Black stool usually comes from something you ate, drank, or took as a supplement or medication. In most cases, it’s harmless and clears up within a day or two. But black stool can also signal bleeding in the upper digestive tract, which needs prompt medical attention. The key is knowing how to tell the difference.
Common Harmless Causes
The most frequent reason for unexpected black poop is something in your diet or medicine cabinet. These causes produce stool that looks dark but has a normal texture and smell.
Iron supplements are one of the top culprits. Iron reacts with digestive juices in your gut, creating a dark or black pigment. This happens with both iron pills and liquid iron, and it’s completely expected. Splitting your daily dose into two smaller amounts can reduce the intensity of the color change along with other stomach side effects.
Pepto-Bismol and similar bismuth-based medications reliably turn stool black. The bismuth in the medicine combines with trace amounts of sulfur naturally present in your saliva and digestive system, forming a black compound called bismuth sulfide. As it works its way through you, your stool turns jet black. This can last a few days after your last dose and is nothing to worry about.
Certain foods can also do it. Blueberries, especially in large quantities, can make stool so dark it looks nearly black. Black licorice is another well-known one. Even a mix of brightly colored candies can combine in your gut to produce a surprisingly dark result. Beets won’t typically cause black stool, but their deep red pigment can create an alarming blood-red color that sends people searching for answers too.
When Black Stool Means Bleeding
Black stool caused by bleeding in the upper digestive tract (the stomach or upper intestine) has a medical name: melena. It looks and feels distinctly different from stool that’s been darkened by food or supplements, and recognizing those differences matters.
Classic melena is jet black with a tarry, sticky consistency, almost like tar or thick motor oil. It also has a particularly strong, foul smell that’s noticeably worse than normal. That odor comes from blood being broken down as it travels through the digestive tract. The longer the blood has been in your system, the darker and more pungent the stool becomes. A small amount of bleeding may look more dark brown than truly black, and some causes of upper digestive bleeding also produce diarrhea, making the stool wetter than the classic tarry texture.
If your black stool has a normal consistency and doesn’t have an unusually offensive smell, it’s far more likely to be from food, iron, or medication. Stool that’s simply been stained black by something you consumed won’t have that distinctive tarry quality or the sharp odor that comes from digested blood.
Causes of Upper Digestive Bleeding
When melena does occur, the bleeding source is almost always somewhere between the esophagus and the upper part of the small intestine. Peptic ulcers, which are open sores in the stomach lining or the first section of the small intestine, are the most common cause. Gastritis (inflammation of the stomach lining) is another frequent one, often related to heavy alcohol use, long-term use of anti-inflammatory painkillers like ibuprofen, or bacterial infection. Less commonly, enlarged veins in the esophagus can rupture and bleed, typically in people with liver disease.
The reason the blood turns black rather than staying red is simple: stomach acid and digestive enzymes break it down as it moves through the tract. By the time it reaches the toilet, it’s been chemically transformed into that dark, tarry substance.
Signs That Need Immediate Attention
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms mean you should seek care right away rather than waiting for an appointment. Feeling dizzy or lightheaded suggests enough blood loss to affect your blood pressure. Vomiting blood, whether it looks bright red or resembles dark coffee grounds, points to active bleeding. Severe cases involving significant blood loss may require hospitalization.
If you’re taking iron or bismuth and you’re confident that’s the cause, you can test your theory by stopping the supplement or medication for a couple of days. If the color returns to normal, you have your answer. If it doesn’t, or if the stool has that tarry, sticky quality, it’s worth getting checked out.
Black Stool in Newborns and Infants
If you’re a new parent noticing very dark stool in your baby’s diaper, context matters. Newborns pass a substance called meconium in their first day or two of life. It’s typically dark brown or green, very sticky, and completely normal. Most babies pass it within the first 24 hours, though it can take up to 48 hours. After that, as milk feeding gets established, stool transitions to a more yellowish-green color.
Once babies start solid foods around four to six months, stool color can shift dramatically depending on what they eat. Dark-colored foods like blueberries or spinach can produce surprisingly dark diapers. However, truly black stool in an infant outside the meconium stage is not considered normal and should be evaluated by a pediatrician.
How to Tell the Difference at a Glance
- Food or supplement stain: Dark or black color, but normal texture and typical smell. Clears up once you stop the food, iron, or medication.
- Melena (digested blood): Jet black, tarry, sticky consistency with a distinctively foul odor. Does not resolve on its own and often comes with other symptoms like fatigue, dizziness, or stomach pain.
Think back over the last 24 to 48 hours. Did you eat a bowl of blueberries, take an iron pill, or chew some Pepto-Bismol tablets? If the answer is yes and your stool looks dark but otherwise normal, that’s almost certainly your explanation. If you can’t identify an obvious dietary or medication cause, or if the stool is sticky and unusually foul-smelling, that’s the signal to get it evaluated.