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 on its own within a few days. But black stool can also signal bleeding in the upper digestive tract, which needs prompt medical attention. The key is figuring out which category you fall into.
Foods and Supplements That Turn Stool Black
Several everyday foods can darken your stool enough to make it look black. Black licorice, blueberries, blood sausage, and dark-colored beets are common culprits. The pigments in these foods aren’t fully broken down during digestion and pass through intact, temporarily changing stool color. This type of color change is completely harmless and resolves once the food works its way out of your system, typically within a day or two.
Iron supplements are one of the most frequent non-food causes of black stool. Iron that isn’t absorbed in the small intestine reacts with digestive enzymes as it moves through the gut, producing a dark black or greenish-black color. Activated charcoal, sometimes taken for gas or bloating, does the same thing. If you recently started either of these, that’s very likely your explanation.
Pepto-Bismol and Bismuth Medications
Pepto-Bismol and similar over-the-counter stomach remedies contain bismuth subsalicylate, which is notorious for turning stool jet black. The active ingredient, bismuth, combines with small amounts of sulfur naturally present in your saliva and digestive system. Together they form bismuth sulfide, a black-colored compound that coats stool and can also darken your tongue.
This side effect is harmless. Dark stools from bismuth can last for several days after you stop taking the medication. Give it a few more days after your last dose and the color should return to normal. One thing worth knowing: iron supplements and bismuth products can interfere with fecal occult blood tests, the screening tests that check for hidden blood in stool. If you’re scheduled for one, your doctor may ask you to stop these products beforehand to avoid a false result.
When Black Stool Means Bleeding
Black stool that isn’t explained by food, supplements, or medication could indicate bleeding somewhere in the upper digestive tract, such as the esophagus, stomach, or the first part of the small intestine. This type of stool has a specific appearance: it’s not just dark but tarry, sticky, and often has a distinctly foul smell that’s different from normal stool. The medical term for it is melena.
The reason bleeding produces black stool rather than red is that blood changes as it travels through the digestive tract. Hemoglobin, the protein that makes blood red, gets broken down by digestive enzymes along the way. By the time it reaches the colon, it’s turned dark black. This is what distinguishes upper GI bleeding from lower GI bleeding, which tends to produce bright red blood in the stool because it hasn’t had time to be digested.
Common Causes of Upper GI Bleeding
Peptic ulcers are one of the most common sources. These are open sores in the lining of the stomach or upper small intestine, often caused by a bacterial infection or long-term use of anti-inflammatory painkillers like ibuprofen or aspirin. When an ulcer erodes into a blood vessel, it can bleed slowly enough that you wouldn’t feel acute pain but steadily enough to produce black, tarry stools over several days.
Esophageal varices are another serious cause. These are swollen veins in the esophagus that develop when blood flow to the liver is blocked, most often by scar tissue from chronic liver disease. These enlarged veins can leak or rupture, causing significant bleeding that shows up as black or tarry stools, sometimes alongside vomiting blood. Bleeding varices are a medical emergency.
Other possible sources include tears in the esophageal lining (often from forceful vomiting), stomach inflammation from alcohol or medications, and, less commonly, tumors in the stomach or upper intestine.
How to Tell the Difference
Start with the simplest explanation. Think back over the last 24 to 48 hours. Did you eat blueberries, take an iron pill, or use Pepto-Bismol? If so, that’s the most likely answer. Try stopping the suspected cause and see if your stool returns to its normal brown color within a few days.
Melena from bleeding looks and smells different from food-related darkening. It’s characteristically tarry and sticky, almost like asphalt, and has an unusually strong odor. Food-related black stool, by contrast, tends to look dark but maintains a more normal consistency.
Pay attention to how you feel overall. Black stool from blueberries doesn’t come with other symptoms. Black stool from internal bleeding often does. Watch for vomiting blood or material that looks like coffee grounds, feeling dizzy, weak, or lightheaded, heart palpitations, shortness of breath, or several consecutive days of tarry black stools. Any of these combinations warrants a trip to the emergency room, not a wait-and-see approach. Even slow, steady bleeding from the upper GI tract can lead to significant blood loss over time, so persistent tarry stools without an obvious dietary explanation should be evaluated promptly.
What Happens During Evaluation
If your doctor suspects bleeding, the first step is usually a stool test to confirm whether blood is actually present. This is a simple, noninvasive test. From there, the most common next step is an upper endoscopy, where a thin flexible camera is passed through the mouth to visually inspect the esophagus, stomach, and upper small intestine. This allows the source of bleeding to be identified and, in many cases, treated during the same procedure.
Blood tests to check for anemia (low red blood cell count) help determine how much blood has been lost and how urgently treatment is needed. For most people, the cause turns out to be treatable, whether it’s an ulcer that responds to medication or a tear that heals on its own once identified.