Black stool is a bowel movement that appears noticeably darker than your usual brown. It can be completely harmless, caused by something you ate or a supplement you’re taking, or it can signal bleeding in your upper digestive tract. The key distinction is whether the color comes from digested blood or from food, medications, and supplements.
Harmless Causes: Food, Supplements, and Medications
Several everyday substances can turn your stool dark green, very dark brown, or jet black. Iron supplements are one of the most common culprits. The iron oxidizes as it passes through your gut, producing a dark green to blackish color that looks alarming but is nothing to worry about.
Pepto-Bismol and similar stomach remedies containing bismuth are another frequent cause. When bismuth meets the small amounts of sulfur naturally present in your saliva and digestive system, they combine to form bismuth sulfide, a black-colored compound. This can turn both your tongue and your stool black. The NHS confirms this is harmless and resolves once you stop taking the medication.
Foods can do it too. Blueberries eaten in large quantities can produce stool so dark it looks almost black. Black licorice, dark chocolate, and even handfuls of brightly colored candy (where the dyes mix together) can have the same effect. These color changes typically last a day or two after you stop eating the food in question.
If you recently started an iron supplement, took an antacid with bismuth, or ate a large serving of blueberries, that’s very likely your explanation. The stool will look dark but otherwise normal in texture, and you won’t have other symptoms.
When Black Stool Means Bleeding
Black stool caused by bleeding in the upper digestive tract (the stomach, esophagus, or first section of the small intestine) has a medical name: melena. It looks and feels different from stool darkened by food or supplements. Melena is jet black, has a tarry or sticky consistency, and carries a distinctly foul smell that patients often describe as unforgettable. The stickiness is a characteristic many people remember specifically.
The color comes from blood that has been partially digested as it travels through the gut. By the time it reaches the colon, the hemoglobin has been broken down by stomach acid and digestive enzymes, turning it from red to black. This process requires time and distance, which is why melena points to bleeding in the upper digestive tract rather than lower down. Bleeding from the lower colon or rectum typically produces red or maroon-colored blood instead.
Conditions That Cause Upper GI Bleeding
Peptic ulcers are the most common cause of upper gastrointestinal bleeding. These are open sores on the lining of the stomach or the upper part of the small intestine, usually caused by a bacterial infection (H. pylori) or regular use of anti-inflammatory medications like ibuprofen, aspirin, or naproxen. If you take these painkillers frequently, especially without food, you’re at higher risk.
Other causes include:
- Mallory-Weiss tears: small tears in the lining of the esophagus, often from forceful vomiting or retching, which can produce significant bleeding
- Esophageal varices: swollen veins in the esophagus caused by restricted blood flow through the liver, most often seen in people with liver disease
- Gastritis: inflammation of the stomach lining, which can erode enough to bleed
- Hiatal hernia: larger hernias can cause erosions in the stomach that lead to slow bleeding
- Growths: rarely, cancerous or noncancerous tumors in the upper digestive tract can bleed
How to Tell the Difference
The simplest first step is to think about what you’ve consumed in the past 48 hours. If you’ve been taking iron supplements, bismuth-based antacids, or eating dark-colored foods, try stopping them for two to three days and see if the color returns to normal. Stool darkened by these substances looks dark but has a normal texture and no unusual smell.
Melena, by contrast, is unmistakably tarry. It’s sticky enough to smear, intensely black (think the color of asphalt or coal, not just dark brown), and has a strong, distinctive odor. If you’re unsure whether your stool is truly black, compare it mentally to a black object rather than just “darker than usual.”
Your doctor can also run a stool test to check for hidden blood. The most common options are the guaiac-based fecal occult blood test and the fecal immunochemical test (FIT). FIT is generally preferred because it specifically detects human blood and doesn’t require dietary restrictions beforehand. It picks up colorectal bleeding with roughly 70 to 85 percent sensitivity, compared to about 50 to 70 percent for the older guaiac test. The guaiac test can also produce false positives from red meat, broccoli, and other foods with natural enzyme activity, making it less reliable without careful preparation.
Symptoms That Signal an Emergency
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms mean you should go to urgent care or an emergency room right away:
- Vomiting blood or vomit that looks like coffee grounds (another sign of digested blood)
- Dizziness, weakness, or lightheadedness, which suggest enough blood loss to affect your circulation
- Rapid heartbeat or shortness of breath without exertion
- Several consecutive days of black, tarry stools
These symptoms point to significant or ongoing blood loss. Severe cases that cause a dangerous drop in blood pressure may require hospitalization.
Black Stool in Newborns
If you’re a new parent, it’s worth knowing that a newborn’s first several bowel movements are normally black and tar-like. This substance, called meconium, is made up of amniotic fluid, skin cells, and other material swallowed in the womb. It typically passes within the first two to three days of life and gradually transitions to green, then yellow or brown stool.
Black or bloody stool that appears after the meconium stage, or that contains bright red blood, is different. In newborns, this can range from something benign like swallowed maternal blood during delivery to more serious conditions. A simple lab test called the Apt test can determine whether blood in a newborn’s stool is the baby’s own or the mother’s. If your newborn has bloody or persistently black stool beyond the first few days, that needs prompt evaluation.
What to Expect at the Doctor
If your doctor suspects upper GI bleeding, the standard next step is an upper endoscopy, where a thin, flexible camera is passed through your mouth to examine your esophagus, stomach, and the beginning of your small intestine. This procedure both identifies the source of bleeding and, in many cases, allows the doctor to treat it during the same session. You’ll typically be sedated and won’t feel it.
For most people searching this topic, the answer will turn out to be something they ate or a supplement they’re taking. But if you see jet-black, sticky, foul-smelling stool with no obvious dietary explanation, especially alongside fatigue, dizziness, or vomiting, that combination deserves same-day medical attention.