A black bowel movement can mean anything from a harmless side effect of something you ate or a supplement you’re taking, to a sign of bleeding in your upper digestive tract. The key distinction is whether the stool is also tarry, sticky, and has a noticeably foul smell. That combination points toward digested blood and needs prompt medical attention, while a dark stool that looks otherwise normal is more likely caused by food, iron supplements, or certain medications.
Why Blood Turns Stool Black
When bleeding happens in the upper part of the digestive system, such as the stomach or the first section of the small intestine, the blood doesn’t stay red. Stomach acid converts the hemoglobin in blood into a brownish-black compound, and as the blood continues through the intestines, further oxidation darkens it. By the time it reaches the end of its journey, the blood has transformed into a black, tar-like substance that mixes with the stool.
This type of stool has a medical name: melena. It’s distinctly sticky, almost like roofing tar, and carries a sharp, unusually foul odor that most people notice immediately. The texture and smell are what separate melena from a stool that’s simply dark in color. Even bleeding from the lower small intestine or the right side of the colon can produce melena if it moves slowly enough through the gut for oxidation to occur.
Common Harmless Causes
Several everyday foods, supplements, and medications can turn your stool black without any bleeding involved. These are the most frequent culprits:
- Iron supplements. Dark or black stools are a well-known side effect of oral iron tablets. The color change is expected and not dangerous on its own. However, if you notice red streaks in the stool while taking iron, that’s worth reporting to your doctor.
- Bismuth-based medications. Products like Pepto-Bismol contain bismuth, which reacts with small amounts of sulfur naturally present in your saliva and digestive system. The reaction creates bismuth sulfide, a black substance that can darken both your tongue and your stool. It clears up once you stop taking the medication.
- Certain foods. Black licorice, blueberries, blood sausage, and activated charcoal can all produce noticeably dark stools. The effect is temporary and resolves once the food passes through your system.
If you can trace your black stool to one of these causes and the stool has a normal consistency (not tarry or unusually sticky), there’s generally nothing to worry about. The color change should resolve within a day or two of stopping the food, supplement, or medication.
Medical Conditions That Cause Black Stool
When black, tarry stool is caused by actual bleeding, the source is usually in the upper digestive tract. The most common conditions include peptic ulcers (open sores in the stomach lining or the first part of the small intestine), gastritis (inflammation of the stomach lining), and tears in the esophagus. Chronic use of anti-inflammatory painkillers is one of the leading causes of these types of ulcers.
A more serious cause is esophageal varices, which are swollen veins in the lining of the esophagus. These develop when blood flow to the liver is blocked, most often by scar tissue from liver disease (cirrhosis). Bleeding from esophageal varices is a medical emergency. People with this condition may vomit large amounts of blood, pass black or bloody stools, and feel lightheaded from rapid blood loss.
Stomach or esophageal cancers can also cause slow, chronic bleeding that shows up as black stool. This is less common than ulcers, but it’s one of the reasons persistent black stool that can’t be explained by diet or medication should always be evaluated.
Black Stool vs. Bright Red Blood
The color of blood in your stool tells a story about where the bleeding is happening and how fast. Black, tarry stool typically points to the upper digestive tract because the blood has had time to be broken down by acid and enzymes during its long transit. Bright red blood passed from the rectum usually means the bleeding source is lower, in the colon or rectum, where there isn’t enough transit time for the blood to darken.
There’s an important exception. Very heavy bleeding in the upper digestive tract can move through the intestines so quickly that it comes out bright red rather than black. So while color is a useful clue, it’s not a perfect indicator of location. Doctors use it as a starting point, not a final answer.
Warning Signs That Need Immediate Attention
A single episode of dark stool after eating blueberries is one thing. But certain combinations of symptoms suggest active internal bleeding that requires urgent care:
- Tarry, sticky consistency with a distinctly foul smell, especially if you haven’t taken iron or bismuth
- Lightheadedness or dizziness when standing up, which can signal significant blood loss
- Vomiting blood or material that looks like coffee grounds
- Rapid heartbeat, weakness, or fainting
- Pale skin or unusual fatigue developing alongside the dark stools
If your stool is black and you also experience any of these symptoms, that combination warrants emergency medical evaluation rather than a wait-and-see approach.
How Doctors Identify the Cause
If you visit a doctor about black stool, one of the first steps is figuring out whether actual blood is present. A fecal occult blood test can detect hidden blood in a stool sample, though it only confirms whether blood is there or not. It can’t pinpoint where the bleeding is coming from. If the test is positive, the next step is typically a colonoscopy or an upper endoscopy, where a small camera is used to look directly at the lining of the digestive tract and find the bleeding source.
Your doctor will also ask about medications, supplements, and recent diet to rule out the harmless causes first. Being specific about what you’ve eaten or taken in the last few days can save you from unnecessary testing.
Black Stool in Newborns and Infants
For new parents, seeing black stool in a newborn’s diaper is completely normal in the first few days of life. This is meconium, a thick, tar-like substance made up of everything the baby swallowed in the womb: amniotic fluid, mucus, bile, and shed cells. It typically passes within the first 48 hours. If you don’t see it within that window, let your pediatrician know.
After the first three days, though, black stool in a baby is no longer expected. Once meconium has passed, stools should transition to lighter colors. If black, tarry stools reappear after day three, it could indicate that blood has entered the baby’s digestive tract, and a pediatrician should evaluate it promptly. The same goes for white, clay-colored, or visibly bloody stools at any age in infancy.