Black stool has two broad explanations: something you ate, drank, or swallowed colored it that way, or blood from your upper digestive tract is mixing with stomach acid and turning dark as it passes through. The first is harmless. The second can be a medical emergency. Telling them apart is usually straightforward once you know what to look for.
Foods, Supplements, and Medications That Turn Stool Black
The most common reason for an unexpected black bowel movement is something completely benign that you recently put in your mouth. Black licorice, blueberries, blood sausage, and very dark chocolate can all darken stool enough to be alarming. The color change usually appears within a day or two of eating the food and clears up just as quickly once you stop.
Iron supplements are another frequent culprit. Black stools are a normal side effect of iron tablets, and since the standard tablet is 325 mg of ferrous sulfate (a dose many people take daily for anemia), this catches a lot of people off guard. The color can range from dark green to true black and persists for as long as you’re taking the supplement.
Bismuth, the active ingredient in Pepto-Bismol, also causes black stool. When bismuth meets the small amounts of sulfur naturally present in your saliva and digestive tract, it forms bismuth sulfide, a black compound that coats stool on its way out. Your tongue can turn black too. This side effect typically fades within several days after you stop taking the medication. Activated charcoal, sometimes taken for gas or as a “detox” supplement, produces the same jet-black color for similar reasons.
In all of these cases, the stool may be dark but it will have a normal texture and a normal (or at least not dramatically worse) smell. That distinction matters, as you’ll see below.
How Bleeding Turns Stool Black
When blood enters your stomach or upper small intestine, it doesn’t stay red for long. Stomach acid breaks down the hemoglobin in red blood cells and converts it into a brown compound called hematin. As that partially digested blood continues through the rest of the intestine, further oxidation turns it progressively darker. By the time it reaches the toilet, the result is a distinctive jet-black, tarry stool that doctors call melena.
The key point: the bleeding source is almost always above the lower intestine, usually in the stomach or the first section of the small intestine. The longer the blood travels through the digestive tract, the darker and more altered it becomes.
Medical Conditions That Cause Black Stool
The most common medical cause of melena is a peptic ulcer, a sore on the lining of the stomach or the upper part of the small intestine. Peptic ulcers develop mainly from two things: infection with the bacterium H. pylori and regular use of anti-inflammatory painkillers like ibuprofen, naproxen, or aspirin, especially when combined with blood thinners. These medications weaken the protective mucus layer that shields the stomach lining, letting acid eat into tissue and eventually into small blood vessels.
Gastritis, a more generalized inflammation of the stomach lining, can produce the same outcome. It shares many of the same triggers: H. pylori, painkillers, blood thinners, and heavy alcohol use. Even without a full-blown ulcer, inflamed tissue can develop shallow breaks that bleed enough to darken stool.
Other upper GI causes include esophagitis (inflammation of the esophagus, often from chronic acid reflux), Mallory-Weiss tears (small rips in the lower esophagus caused by severe or prolonged vomiting), and esophageal or stomach varices (swollen veins that can burst, most often linked to liver cirrhosis). Benign tumors and cancers of the esophagus or stomach can also weaken the GI lining and expose blood vessels, though these are less common than ulcers and gastritis.
How to Tell Harmless Black Stool From a GI Bleed
There are two reliable clues: texture and smell.
Melena from a GI bleed is jet black with a tarry, sticky consistency, almost like roofing tar. It tends to cling to the bowl. Stool that’s been stained by food, iron, or bismuth is dark but keeps its normal shape and texture. It flushes away without much fuss.
The other giveaway is odor. Melena has a distinctly foul smell that’s noticeably worse than a typical bowel movement. That smell comes from blood being broken down and partially digested as it travels through the intestine. The longer it has traveled, the stronger the odor. Stool that’s simply been dyed black by blueberries or Pepto-Bismol won’t have that same unusually offensive quality.
A small amount of upper GI bleeding may produce stool that looks more dark brown than true black, and if bleeding is also causing faster transit through the gut, the stool can be looser or watery rather than classically tarry. So texture alone isn’t a perfect guide, but combined with smell and context, it gives you a reasonable starting point.
Symptoms That Signal an Emergency
Black, tarry stool on its own warrants medical attention, but certain accompanying symptoms mean you should seek care immediately:
- Lightheadedness, dizziness, or fainting. These suggest enough blood loss to drop your blood pressure.
- A racing heartbeat. Your heart speeds up to compensate when blood volume drops.
- Vomiting blood or material that looks like coffee grounds. This confirms active upper GI bleeding.
- Severe abdominal pain. This can indicate a perforated ulcer or another acute problem.
- Weakness, confusion, or pale skin. All signs of significant blood loss.
Acute upper GI bleeding can begin suddenly and become severe quickly. If you notice tarry black stool alongside any of the symptoms above, that combination points to active bleeding that may need urgent treatment.
What Happens at the Doctor’s Office
The first step is usually a stool test that checks for hidden blood (an occult blood test). This is a simple, quick screen that confirms whether the dark color is actually from blood or from something harmless. If the test is positive, or if your symptoms are concerning enough, the next step is typically an upper endoscopy, where a thin, flexible camera is passed through the mouth into the stomach and upper intestine to locate the source of bleeding. In many cases, bleeding can be stopped during the same procedure.
Blood tests help gauge how much blood you’ve lost. Doctors look at hemoglobin levels and other markers to assess severity and decide whether you need further intervention or can be managed as an outpatient.
A Simple Way to Think Through It
If you’re seeing black stool and you’ve recently taken iron supplements, Pepto-Bismol, activated charcoal, or eaten a pile of blueberries or black licorice, the explanation is almost certainly dietary. Stop the suspect item and see if the color returns to normal within a day or two.
If you can’t connect the color to anything you’ve eaten or taken, or if the stool is sticky and tarry with an unusually strong smell, that pattern points toward blood in the GI tract. The risk is higher if you regularly take anti-inflammatory painkillers, blood thinners, or drink alcohol heavily. In that situation, getting evaluated promptly matters, because upper GI bleeding is far easier to manage when it’s caught early.