Dark or black stools usually come from something you ate, drank, or took as a supplement. Iron pills, bismuth-based medications like Pepto-Bismol, and foods like blueberries or black licorice are the most common culprits. But dark stools can also signal bleeding in the upper digestive tract, which is a serious medical issue worth understanding so you can tell the difference.
Foods That Turn Stool Dark
Several everyday foods can change your stool to a noticeably darker shade, sometimes dark enough to look black. Blueberries are one of the most common offenders. Eat enough of them and the deep pigment can tint your stool so dark it looks almost black. Black licorice does the same, as can blood sausage and other dark-colored processed meats.
Beets deserve a special mention because they contain a red pigment called betanin that can make stool appear dark red or blood-red, which understandably alarms people. Heavily dyed candies and foods can also contribute. If you’ve recently eaten a lot of any deeply colored food, that’s likely your explanation. The color change is harmless and clears up within a day or two once you stop eating the food in question.
Medications and Supplements
Iron supplements are one of the most frequent non-food causes of black stool. Your body doesn’t absorb all the iron you swallow, and the unabsorbed iron oxidizes as it moves through your gut, turning stool dark green to black. This is completely expected and not a reason to stop taking your supplement.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar products, also turns stool black. The mechanism is straightforward: bismuth reacts with sulfur compounds naturally produced by bacteria in your gut, forming a dark substance called bismuth sulfide. This same reaction can temporarily darken your tongue, which catches people off guard but is harmless. Activated charcoal, sometimes taken for gas or bloating, will also turn stool jet black.
If you’ve recently started any of these and notice darker stool with no other symptoms, the medication or supplement is almost certainly the cause.
When Dark Stool Means Bleeding
The serious cause of black stool is bleeding somewhere in the upper digestive tract: the esophagus, stomach, or the first section of the small intestine. When blood is exposed to stomach acid, the hemoglobin in red blood cells breaks down and oxidizes. By the time it passes through the rest of the intestines, it turns stool a distinctive tarry black. Doctors call this melena.
Melena looks and feels different from stool that’s dark because of food or medication. It tends to be loose or liquid, has a shiny, tar-like appearance, and carries a notably foul smell that’s hard to miss. If your dark stool is solid, formed, and doesn’t have these characteristics, bleeding is much less likely.
Peptic ulcers are the single most common cause of upper digestive bleeding, responsible for roughly a third of cases. Esophagitis (inflammation of the esophagus, often from acid reflux) accounts for about 24% of cases, and gastritis (inflammation of the stomach lining) another 18 to 22%. Esophageal varices, which are swollen veins in the esophagus typically linked to liver disease, cause about 11%. Less common causes include Mallory-Weiss tears, which are small rips in the lining of the esophagus from forceful vomiting or retching.
How to Tell the Difference
The first thing to do when you notice dark stool is think back over the past 24 to 48 hours. Did you take iron, Pepto-Bismol, or activated charcoal? Did you eat a large serving of blueberries, black licorice, or beets? If the answer is yes and you feel otherwise fine, a harmless cause is overwhelmingly likely.
Bleeding-related dark stool rarely shows up in isolation. It typically comes with other warning signs: lightheadedness or dizziness, unusual fatigue, abdominal pain, nausea, or vomiting that may contain what looks like coffee grounds (which is also partially digested blood). A rapid heart rate or feeling faint when standing up can indicate enough blood loss to affect circulation. Any of these alongside black, tarry stool warrants urgent medical evaluation.
Doctors can test stool for hidden blood using a fecal occult blood test, though these have limitations. The older version of the test (guaiac-based) can produce false positives if you’ve recently eaten red meat or certain raw vegetables, because it reacts to similar compounds in those foods. Newer immunochemical versions are more specific to human blood but are less reliable for detecting bleeding from the upper digestive tract, since the blood proteins partially break down during digestion before reaching the stool.
Dark Stool in Newborns
If you’re a new parent noticing very dark stool in your baby’s first diapers, that’s almost certainly meconium. Meconium is a newborn’s first stool, made up of materials your baby swallowed in the womb, including amniotic fluid. It’s dark, thick, sticky, and tar-like, and it’s completely normal. Most newborns pass meconium within 24 to 48 hours of birth, and stool gradually transitions to lighter colors over the following days as feeding begins.
If meconium is abnormally thick and your baby hasn’t passed it within that expected window, the pediatric team will evaluate for potential blockage. In some cases, unusually thick meconium is an early sign of cystic fibrosis, a genetic condition that affects the consistency of intestinal secretions. But for most newborns, dark first stools are simply part of the transition to life outside the womb.
What to Watch For Over Time
A single episode of dark stool with an obvious dietary or medication explanation is nothing to worry about. Recurrent dark stools without a clear cause deserve attention, especially if you notice them becoming tarry in texture or accompanied by fatigue, unexplained weight loss, or abdominal discomfort. Chronic low-level bleeding in the upper digestive tract can lead to iron-deficiency anemia over time, sometimes before you notice any visible change in stool color at all.
People who regularly use nonsteroidal anti-inflammatory drugs like ibuprofen or aspirin have a higher risk of peptic ulcers and stomach lining irritation, both of which can cause upper GI bleeding. Heavy alcohol use increases the risk of gastritis and liver-related causes like esophageal varices. If you fall into either category and notice unexplained dark stools, that context matters and is worth mentioning to your doctor.