Dark stool usually indicates something you ate, a supplement you’re taking, or bleeding somewhere in your upper digestive tract. The cause can be completely harmless or potentially serious, so the key is knowing which signs point to a dietary explanation and which ones suggest you need medical attention.
Harmless Causes That Darken Your Stool
Several everyday foods and over-the-counter products can turn your stool dark brown or even black without any cause for concern. Iron supplements are one of the most common culprits. Bismuth subsalicylate, the active ingredient in Pepto-Bismol, reliably turns stool black by reacting with trace amounts of sulfur in your digestive system. Dark-colored foods like blueberries, black licorice, beets, and dark leafy greens can also shift stool color noticeably.
The important distinction here: stool darkened by food or supplements looks dark but still has a normal texture and smell. It’s firm or soft, not sticky or unusually foul-smelling. If you recently started an iron supplement or ate a large serving of blueberries, that’s almost certainly your answer. The color returns to normal once you stop the food or supplement.
When Dark Stool Means Bleeding
Black, tarry stool with a distinctly foul smell has a specific medical name: melena. It looks different from stool darkened by food. Melena is sticky, almost like tar in consistency, and has a sharp, unmistakable odor that’s hard to ignore. This appearance happens because blood from the upper digestive tract (the esophagus, stomach, or the first part of the small intestine) gets broken down by digestive enzymes as it travels through you. Hemoglobin, the protein that makes blood red, turns dark as those enzymes work on it, which is why the stool comes out black rather than bright red.
Bright red blood in stool, by contrast, typically comes from lower in the digestive tract, closer to the exit. Black, tarry stool points higher up.
Common Causes of Upper GI Bleeding
Peptic ulcers are the single most common cause, responsible for roughly 32 to 36% of hospitalizations for upper GI bleeding. These are sores that develop in the stomach lining or the upper portion of the small intestine. The two biggest drivers of peptic ulcers are infection with a bacterium called H. pylori and regular use of common pain relievers like aspirin, ibuprofen, and naproxen. If you’ve been taking these medications frequently, especially without food, that’s a meaningful risk factor.
Inflammation of the stomach lining (gastritis) and inflammation of the esophagus account for up to 24% of cases. Gastritis can result from the same H. pylori infection, heavy alcohol use, or frequent pain reliever use. Esophageal inflammation is often driven by chronic acid reflux, where stomach acid repeatedly damages the lower esophagus and eventually causes it to bleed.
Enlarged veins in the esophagus or stomach, a condition linked to liver cirrhosis, cause about 11% of upper GI bleeding hospitalizations overall. In people with liver cirrhosis specifically, these enlarged veins are responsible for 90% of bleeding episodes. Tears in the lower esophagus from severe or prolonged vomiting can also bleed enough to produce dark stool. Less commonly, tumors in the esophagus or stomach can weaken the lining and expose blood vessels.
Who Is Most at Risk
Upper GI bleeding becomes significantly more common with age. Hospitalization rates roughly double between your mid-60s and your late 70s, jumping from about 197 per 100,000 people in the 66-to-75 age group to over 425 per 100,000 in people older than 75. Older adults are also more likely to be taking blood thinners or daily aspirin, which compounds the risk. In people over 65, ulcers and esophageal inflammation together account for about 80% of bleeding episodes.
Warning Signs That Need Urgent Attention
Dark stool on its own, without other symptoms, could still be dietary. But certain accompanying symptoms indicate significant blood loss and require urgent care:
- Dizziness or lightheadedness, especially when standing up, which signals a drop in blood pressure from blood loss
- Vomiting blood or vomit that looks like dark coffee grounds
- Heart palpitations or shortness of breath at rest or with minimal effort
- Feeling unusually weak or fatigued beyond what’s normal for you
- Several consecutive days of black, tarry stool
Severe bleeding can cause blood pressure to drop dangerously and may require hospitalization. If you’re experiencing dark stool along with any of these symptoms, that combination warrants an emergency room visit rather than a scheduled appointment.
How Doctors Identify the Cause
If the cause isn’t obvious from your diet or medications, the first step is usually a stool test that checks for hidden blood. These fecal occult blood tests detect tiny amounts of blood invisible to the naked eye. There are two main versions. One uses a chemical reaction and requires you to collect samples from three separate bowel movements. The other is simpler, needing just one sample collected with a small stick from the test kit.
These tests have limitations worth knowing about. They can only tell whether blood is present, not where it’s coming from. False positives happen when bleeding comes from something minor like a stomach irritation or even swallowed blood from a nosebleed. False negatives can occur if a polyp or growth is present but isn’t actively bleeding at the time of the test. Certain foods, vitamin C, iron supplements, and pain relievers like aspirin and ibuprofen can all skew results, which is why you may be asked to avoid them for a few days before testing.
When a stool test comes back positive or the suspicion for bleeding is high, the next step is typically an endoscopy, where a thin, flexible camera is passed through the mouth to visually inspect the esophagus, stomach, and upper small intestine. This allows doctors to both find the source and treat it in the same procedure.
What Treatment Looks Like
If an ulcer or other bleeding source is found during the endoscopy, it can often be sealed during the same procedure using heat, small clips, or other techniques applied through the scope. This is effective for most cases. Afterward, you’ll typically take acid-reducing medication for at least two weeks and sometimes longer to let the area heal and prevent re-bleeding.
If the bleeding can’t be controlled through the scope, a procedure called arterial embolization can block the blood vessel feeding the site. Surgery is reserved for the most severe cases where blood loss is rapid and other approaches haven’t worked.
For bleeding caused by H. pylori infection, a course of antibiotics clears the underlying problem. For bleeding linked to pain relievers, stopping or switching those medications is a critical part of preventing recurrence.
Newborns and Infants
In newborns, very dark or black-green stool in the first day or two of life is completely normal. This is meconium, a substance made up of materials your baby ingested in the womb. It passes within the first few days and is replaced by lighter stool as feeding begins. Dark stool that appears later in infancy, after meconium has already cleared, is worth mentioning to your pediatrician, especially if it has a tarry texture.