Black poop is not always bad. In most cases, it’s caused by something you ate or a medication you took, and it resolves on its own. 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.
Harmless Causes of Black Stool
Several everyday foods and medications can turn your stool black without any bleeding involved. The most common culprits include iron supplements, bismuth-based antacids (like Pepto-Bismol), activated charcoal, black licorice, blueberries, and blood sausage. These color changes are temporary and harmless.
Pepto-Bismol is one of the most frequent causes people notice. The bismuth in the medication reacts with small amounts of sulfur naturally present in your saliva and digestive system, forming a black compound called bismuth sulfide. This can turn both your tongue and your stool black for a day or two after use. It looks alarming but means nothing medically.
If you recently started an iron supplement, expect dark green to black stools for as long as you’re taking it. This is normal and not a reason to stop.
When Black Stool Means Bleeding
Black stool caused by bleeding in the stomach, esophagus, or upper small intestine has a medical name: melena. It takes about 50 milliliters of blood, roughly a shot glass worth, to turn stool black. The blood darkens as it travels through your digestive tract and gets broken down by stomach acid and digestive enzymes.
Melena looks and feels distinctly different from stool that’s been stained by food or medication. It is jet black with a tarry, sticky consistency, almost like roofing tar. It also has a notably strong, foul odor that’s hard to miss. That smell is a byproduct of blood being digested as it moves through the intestines. The longer the blood has traveled, the darker and smellier the stool becomes. A smaller amount of bleeding may produce stool that looks more dark brown than true black.
Stool that’s been darkened by food or supplements won’t have that distinctive tarry texture or the unusually strong smell. That difference is the most reliable clue you have at home.
Common Medical Causes
The most frequent cause of true melena is a peptic ulcer, a sore in the lining of the stomach or the first part of the small intestine. Ulcers bleed when they erode into a blood vessel beneath the surface. This can range from a slow ooze from small capillaries to more significant bleeding from larger vessels. Other causes include inflammation of the stomach lining (gastritis), tears in the esophagus, and enlarged veins in the esophagus or stomach.
Regular use of common pain relievers like ibuprofen, naproxen, and aspirin significantly raises the risk of upper digestive bleeding. These medications reduce the protective mucus barrier in the stomach and small intestine, making the lining more vulnerable to acid damage. The risk varies by medication: naproxen carries about a 5.6 times higher relative risk of upper GI bleeding compared to non-users, while ibuprofen sits lower at about 2.7 times the baseline risk. Heavy alcohol use compounds this effect by further irritating the stomach lining.
If you take these pain relievers regularly, especially in combination with alcohol or blood thinners, black tarry stools are something to take seriously.
How to Tell the Difference
Start by thinking about what you’ve eaten or taken in the last 24 to 48 hours. If you had Pepto-Bismol, started iron pills, or ate a bowl of blueberries, that’s likely your answer. Stop the food or medication and see if your stool returns to its normal color within a day or two.
If you can’t trace it to a food or medication, pay attention to the stool itself. Ask yourself three questions: Is it jet black rather than just dark? Is it sticky or tarry? Does it smell significantly worse than usual? If the answer to those is yes, you’re likely looking at digested blood.
Your doctor can confirm or rule out bleeding with a fecal occult blood test, a simple test that detects hidden blood in a stool sample. The test only tells you whether blood is present, not where it’s coming from. Certain foods, supplements, and medications can cause false positives or false negatives, so your doctor may ask you to avoid specific things before taking the test. If blood is confirmed, a colonoscopy or upper endoscopy is typically the next step to find the source.
Symptoms That Signal an Emergency
Black tarry stool on its own warrants a call to your doctor, but certain additional symptoms suggest more significant blood loss that needs immediate attention. These include feeling dizzy or lightheaded when standing, a rapid heartbeat, unusual fatigue or weakness, vomiting blood or material that looks like coffee grounds, and abdominal pain. These signs suggest enough blood loss to affect your circulation, and that’s a situation for the emergency room rather than a scheduled appointment.
Even without those symptoms, black tarry stool that persists for more than a couple of days, or that you can’t explain with diet or medication, is worth getting checked. Upper GI bleeding can start slowly and worsen over time, sometimes causing iron deficiency anemia before any dramatic symptoms appear.