulfur naturally present in your saliva and digestive tract, forming a compound called bismuth sulfide. That substance is jet black, and as it passes through your system, it darkens both your stool and sometimes your tongue. The effect is temporary and clears once the medication leaves your body, usually within a couple of days.
Other medications that can darken stool include activated charcoal and certain combinations of antacids. If you recently started any new medication or supplement and noticed the change, that connection is worth noting.
When Dark Stool Means Bleeding
The type of dark stool that signals a medical problem has a specific name: melena. It looks and feels different from stool that’s been darkened by food or supplements. Melena is black, sticky, and tarry in texture, and it has a distinctly strong, foul odor that’s noticeably different from normal stool. It results from blood being digested as it travels through the intestines, and it takes a relatively small amount of blood (roughly half a cup to a cup) to produce it.
The bleeding that causes melena typically originates in the upper digestive tract: the esophagus, stomach, or the first section of the small intestine. The most common causes include peptic ulcers (open sores in the stomach lining or upper intestine, often related to a bacterial infection or prolonged use of anti-inflammatory painkillers like ibuprofen or aspirin) and swollen, ruptured veins in the esophagus or stomach, which are associated with liver disease.
Melena rarely shows up in isolation. You might also notice lightheadedness or dizziness, unusual fatigue, abdominal pain, nausea, or a general sense that something is off. If your dark stool is tarry and sticky, especially if it appeared without an obvious dietary explanation and comes with any of these symptoms, that combination warrants urgent medical evaluation. Blood loss from the upper GI tract can escalate quickly.
How to Tell the Difference
The practical question is whether you’re looking at digested blood or digested blueberries, and a few simple observations help sort that out. Start with the obvious: did you recently eat dark-colored foods, start an iron supplement, or take a bismuth-based stomach medicine? If yes, that’s very likely your answer. Stop the suspected cause and watch whether the color returns to brown within a few days.
Texture is the strongest clue when diet doesn’t explain it. Normal stool that’s been darkened by food or supplements still has a typical consistency. Melena is distinctly tarry, almost like the consistency of roofing tar, and it tends to stick to the toilet bowl. The smell is also markedly worse than usual.
If you’re unsure, your doctor can run a simple stool test called a fecal occult blood test, which detects hidden blood invisible to the naked eye. The fecal immunochemical version of this test is the more sensitive option. One important caveat: certain foods, vitamin C, iron supplements, and pain relievers like aspirin and ibuprofen can all interfere with results, so your doctor may ask you to avoid these for a few days before testing to prevent a false positive.
Common Causes at a Glance
- Blueberries, blackberries, black licorice, dark chocolate: Deeply pigmented foods that darken stool harmlessly. Resolves in 1 to 3 days after stopping.
- Iron supplements: Expected side effect. Stool stays dark for as long as you take them.
- Bismuth subsalicylate (Pepto-Bismol): Chemical reaction with sulfur in the gut creates a black compound. Clears within a couple of days.
- Beets: Can produce alarming red or dark red stool due to the pigment betanin. Not blood.
- Upper GI bleeding: Produces tarry, sticky, foul-smelling black stool (melena). Requires medical attention.
If your stool has been dark for more than a few days and you can’t connect it to anything you’ve eaten or taken, or if the texture is unusually tarry and accompanied by pain, dizziness, or fatigue, that’s a situation that benefits from a medical evaluation rather than waiting it out.