A black bowel movement usually means one of two things: something you ate or swallowed changed the color, or there is bleeding somewhere in your upper digestive tract. The difference matters a lot, and you can often tell which one you’re dealing with based on the texture, smell, and other symptoms happening at the same time.
Harmless Causes: Foods and Medications
Several everyday foods and over-the-counter products turn stool black without any bleeding involved. Blueberries are a common culprit, especially if you eat them in large quantities. The deep pigment can darken stool enough to look almost black. Black licorice does the same thing, as can dark-colored candies when the dyes mix together in your gut. Beets, dark leafy greens, and foods with dark food coloring can all shift stool color temporarily.
Medications are another frequent cause. Pepto-Bismol (bismuth subsalicylate) is one of the most well-known. When bismuth reaches your digestive system, it reacts with small amounts of sulfur naturally present in your saliva and gut, forming bismuth sulfide, a harmless black compound. This can turn both your stool and your tongue black for a day or two after taking it.
Iron supplements are the other big one. In studies of people taking oral iron, between 8% and 31% developed black stools, depending on the type and dose of iron. Higher doses caused it more often. If you recently started an iron supplement and noticed the change, that’s very likely the explanation. The color shift is a normal side effect and not a sign of bleeding.
In all of these cases, the stool may be dark or black, but it looks and feels like a normal bowel movement. It’s formed, it doesn’t have an unusual stickiness, and it doesn’t smell dramatically worse than usual. That texture difference is key.
When Black Stool Signals Bleeding
The medical term for black, tarry stool caused by bleeding is melena. It happens when blood enters the digestive tract somewhere above the small intestine, typically in the esophagus, stomach, or the first section of the small intestine. Digestive chemicals, including stomach acid and enzymes, break down the blood during its long journey through the gut. That chemical process transforms red blood into a dark, tar-like substance.
The most common causes of this type of bleeding include peptic ulcers (open sores in the stomach lining or upper small intestine), inflammation of the stomach lining (gastritis), and swollen veins in the esophagus that can rupture. Heavy use of anti-inflammatory painkillers like ibuprofen or aspirin can erode the stomach lining over time and trigger bleeding that shows up as black stool.
Melena looks and smells distinctly different from stool that’s simply been darkened by food. It tends to be sticky and tar-like, almost like roofing tar in consistency. It also carries a notably foul odor, sharper and more pungent than a typical bowel movement. If what you’re seeing in the toilet is loose, sticky, jet-black, and smells unusually bad, that pattern points toward digested blood rather than a dietary cause.
How to Tell the Difference
Start with the simplest question: did you eat anything dark or take any medication in the last day or two? If you had a large serving of blueberries, took Pepto-Bismol, or started iron supplements, the answer is probably straightforward. Stop the food or supplement and see if stool color returns to normal within a day or two. It almost always does.
If you can’t point to a dietary or medication cause, pay attention to texture and smell. Normal stool that happens to be dark will still hold its shape and won’t coat the toilet bowl. Melena, on the other hand, is often loose, smears easily, and sticks to surfaces. The smell is hard to miss.
Also consider how you feel overall. Black stool from blueberries doesn’t come with other symptoms. Black stool from internal bleeding often does. Lightheadedness, feeling unusually tired, a fast heartbeat, abdominal pain, nausea, or vomiting (especially vomiting material that looks like coffee grounds) are all signs that blood loss may be involved. Any combination of black stool with those symptoms warrants urgent medical attention.
What Happens During Evaluation
If your doctor suspects the black stool is from bleeding, they’ll typically start with blood work and a stool test. The blood tests check your red blood cell count (to gauge how much blood you may have lost), how quickly your blood clots, and how well your liver is functioning. A stool sample can be tested for hidden blood that isn’t visible to the naked eye.
The most informative test for upper digestive bleeding is an upper endoscopy, where a thin, flexible tube with a camera is passed down the throat to examine the esophagus, stomach, and upper small intestine directly. This lets doctors see the source of bleeding and often treat it at the same time. If the source isn’t found there, a colonoscopy examines the lower digestive tract. In harder-to-find cases, imaging like a CT scan or a procedure that injects dye into blood vessels (angiography) can pinpoint a bleeding site deeper in the small intestine.
Most people won’t need all of these tests. The evaluation usually narrows the cause quickly, especially when the bleeding source is a common one like an ulcer.
Why Upper Bleeding Looks Different From Lower Bleeding
The color of blood in stool depends on where the bleeding starts and how far the blood travels before leaving the body. A natural dividing line exists at a ligament in the upper small intestine called the suspensory ligament of the duodenum. Bleeding above this point, in the stomach or esophagus, produces black stool because the blood spends hours being broken down by acid and enzymes. Bleeding below this point, in the lower intestine or colon, produces bright red or maroon-colored stool because the blood passes through more quickly with less chemical processing.
This is why stool color gives doctors an immediate clue about where to look. Black and tarry points upward. Bright red points downward. Both deserve evaluation, but they lead to different diagnostic paths.
What to Do Next
If your stool is black but formed normally, and you recently consumed dark-colored foods, supplements, or bismuth-containing medications, try eliminating those for 48 to 72 hours. Your stool should return to its usual brown. If it does, you have your answer.
If your stool is black and tarry with an unusually strong odor, or if the color change persists after removing potential dietary causes, get it evaluated. If you also feel dizzy, weak, short of breath, or notice vomiting that resembles dark coffee grounds, seek care immediately. These signs suggest active blood loss that may need prompt treatment. Even a small amount of ongoing bleeding can add up over days and lead to significant anemia, so persistent unexplained black stool is always worth investigating even if you feel fine otherwise.