Black poop usually means one of two things: something you ate, drank, or swallowed (like a supplement or medication) changed the color, or there’s bleeding somewhere in your upper digestive tract. The harmless causes are far more common, but telling them apart matters because upper GI bleeding needs prompt medical attention.
Harmless Causes That Turn Stool Black
The most common reason for black stool is something you recently consumed. Iron supplements are a frequent culprit, and the effect is dose-dependent. At 25 mg of supplemental iron, roughly 8% of people notice black stools. At 50 mg, that jumps to 31%. If you recently started an iron supplement or prenatal vitamin, that’s very likely your answer.
Pepto-Bismol and similar bismuth-based medications are another classic cause. When bismuth meets the small amounts of sulfur naturally present in your digestive system, it forms bismuth sulfide, a black compound. This is completely harmless, though it can also turn your tongue black. The discoloration clears up on its own after you stop taking the medication, typically within a few days as the substance works its way through your system.
Certain foods do it too: black licorice, blueberries, blood sausage, and activated charcoal (now common in smoothies and supplements) can all produce dark or black stools. If you can trace the timing back to one of these, there’s nothing to worry about.
When Black Stool Means Bleeding
The concerning type of black stool has a medical name: melena. It looks distinctly different from stool that’s dark because of food or supplements. Melena is jet black, sticky, and tarry in texture, and it has a noticeably foul smell that’s different from normal stool. If that description matches what you’re seeing, it points to bleeding in the upper part of your digestive tract: the esophagus, stomach, or the first section of the small intestine.
The reason the blood looks black rather than red is chemistry. As blood travels through the stomach, gastric acid converts hemoglobin (the red pigment in blood) into a dark brown or black substance called hematin. By the time it reaches the toilet, it no longer looks like blood at all.
The most common causes of upper GI bleeding include:
- Peptic ulcers: open sores in the stomach lining or the upper small intestine, often linked to long-term use of anti-inflammatory painkillers or a bacterial infection
- Gastritis or esophagitis: severe inflammation of the stomach or esophageal lining
- Esophageal varices: swollen veins in the esophagus that can rupture, most often seen in people with liver disease
- A Mallory-Weiss tear: a tear in the esophagus caused by forceful or prolonged vomiting
- Cancers of the stomach, esophagus, or pancreas: less common but serious possibilities, particularly in older adults
How to Tell the Difference
Start with the simplest question: have you taken iron, bismuth, activated charcoal, or eaten any dark-colored foods in the last day or two? If yes, stop taking or eating the suspected cause and watch whether your stool returns to its normal brown within two to three days. That’s usually all you need to confirm a harmless explanation.
If none of those apply, pay attention to how the stool looks and how you feel. Melena from bleeding is distinctly tarry and sticky, almost like roofing tar, and the smell is unusually strong. Stool that’s simply dark brown or black from supplements tends to be firmer and more normal in texture. Accompanying symptoms are also telling. If your black stool comes with lightheadedness, weakness, dizziness when standing, abdominal pain, vomiting (especially vomit that looks like coffee grounds), or a racing heartbeat, those are signs of active blood loss and you should get to an emergency room.
What Happens at the Doctor’s Office
If your doctor suspects bleeding, the workup typically starts with blood tests to check for anemia and see how well your blood is clotting. A stool test can detect hidden blood that isn’t visible to the naked eye, which helps confirm whether bleeding is actually happening.
The most informative test is an upper endoscopy, where a thin, flexible tube with a camera is passed down your throat to visually inspect the esophagus, stomach, and upper small intestine. This is typically done within 24 hours of presentation for patients admitted with suspected upper GI bleeding. The advantage of endoscopy is that it’s both diagnostic and therapeutic: if a bleeding ulcer or ruptured vessel is found, the doctor can often treat it during the same procedure.
In some cases, a colonoscopy is done instead or in addition, particularly if the source of bleeding isn’t found in the upper tract. Another option is capsule endoscopy, where you swallow a vitamin-sized capsule containing a tiny camera that photographs your entire digestive tract as it passes through.
Black Stool in Newborns
If you’re a new parent, black stool in a newborn is usually normal. A baby’s first few bowel movements consist of meconium, a thick, dark, tar-like substance that built up in the intestines before birth. This clears within the first few days of life and transitions to the yellow or mustard-colored stool typical of breastfed infants.
Newborns can also have dark stools from swallowing small amounts of maternal blood during delivery or breastfeeding (if the mother has cracked nipples). This is generally harmless. However, black or bloody stool in an infant that persists beyond the first few days, or that appears alongside poor feeding, a swollen belly, or signs of distress, warrants immediate medical evaluation. Conditions like necrotizing enterocolitis affect up to 10% of full-term infants who present with it, and early detection makes a significant difference in outcomes.
The Bottom Line on Timing
If you’ve identified a dietary or supplement cause, your stool color should normalize within two to three days of stopping that food or medication. If black, tarry stools persist beyond that window with no obvious dietary explanation, or if they appear suddenly without any change in what you’re eating or taking, that’s when you need a medical evaluation. The sooner upper GI bleeding is identified, the simpler it is to treat.