Dark stool usually comes from something you ate, a supplement you’re taking, or a medication. The brown color of normal stool comes from a pigment called stercobilin, which forms when bacteria in your gut break down bile. When that process is altered, or when certain substances pass through your digestive tract, the color can shift noticeably darker, from deep brown to fully black.
How Stool Gets Its Normal Color
Your liver produces bile, a greenish fluid that helps digest fats. As bile moves through the intestines, gut bacteria chemically reduce it into stercobilin, an orange-brown pigment. Stercobilin is the main reason healthy stool is brown. The shade varies day to day depending on how long food spends in your digestive tract, what you’ve eaten, and how much bile your liver releases. Stool that moves through quickly tends to be lighter or greenish, while slower transit generally produces a darker brown.
Foods That Turn Stool Dark or Black
Several common foods can make stool look much darker than usual, sometimes fully black. The most frequent culprits are blueberries, black licorice, and blood sausage. Dark leafy greens, blackberries, and foods with dark artificial dyes can also darken stool significantly. Beets and red food coloring, meanwhile, can produce a dark reddish stool that people sometimes mistake for blood.
Food-related color changes are harmless and temporary. They typically resolve within a day or two after you stop eating the food. The key feature of diet-related darkening is that the stool looks and smells normal otherwise. It won’t be sticky or tarry, and it won’t have an unusually foul odor.
Medications and Supplements
Iron supplements are one of the most common causes of very dark or black stool. Iron oxidizes as it passes through the digestive tract, and the result is a stool that can look almost ink-black. This is a well-known, harmless side effect.
Pepto-Bismol and similar medications containing bismuth can also turn stool black. The active ingredient, bismuth, reacts with trace amounts of sulfur in your saliva and digestive system to form bismuth sulfide, a black-colored compound. This reaction can also temporarily darken your tongue. The effect clears up once you stop taking the medication.
Activated charcoal, sometimes used for digestive issues or detox products, will also produce black stool for the simple reason that the charcoal itself is black and passes through mostly unabsorbed.
When Dark Stool Signals Bleeding
The concerning cause of dark stool is bleeding somewhere in the upper digestive tract: the esophagus, stomach, or upper small intestine. When blood travels through the full length of the gut, digestive enzymes and bacteria break it down, turning it black. The medical term for this is melena, and it looks distinctly different from stool that’s dark because of food or supplements.
Classic melena is jet black with a tarry, sticky consistency. It clings and smears in a way that normal dark stool does not. It also has a particularly strong, offensive odor that’s hard to miss. That smell is a byproduct of blood being digested as it moves through the intestines. The longer the blood has traveled, the darker and more pungent the stool becomes. A smaller amount of bleeding may look more dark brown than fully black, which can make it harder to distinguish from dietary causes.
Common causes of upper GI bleeding include stomach ulcers, inflammation of the stomach lining, esophageal tears (often from severe vomiting), and enlarged veins in the esophagus. Some of these conditions also cause diarrhea alongside the dark stool, which changes the typical tarry consistency and can be confusing.
How to Tell the Difference
If you notice unusually dark stool, the first thing to consider is what you’ve eaten or taken in the past 24 to 48 hours. Iron supplements, bismuth medications, blueberries, and black licorice are the usual explanations. If one of those applies, and the stool has a normal texture and smell, there’s little reason for concern.
The features that distinguish bleeding from harmless causes are texture, odor, and accompanying symptoms. Melena is sticky and tar-like. It has a distinctly foul smell you won’t notice with food-stained stool. And bleeding significant enough to darken your stool often comes with other warning signs: lightheadedness, fatigue, abdominal pain, or vomiting that looks like coffee grounds. If your dark stool is accompanied by any of these, that combination points toward active bleeding rather than last night’s blueberry smoothie.
If there’s uncertainty, a fecal occult blood test can detect hidden blood in a stool sample. A positive result typically leads to a colonoscopy or upper endoscopy to locate the source of bleeding.
Dark Stool in Newborns
If you’re a new parent noticing very dark stool in your baby’s diaper, it’s almost certainly meconium. This is the thick, dark green-to-black, tar-like substance that builds up in a baby’s intestines before birth. Babies pass meconium within the first 24 to 48 hours of life. Once they begin drinking breast milk or formula, their digestive system pushes out the remaining meconium, and stool gradually transitions to the lighter yellow or mustard color typical of newborn poop. This transition is completely normal and expected.
Stool Color and Transit Time
Beyond specific foods and medications, how long stool spends in your colon affects its shade. Slower transit gives gut bacteria more time to act on bile pigments, which tends to produce a darker brown. Constipation, for instance, often results in harder, darker stool simply because it sat in the colon longer. Faster transit, whether from illness, certain foods, or a naturally quick digestive system, can produce lighter or even greenish stool because bile hasn’t been fully broken down.
This means that a stretch of darker-than-usual stool can reflect nothing more than a change in fiber intake, hydration, or physical activity, all of which influence how quickly food moves through your gut. These day-to-day shifts in shade are normal and not a sign of a problem on their own.