Super dark or black stool is usually caused by something you ate, drank, or took as a supplement or medication. In most cases, it’s completely harmless and clears up within a day or two. Less commonly, very dark stool signals bleeding somewhere in the digestive tract, which needs medical attention. The key is knowing which situation you’re in.
Foods That Turn Stool Dark or Black
Several everyday foods contain pigments intense enough to change your stool color dramatically. Blueberries are one of the most common culprits. The deep pigments in their skin can tint stool so dark it looks almost black, especially if you eat a large amount. Dark leafy greens like spinach and kale can push stool toward a very dark green that’s easy to mistake for black.
Black licorice, beets, and foods with heavy artificial coloring can do the same. If you eat a handful of brightly colored candy, the dyes can mix together in your gut and produce black stool. Dark chocolate, black beans, and even grape juice round out the list. The effect is temporary and typically resolves once the food passes through your system, usually within one to three days.
Iron Supplements and Medications
If you recently started taking iron supplements, that’s almost certainly the answer. Iron turns stool dark green to black in nearly everyone who takes it. This is normal and expected. The color change happens because your body doesn’t absorb all the iron you swallow, and the excess oxidizes as it moves through the intestines.
The stool from iron supplements is dark but should look normal in texture. If it starts looking tarry, sticky, or has red streaks, that’s a different situation worth mentioning to your doctor.
Pepto-Bismol is the other big one. The active ingredient, bismuth, reacts with trace amounts of sulfur naturally present in your saliva and digestive system. Together they form a black compound called bismuth sulfide, which can turn your stool jet black and even darken your tongue. This is harmless and fades once you stop taking the medication.
When Dark Stool Means Bleeding
This is the possibility that matters most. When bleeding occurs high in the digestive tract (the stomach or the first part of the small intestine), blood doesn’t stay red. As it travels through the gut, digestive enzymes break down the hemoglobin in blood cells, turning it progressively darker. By the time it reaches the other end, it produces stool that’s black, sticky, and tarry. Doctors call this melena, and it takes roughly 100 to 200 milliliters of blood in the upper digestive tract to cause it.
The texture is the critical clue. Stool that’s dark from food or supplements looks dark but otherwise normal. Melena has a distinct tar-like consistency and a notably foul smell that’s different from typical stool odor. It also tends to be unusually sticky. If your dark stool matches that description and you haven’t taken iron or Pepto-Bismol, bleeding is a real possibility.
By contrast, bleeding from the lower digestive tract (the colon or rectum) usually shows up as bright red blood on or mixed into stool, because it hasn’t had time to be broken down by digestive enzymes.
Common Causes of Upper GI Bleeding
Peptic ulcers are the most frequent source. These are open sores in the lining of the stomach or the upper small intestine, often caused by a bacterial infection or long-term use of anti-inflammatory painkillers like ibuprofen or aspirin. When an ulcer erodes into a blood vessel, it bleeds into the digestive tract. The bleeding can be slow and chronic, producing dark stool over days, or sudden and severe.
Gastritis, which is inflammation of the stomach lining, can also cause enough bleeding to darken stool. Heavy alcohol use, certain medications, and chronic stress on the stomach lining are common triggers. Less frequently, dark stool can result from esophageal tears (often from forceful vomiting), abnormal blood vessels in the gut, or tumors.
How to Tell If It’s Serious
Start with the simplest explanation. Think back over the last 24 to 48 hours. Did you eat blueberries, black licorice, or beets? Start an iron supplement? Take Pepto-Bismol? If yes, that’s likely the cause. Stop the food or medication and see if the color returns to normal within a couple of days.
If none of those apply, pay attention to the stool’s texture and your body’s other signals. Tarry, sticky, unusually foul-smelling stool combined with any of the following warrants urgent medical evaluation:
- Dizziness, weakness, or lightheadedness, which can indicate significant blood loss
- Vomiting blood or material that looks like coffee grounds
- Heart palpitations or shortness of breath
- Abdominal pain, especially in the upper stomach area
- Dark stool persisting for several days without an obvious dietary explanation
These symptoms together suggest active bleeding that may need treatment. Even without those accompanying symptoms, unexplained black tarry stool that persists for more than a couple of days is worth getting checked. A simple stool test can confirm whether blood is present, and that distinction tells you and your doctor everything you need to know about next steps.