Black stool is often normal and caused by something you recently ate or took, like iron supplements, bismuth medications (Pepto-Bismol), or certain foods. However, black stool can also signal bleeding in the upper digestive tract, which needs prompt medical attention. The key difference comes down to what the stool looks like, how it smells, and whether you have other symptoms.
Common Harmless Causes
Several everyday foods and supplements turn stool noticeably dark or black. Iron supplements are one of the most common culprits. Dark or black stools are a typical side effect of oral iron tablets, and the color change happens because unabsorbed iron oxidizes as it moves through your digestive tract. The stool may look dark green to black, but it won’t have an unusually foul smell or sticky texture.
Bismuth-containing medications like Pepto-Bismol also cause temporary black discoloration of both stool and tongue. This happens when bismuth reacts with trace amounts of sulfur in your saliva and digestive tract. The color change is harmless and clears up after you stop taking the product, typically within a few days.
Foods that can darken stool include black licorice, blueberries, blood sausage, and activated charcoal. If you can trace the color change to something you consumed in the last day or two, that’s usually the explanation.
When Black Stool Signals Bleeding
Black stool caused by bleeding in the upper digestive tract (the stomach or upper small intestine) has a medical name: melena. It looks and feels distinctly different from stool darkened by food or supplements. Classic melena is jet black with a tarry, sticky consistency. It also has a particularly strong, offensive odor, which is a byproduct of blood being broken down by stomach acid and digestive enzymes as it passes through the gut.
It takes about 50 milliliters of blood in the stomach (roughly 3 tablespoons) to turn stool black. By the time blood travels from the upper digestive tract to the colon, stomach acid has chemically altered it, converting the red hemoglobin into a dark compound. That’s why upper GI bleeding produces black stool rather than red. If bleeding is very rapid, though, blood can move through the system fast enough to still appear red when it comes out.
Common causes of upper GI bleeding include stomach ulcers (often related to long-term use of pain relievers like ibuprofen or aspirin), inflammation of the stomach lining, and tears in the esophagus. Regular NSAID use significantly increases bleeding risk in the digestive tract, particularly for people over 65.
How to Tell the Difference
The texture and smell are your most reliable clues. Stool darkened by iron supplements or food is typically firm or normal in consistency, without an unusual odor. Melena is sticky and tar-like. People who experience it often remember the stickiness because it clings to the toilet bowl and is difficult to flush.
Context matters too. If you started iron supplements three days ago and your stool turned dark, the connection is straightforward. If you haven’t taken anything that would explain the color, and the stool is black, tarry, and foul-smelling, that pattern points toward bleeding.
Other symptoms that often accompany GI bleeding include lightheadedness or dizziness, fatigue, pale skin, nausea, vomiting (sometimes with material that looks like coffee grounds), and abdominal pain. If you notice black tarry stool along with any of these, that combination warrants urgent evaluation.
Black Stool in Newborns
If you’re a new parent, black stool in your baby’s first few days is completely expected. Newborns pass meconium, a thick, dark green to black substance, typically within the first 24 to 48 hours after birth. Meconium is made up of materials the baby ingested in the womb, including amniotic fluid and skin cells. It signals that the digestive tract is working properly. Over the next few days, stool transitions to a lighter green and then yellow color as the baby begins digesting milk. If a newborn hasn’t passed meconium within 48 hours, that may indicate a bowel obstruction that needs evaluation.
How Doctors Confirm the Cause
If the cause of black stool isn’t obvious from your diet or medications, a simple stool test can check for hidden blood. The two most common versions are a guaiac-based test, which detects blood through a chemical reaction, and a fecal immunochemical test (FIT), which uses antibodies to identify human blood specifically. FIT tends to be more accurate because it’s less likely to be thrown off by foods or supplements. A positive result on either test typically leads to further investigation, often with a scope exam to look directly at the lining of the upper digestive tract.
What to Watch For
A single episode of dark stool after eating blueberries or taking Pepto-Bismol is not a concern. The color should return to normal within a day or two after you stop consuming whatever caused it. With iron supplements, expect dark stools for as long as you’re taking them.
The pattern that deserves attention is black, tarry, sticky stool with a strong odor that you can’t link to a food, supplement, or medication. This is especially true if it happens more than once, if you take NSAIDs regularly, if you have a history of stomach ulcers, or if you’re experiencing dizziness, weakness, or stomach pain alongside the color change. In those cases, getting evaluated quickly matters because ongoing bleeding can lead to significant blood loss before other symptoms become obvious.