Dark stool is normal in most cases, especially if you’ve recently eaten certain foods, taken iron supplements, or used an over-the-counter stomach medication like Pepto-Bismol. These are the most common reasons stool turns dark or black, and the color change is harmless. However, stool that is black, sticky, and tar-like can signal bleeding in the upper digestive tract, which needs prompt medical attention. The key is knowing what else is going on in your body and what you’ve consumed in the last day or two.
Foods That Darken Your Stool
Several everyday foods can turn your stool noticeably darker. Blueberries, blackberries, beets, dark leafy greens, and black licorice are common culprits. The pigments in these foods pass through your digestive system largely intact, tinting your stool dark brown, dark green, or even black. This is completely harmless and typically resolves within a day or two after you stop eating the food in question.
Medications and Supplements
Iron supplements are one of the most frequent causes of black stool. Unabsorbed iron oxidizes as it moves through the gut, turning stool dark green to black. The effect depends on the dose and the type of iron you’re taking. In studies comparing different iron formulas, the percentage of people who developed black stools ranged from about 8% on lower-dose forms to 31% on standard ferrous sulfate. If you’ve just started an iron supplement and notice the change, that’s almost certainly the explanation.
Pepto-Bismol (bismuth subsalicylate) is another well-known cause. The active ingredient, bismuth, reacts with trace amounts of sulfur in your saliva and digestive tract to form a black compound called bismuth sulfide. This can darken both your tongue and your stool. The effect is temporary and clears as the medication works its way out of your system, usually within a few days of your last dose.
What Tarry Black Stool Looks Like
The kind of dark stool that signals a medical problem has a distinct appearance. It’s called melena, and it looks and feels different from stool that’s simply dark in color. Melena is jet black, has a tar-like consistency, and is notably sticky. It also has a strong, foul odor that most people find immediately noticeable and different from normal stool.
This happens when blood from the upper digestive tract (the esophagus, stomach, or first part of the small intestine) is partially digested as it travels through the gut. The digestive process breaks down the red blood cells and turns the blood black. By the time it reaches the toilet, the result is unmistakable: dark, tarry, and adhesive in a way that diet-related dark stool simply isn’t.
Medical Causes of Black, Tarry Stool
When melena is present, it typically points to bleeding somewhere in the upper gastrointestinal tract. The three most common sources are:
- Peptic ulcers: open sores on the lining of the stomach or the first section of the small intestine. These are most often caused by a bacterial infection (H. pylori) or regular use of anti-inflammatory painkillers like ibuprofen, aspirin, or naproxen.
- Gastritis: inflammation or damage to the stomach lining, which can develop shallow breaks that bleed. The same culprits apply: H. pylori, painkillers, blood thinners, and heavy alcohol use.
- Esophageal or stomach varices: swollen, fragile veins that can rupture and bleed, most often linked to liver cirrhosis.
Slower bleeding from these conditions produces the classic tarry stool, while a larger, more acute bleed may cause additional symptoms like vomiting blood (sometimes resembling coffee grounds), dizziness, lightheadedness, or abdominal pain.
How Doctors Tell the Difference
If there’s any uncertainty about whether dark stool contains blood, a simple stool test can clarify. The older version, called a guaiac-based test, detects blood through a chemical reaction, but it can be thrown off by certain foods. Raw broccoli, turnips, cauliflower, horseradish, and red meat can all trigger false positives, which is why patients were traditionally asked to avoid these foods for three days before testing.
Newer immunochemical tests (FIT) are more accurate and more convenient. They use antibodies that specifically detect human blood proteins, so they aren’t affected by what you’ve eaten. No dietary restrictions are needed beforehand, and they’re better at catching actual bleeding while avoiding false alarms. Most clinicians now prefer these newer tests for exactly those reasons.
Dark Stool in Newborns and Infants
Newborns pass a thick, black, tar-like stool called meconium in the first few days of life. This is entirely normal. Meconium is made up of materials the baby swallowed in the womb: amniotic fluid, mucus, bile, and shed cells. It should appear within the first 48 hours after birth. If it doesn’t, that’s worth mentioning to a pediatrician.
After those first few days, black stool in a baby is no longer expected. If a baby older than three days passes black, tarry stool, it may indicate blood in the gastrointestinal tract and should be evaluated promptly. The same applies to white, clay-colored, or bloody stools at any age in infancy.
Symptoms That Need Immediate Attention
Dark stool on its own, without other symptoms and with a clear dietary or medication explanation, is rarely cause for concern. But certain combinations of symptoms suggest active bleeding and warrant urgent evaluation:
- Vomiting blood or material that looks like dark coffee grounds
- Dizziness or lightheadedness, which can indicate significant blood loss
- Abdominal pain, especially if sudden or severe
- Fever, bloating, or diarrhea alongside the color change
If you can trace your dark stool to blueberries, iron pills, or Pepto-Bismol, and you feel fine otherwise, the color should return to normal within a couple of days. If it persists beyond that, or you develop any of the symptoms above, that’s when it becomes important to get checked.