Dark stool is poop that appears black, very dark brown, or tarry. In most cases, it’s caused by something you ate or a supplement you’re taking. But when dark stool is sticky, tar-like, and has a strong foul smell, it can signal bleeding somewhere in your upper digestive tract, a condition doctors call melena.
The key distinction is between stool that’s simply darker than usual and stool that looks and feels like tar. Understanding the difference helps you figure out whether your dark stool is harmless or something that needs medical attention.
Common Harmless Causes
Several everyday foods, supplements, and over-the-counter medications can turn your stool noticeably dark or even jet black. These changes are cosmetic, meaning there’s nothing wrong with your digestive tract. The color shift usually starts within a day or two of consuming the substance and clears up once you stop.
The most common culprits include:
- Iron supplements, one of the most frequent causes of black stool
- Bismuth subsalicylate (the active ingredient in Pepto-Bismol)
- Activated charcoal
- Black licorice, blueberries, blood sausage, and other very dark-colored foods
If you recently started an iron supplement and notice black stool, that’s almost certainly the explanation. The stool may look alarming, but it won’t have the sticky, tar-like consistency or the distinctly foul odor associated with digested blood. It also shouldn’t come with any other symptoms like lightheadedness or abdominal pain.
When Dark Stool Means Bleeding
True melena, the medical term for black tarry stool caused by bleeding, has a distinctive appearance. It’s not just dark. It’s sticky, shiny, and has a thick consistency that people often compare to roofing tar or motor oil. It also carries a particularly strong, unpleasant smell that’s different from normal stool odor.
This happens because blood from your upper digestive tract (your esophagus, stomach, or the first part of your small intestine) gets partially digested as it travels through your system. The iron in your blood oxidizes during that journey, turning the blood black by the time it exits your body. The higher up the bleeding originates, the darker and more tar-like the stool tends to be.
Stool color actually provides a rough map of where bleeding is happening. Black, tarry stool typically points to bleeding in the stomach. Dark red or maroon stool suggests bleeding higher in the colon or small intestine. Bright red blood on or in your stool usually means the source is lower, in the colon, rectum, or anus.
Conditions That Cause Tarry Stool
The most common cause of true melena is a peptic ulcer, an open sore in the lining of your stomach or the upper part of your small intestine. These ulcers can bleed slowly over time, producing dark stool that you might not immediately notice, or they can bleed more heavily and cause obvious symptoms.
Gastritis, which is inflammation of the stomach lining, can also cause slow bleeding that darkens your stool. This is sometimes triggered by long-term use of anti-inflammatory painkillers like ibuprofen or aspirin, heavy alcohol use, or bacterial infections.
A more serious cause is esophageal varices, which are swollen veins in the esophagus. These develop most often in people with significant liver disease, particularly cirrhosis. When scar tissue in the liver blocks normal blood flow, pressure builds in surrounding veins, causing them to enlarge and sometimes rupture. Bleeding from esophageal varices is a medical emergency.
Less common causes include tumors or growths in the stomach or upper intestine, tears in the esophageal lining (sometimes from severe vomiting), and blood that’s been swallowed from a nosebleed or dental procedure.
Symptoms That Signal an Emergency
Dark stool on its own, without other symptoms, is often benign. But certain accompanying signs suggest active internal bleeding that needs immediate attention:
- Dizziness or lightheadedness, especially when standing up
- Rapid heartbeat or feeling faint
- Vomiting blood, which may look red or resemble dark coffee grounds
- Severe abdominal pain
- Pale skin, unusual fatigue, or confusion
These symptoms suggest enough blood loss to affect your circulation. If you have black, tarry stool along with any of these, that warrants emergency care. Bleeding from esophageal varices in particular can become life-threatening quickly.
How Doctors Test for Blood in Stool
If your stool is dark but the cause isn’t obvious, your doctor can test for hidden blood using a fecal occult blood test. The older version of this test, called a guaiac-based test, detects blood through a chemical reaction. It picks up colorectal cancer with a sensitivity ranging from 51% to 100%, depending on how often testing is repeated.
One limitation of the guaiac test is that it reacts to any blood, not just human blood. Red meat, certain raw vegetables, and some medications can trigger false positives. A newer version called the fecal immunochemical test is more specific to human blood and less prone to interference from diet. If either test comes back positive, the next step is typically a colonoscopy or upper endoscopy to find the source.
Dark Stool in Newborns
If you’re a new parent noticing very dark stool in your baby’s diaper, there’s a good chance it’s completely normal. A newborn’s first bowel movements consist of meconium, a thick, sticky, blackish-green substance that resembles tar. Meconium is made up of materials your baby ingested in the womb, and it’s expected within the first 24 to 48 hours after birth.
Once your baby starts feeding on breast milk or formula, their digestive system pushes out the remaining meconium. Over the next few days, stool transitions from black-green to a greenish-brown, and eventually to the yellow or mustard color typical of breastfed infants (or tan-brown for formula-fed babies). If dark, tarry stools persist beyond the first week, or return after stool color has already lightened, that’s worth bringing up with your pediatrician.
What to Do When You Notice Dark Stool
Start by thinking about what you’ve eaten or taken in the past day or two. If you’re on iron supplements, taking Pepto-Bismol, or recently ate a large amount of dark-colored food, that’s the most likely explanation. Try stopping the suspected cause for two to three days and see if your stool returns to its normal color.
If the dark color persists without an obvious dietary explanation, pay attention to the texture and smell. Normal stool that happens to be dark will still have a typical formed consistency. Melena is distinctly sticky and tar-like, and the odor is noticeably worse than usual. If your stool fits that description, or if you’re experiencing any of the emergency symptoms listed above, get medical evaluation promptly rather than waiting to see if it resolves on its own.