Black stool is usually caused by something you ate or a medication you’re taking, and it resolves on its own once that substance clears your system. The most common culprits are iron supplements, bismuth-based medications like Pepto-Bismol, black licorice, blueberries, and blood sausage. However, jet-black stool that looks tarry and feels sticky can signal bleeding in the upper digestive tract, which needs medical attention.
The fix depends entirely on the cause. Here’s how to figure out what’s behind it and what to do next.
Check Your Medications and Diet First
Before worrying, think back over the last day or two. The most common causes of harmless black stool are:
- Iron supplements: Black stool from iron is actually unabsorbed iron passing through. It happens when the dose exceeds what your intestines can absorb, or when the formula has low bioavailability.
- Bismuth medications: Pepto-Bismol and similar products turn stool black through a chemical reaction with sulfur in your digestive tract.
- Foods: Black licorice, blueberries, blood sausage, and dark-colored foods can temporarily darken stool.
- Activated charcoal: Often taken for digestive issues or detox products, it passes through and darkens everything along the way.
If any of these apply, you’ve almost certainly found your answer. The color change is cosmetic, not dangerous.
How to Fix Iron-Related Black Stool
If you’re taking iron supplements and the dark stool bothers you, you have a few options. The black color means some of the iron isn’t being absorbed, which suggests you may be taking more than your body can use in a single dose. Splitting your dose into smaller amounts taken at different times can help your intestines absorb more and leave less to discolor your stool.
The type of iron matters too. Research comparing different iron formulas found that ferrous bisglycinate produced the lowest rate of black stools, likely because it delivers a smaller amount of elemental iron per dose. Ferrous fumarate and ferrous sulphate, the more common forms, caused black stool at higher and roughly equal rates. If the color change is discouraging you from sticking with your supplements, switching to a bisglycinate formula is worth trying.
That said, don’t stop taking iron supplements just because of stool color. Black stool from iron isn’t harmful. It’s a known side effect that doctors consider separate from actual gastrointestinal problems.
How to Fix Bismuth-Related Black Stool
If Pepto-Bismol or a similar bismuth product is the cause, the fix is simple: stop taking it. According to the NHS, the black color typically fades within several days after your last dose. It can also turn your tongue black, which clears up on the same timeline. If your stool is still black several days after stopping, that’s worth a conversation with your doctor since something else may be going on.
When Black Stool Signals Something Serious
There’s an important difference between stool that’s simply dark-colored and true melena, the medical term for stool caused by bleeding in the upper digestive tract. Melena is jet black, has a tarry consistency, and tends to be noticeably sticky. Clinicians describe it by comparing the color to a black object, not just “dark brown.” It also has a distinctive, unusually foul smell that most people notice right away.
When blood enters the stomach or upper intestine, digestive acids break it down as it travels through the gut. By the time it reaches the colon, the blood has been chemically transformed into a dark, tar-like substance. This is different from bright red blood in the stool, which typically comes from the lower colon or rectum and points to a different set of causes.
Upper GI bleeding can come from ulcers, inflammation of the stomach lining, tears in the esophagus, or other conditions. The bleeding can range from mild to severe.
Symptoms That Need Immediate Attention
If your black stool is tarry and sticky and you haven’t taken iron, bismuth, or eaten anything that explains it, get medical care promptly. Seek emergency help if you also experience any of these:
- Lightheadedness or fainting
- Rapid heartbeat or rapid breathing
- Vomiting blood (which may look like coffee grounds)
- Abdominal pain or chest pain
- Weakness or unusual fatigue
- Nausea
- Pale or grayish skin, lips, or fingernails
These can be signs of significant blood loss. Because the body is losing whole blood, standard blood tests may not show the drop in blood count for four to six hours, so don’t wait for confirmation if you feel unwell.
What Happens if You Need Testing
If a doctor suspects the black stool is from internal bleeding, the process usually starts with blood work and stool tests. Blood tests check for anemia, clotting ability, and liver function. A stool sample can detect hidden blood that isn’t visible to the eye.
The most common next step is an upper endoscopy, where a thin tube with a camera is passed down the throat to examine the esophagus, stomach, and upper small intestine. This lets the doctor both find and often treat the source of bleeding in the same procedure. If the upper tract looks clear, a colonoscopy examines the lower digestive system. For harder-to-reach areas of the small intestine, you might swallow a pill-sized camera that takes thousands of images as it passes through, or undergo a specialized scope procedure.
Most people with upper GI bleeding respond well to treatment once the source is identified. Recovery depends on the cause, but many bleeding ulcers and similar issues can be addressed during the endoscopy itself.
A Quick Way to Sort It Out
Start with the simplest explanation. Review what you’ve eaten and what medications or supplements you’ve taken in the last 48 hours. If you find a clear match (iron, Pepto-Bismol, black licorice, blueberries), stop or adjust the item and wait a few days. Normal brown stool should return once the substance is out of your system.
If you can’t identify an obvious cause, pay attention to the stool’s texture. Dark stool that looks and feels normal is far less concerning than stool that’s tar-like, sticky, and unusually foul-smelling. The latter, especially combined with fatigue, dizziness, or abdominal pain, warrants a prompt medical evaluation rather than a wait-and-see approach.