Black, foul-smelling stool has two main explanations: something you ate or took, or bleeding somewhere in your upper digestive tract. The difference matters a lot, and fortunately it’s usually possible to tell them apart at home before deciding what to do next.
What Makes Stool Turn Black and Smell Worse
The most concerning cause is blood from your stomach or upper small intestine. As blood travels through your digestive tract, digestive chemicals break it down, changing its color from red to black and giving it a sticky, tar-like texture. That breakdown process also produces the distinctly foul smell, sharper and more metallic than normal stool odor. The longer the blood has traveled through your system, the darker and smellier it gets. Doctors call this type of stool “melena.”
But black stool doesn’t always mean bleeding. Several everyday substances can darken your stool without any cause for alarm:
- Bismuth medications (the active ingredient in Pepto-Bismol) react with sulfur compounds in your gut to form a dark substance called bismuth sulfide. About 80% of people taking these medications notice their stool turning black. This is harmless and stops once you quit the medication.
- Iron supplements commonly turn stool dark green to black.
- Dark-colored foods like blueberries, black licorice, and dark chocolate can temporarily darken stool.
How to Tell Harmless Dark Stool From Bleeding
Texture is the biggest clue. Melena from internal bleeding looks and feels like tar: it’s shiny, sticky, and clings to the toilet bowl. It also has a uniquely terrible smell that’s hard to miss, distinctly worse than your normal bathroom experience. Stool that’s simply dark from food or supplements tends to be firmer and more formed, without that sticky quality.
Think about what you’ve consumed in the past 24 to 48 hours. If you recently took Pepto-Bismol, started iron supplements, or ate a pint of blueberries, that’s likely your answer. If you can’t connect the color to anything you ate or took, the stool looks tarry, and the smell is unusually foul, bleeding is a real possibility.
What Causes Bleeding in the Upper Digestive Tract
Peptic ulcers are the most common cause of upper GI bleeding. These are open sores on the lining of your stomach or the upper part of your small intestine, and they affect roughly 10 to 15% of people worldwide. They develop when stomach acid damages the lining, usually because of a bacterial infection (H. pylori) or regular use of anti-inflammatory painkillers like ibuprofen, aspirin, or naproxen. If you’ve been taking these medications frequently, especially without food, that’s a significant risk factor.
Other causes include irritation or inflammation of the stomach lining (gastritis), tears in the esophagus from forceful vomiting, and enlarged veins in the esophagus, which occur most often in people with serious liver disease, commonly from long-term heavy alcohol use. Bleeding from the upper GI tract sends roughly 300,000 people to U.S. hospitals each year and causes an estimated 10,000 to 15,000 deaths annually, so it’s not something to ignore.
Warning Signs That Need Immediate Attention
Black, tarry stool on its own warrants a call to your doctor. But certain accompanying symptoms signal that bleeding may be significant and you should seek care urgently:
- Dizziness or lightheadedness, especially when standing up, which suggests enough blood loss to affect your circulation
- Rapid heartbeat or feeling your heart pounding at rest
- Unusual fatigue or weakness that came on suddenly
- Pale skin
- Vomiting blood or material that looks like coffee grounds
- Abdominal pain, particularly in the upper stomach area
Any combination of black stool with these symptoms points to active bleeding that needs same-day evaluation. If you feel faint or are vomiting blood, go to an emergency room.
What Happens at the Doctor’s Office
Your doctor will likely start with a stool sample to check for hidden blood. This is a simple test that confirms whether the dark color is actually from blood or from something else. If blood is present, the next step is usually an upper endoscopy, where a thin camera is passed through your mouth into your stomach and upper intestine. This lets the doctor see the source of bleeding directly and often treat it during the same procedure.
Blood tests to check your blood count and iron levels help determine how much blood you’ve lost. If the cause turns out to be a peptic ulcer, treatment typically involves acid-reducing medication and, if H. pylori bacteria are involved, a course of antibiotics. Most ulcers heal within a few weeks of starting treatment.
When Black Stool Is Nothing to Worry About
If your stool is dark but formed (not tarry or sticky), the smell isn’t dramatically worse than usual, and you recently took bismuth medication, iron supplements, or ate dark-colored foods, you’re almost certainly fine. Stop the suspected food or supplement and see if your stool returns to its normal color within a day or two. If it does, you have your answer. If dark, tarry, foul-smelling stool persists for more than two or three days without an obvious dietary explanation, get it checked out.