Black stool usually comes from something you ate, drank, or took as a medication. But in some cases, it signals bleeding in the upper digestive tract, which needs medical attention. The key difference comes down to how the stool looks, feels, and smells.
Common Harmless Causes
Several everyday foods, supplements, and over-the-counter medications can turn your stool noticeably dark or black. These include:
- Iron supplements: One of the most common culprits. Iron reliably turns stool dark green to black, typically within a day or two of taking a dose.
- Pepto-Bismol and similar products: The bismuth in these medications reacts with small amounts of sulfur naturally present in your saliva and digestive system. This creates bismuth sulfide, a black compound that darkens both your stool and sometimes your tongue. It’s completely harmless.
- Activated charcoal: Often taken for bloating or gas, it passes through and colors the stool black.
- Foods: Black licorice, blueberries, blood sausage, and other deeply pigmented foods can all do it.
If any of these apply to you, the color change is temporary. Stool typically returns to its normal brown within one to two days after you stop the supplement, medication, or food in question.
How to Tell if It’s Blood
When bleeding happens in the stomach, esophagus, or upper small intestine, the blood gets partially digested as it travels through the rest of the gut. This process turns it black. The medical term for this is melena, and it looks and feels distinctly different from stool that’s been darkened by food or medication.
Melena is jet black, tarry, and sticky. People who have experienced it often remember the stickiness in particular. It also has a strong, offensive smell that’s noticeably different from normal stool odor. That smell comes from blood being broken down by digestive enzymes. If your stool is simply stained dark by blueberries or iron pills, it won’t have that distinctive tarry consistency or foul odor.
It takes roughly 50 milliliters of blood in the stomach (a few tablespoons) to turn stool black. So melena represents a meaningful amount of bleeding, not a trace.
What Causes Bleeding in the Upper Gut
The most common sources of upper gastrointestinal bleeding include stomach ulcers, inflammation of the stomach lining (often from long-term use of anti-inflammatory painkillers like ibuprofen or naproxen), and tears in the esophagus. Enlarged veins in the esophagus, often related to liver disease, can also bleed. Less commonly, tumors in the stomach or upper intestine are the source.
Bleeding can range from slow and chronic to sudden and severe. A slow bleed might produce dark stools for days before you notice other symptoms. A fast bleed can cause lightheadedness, weakness, rapid heartbeat, pale skin, or fainting, sometimes along with vomiting blood that looks like coffee grounds.
Symptoms That Signal an Emergency
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms mean you should seek care immediately:
- Dizziness or lightheadedness when standing up
- Rapid heartbeat or feeling like your heart is racing
- Vomiting blood or material that looks like dark coffee grounds
- Severe abdominal pain
- Feeling faint or confused
- Pale or clammy skin
These symptoms suggest significant blood loss and require urgent evaluation.
How Doctors Confirm the Cause
If you report black stool and there’s no obvious dietary or medication explanation, your doctor will likely start with a stool test to check for hidden blood. The most commonly used version detects human blood proteins and can pick up as little as 0.3 milliliters of blood in a stool sample. A positive result means further evaluation is needed.
For upper GI bleeding specifically, standard stool tests are less reliable because digestive enzymes break down blood proteins as they travel through the gut. A different type of test that detects blood breakdown products is more sensitive for upper tract bleeding, catching roughly 90% of cases. If any test comes back positive, or if symptoms are significant, the next step is usually an endoscopy, where a camera is passed through the mouth to directly examine the esophagus, stomach, and upper intestine.
If you’re already experiencing obvious symptoms like tarry stool with dizziness or vomiting blood, doctors skip the stool test entirely and move straight to direct evaluation.
A Quick Self-Check
Before you worry, run through this checklist. Have you recently taken iron supplements, Pepto-Bismol, or activated charcoal? Have you eaten black licorice, blueberries, or blood sausage in the past day or two? If so, stop the suspect item and watch. Your stool should return to normal within a couple of days.
If the dark color persists after ruling out dietary and medication causes, or if the stool is sticky, tarry, and unusually foul-smelling, that pattern points toward blood in the digestive tract. The combination of color, consistency, and odor is what separates a harmless color change from something that needs medical attention.
Black Stool in Newborns
For new parents: a newborn’s first few bowel movements are normally black and tar-like. This is meconium, a substance made up of everything the baby ingested in the womb, and it’s completely expected. It typically clears within the first few days of life as the baby begins feeding.
After that initial period, black or bloody stool in an infant is not normal. It can sometimes result from swallowed maternal blood during delivery, which is benign, but it can also indicate more serious conditions like vitamin K deficiency bleeding or intestinal problems. Doctors can run a specific test to determine whether the blood in a baby’s stool is maternal or the infant’s own, which helps guide next steps quickly.