Black stool has two broad categories of causes: harmless ones like foods and medications, and serious ones like bleeding in the digestive tract. The key distinction is texture. Stool that’s black but otherwise normal in consistency is almost always from something you ate or a supplement you’re taking. Stool that’s black, tarry, sticky, and has a notably foul smell points to digested blood and needs medical attention.
Foods and Supplements That Turn Stool Black
Several everyday items can darken your stool enough to cause alarm. Black licorice, blueberries, blood sausage, and activated charcoal are common culprits. The color change is harmless and clears up once you stop eating the food in question, usually within a day or two.
Iron supplements are one of the most frequent causes. In a study of 27 healthy volunteers taking iron tablets, every single participant developed dark stools. If you recently started an iron supplement and notice the change, that’s almost certainly the explanation. The unabsorbed iron oxidizes in your gut and turns black on its way out.
Over-the-counter stomach remedies containing bismuth (the active ingredient in Pepto-Bismol) also cause black stool. When bismuth meets trace amounts of sulfur naturally present in your saliva and digestive system, it forms bismuth sulfide, a black compound. This can also temporarily darken your tongue. Both effects are harmless and disappear after you stop taking the medication.
When Black Stool Means Bleeding
The medical term for black, tarry stool caused by bleeding is melena. It happens when blood from somewhere in the upper digestive tract (the esophagus, stomach, or first part of the small intestine) gets broken down by stomach acid during digestion. The acid converts the red hemoglobin in blood into a dark brown or black substance, which mixes with stool and gives it a distinctive tarry, sticky texture.
It takes roughly 100 to 200 milliliters of blood in the upper digestive tract to produce melena. That’s about half a cup to a full cup. The black color can persist for several days even after bleeding has stopped, because residual blood continues moving through the intestines. So even if melena seems to resolve on its own, it still warrants evaluation.
The most common causes of upper GI bleeding include stomach ulcers, inflammation of the stomach lining (gastritis), and tears in the esophagus. Heavy use of anti-inflammatory painkillers like ibuprofen or aspirin increases the risk of stomach ulcers, making this a particularly important connection to be aware of if you take those regularly.
How to Tell the Difference
The distinction between harmless black stool and melena comes down to a few observable qualities:
- Texture: Diet-related black stool has a normal consistency. Melena is notably sticky and tar-like, often difficult to flush.
- Smell: Melena has a distinctly strong, unpleasant odor that’s different from typical stool. If the smell seems unusually foul, that’s a meaningful signal.
- Context: If you can trace the color to blueberries, iron pills, or Pepto-Bismol, and the stool otherwise looks and feels normal, the cause is likely benign.
A simple first step is to stop whatever food, supplement, or medication you suspect is responsible. If your stool returns to its usual color within a couple of days, you have your answer.
Symptoms That Signal an Emergency
Black, tarry stool on its own deserves a call to your doctor. But certain accompanying symptoms mean you should go to urgent care or an emergency room promptly:
- Vomiting blood or vomit that looks like dark coffee grounds
- Dizziness, weakness, or lightheadedness, which can indicate significant blood loss
- Heart palpitations or shortness of breath
- Several consecutive days of black, tarry stool
These signs suggest active or recent bleeding that’s substantial enough to affect your circulation. Blood loss from upper GI bleeding can happen gradually, so you might not feel dramatic symptoms right away. Persistent fatigue or feeling unusually winded during normal activity can be subtler signs of ongoing slow bleeding.
Black Stool in Newborns
If you’re a new parent noticing very dark stool in your baby’s diaper, the first few days are normal. Meconium, the first stool a newborn passes, is typically dark brown or green and very sticky. It usually appears within the first 24 hours of life and continues for a couple of days. As your baby starts feeding, stool transitions to a more yellowish-green color.
If meconium hasn’t passed within 48 hours, or if black stool reappears after the initial transition period, that warrants a visit to the pediatrician. Black stool in an infant beyond the newborn stage is not considered normal.