Dark-colored stool is most often caused by something you ate, drank, or took as a supplement. Iron pills, bismuth-based antacids, black licorice, and blueberries can all turn stool noticeably dark or even black. Less commonly, dark stool signals bleeding somewhere in the upper digestive tract, which requires prompt medical attention. The key is knowing how to tell the difference.
Foods That Darken Stool
Several everyday foods can temporarily change stool color. Black licorice, blueberries, and blood sausage are common culprits. Dark leafy greens, blackberries, and foods with dark artificial coloring can also shift stool from brown toward dark green or black. Beets and red-dyed foods tend to produce a reddish tinge that some people initially mistake for blood.
These color changes typically appear within a day or two of eating the food and resolve just as quickly once you stop. The stool looks darker than usual but otherwise feels normal in texture. There’s no unusual smell, no stickiness, and no accompanying symptoms. If you recently ate any of these foods and your stool darkened, that’s almost certainly the explanation.
Iron Supplements and Medications
Oral iron supplements are one of the most common non-food causes of black stool. Unabsorbed iron passes through the digestive tract and darkens stool considerably, sometimes to a deep black. This is a well-known, expected side effect and not a sign of bleeding. The color change usually starts within the first few days of taking iron and continues for as long as you’re on the supplement.
Bismuth-based medications like Pepto-Bismol also produce strikingly dark stool. The mechanism is straightforward: bismuth reacts with small amounts of sulfur naturally present in your saliva and digestive system, forming bismuth sulfide, a black compound. This same reaction can temporarily darken your tongue. The effect is harmless and clears up within a few days of stopping the medication. Activated charcoal, sometimes used for digestive issues, causes the same kind of color change.
Bleeding in the Upper Digestive Tract
When dark stool isn’t explained by food or supplements, the concern shifts to bleeding. Blood from the upper digestive tract (the esophagus, stomach, or upper small intestine) turns stool jet black because hemoglobin, the protein in red blood cells, gets progressively darker as digestive enzymes break it down during its journey through the gut. The medical term for this is melena.
The most common causes of upper GI bleeding include:
- Stomach or duodenal ulcers: open sores in the stomach lining or the first part of the small intestine, often related to certain bacteria or long-term use of anti-inflammatory painkillers
- Esophageal varices: swollen veins in the lower esophagus that develop when liver scarring (cirrhosis) blocks normal blood flow, forcing blood through smaller, fragile veins that can rupture
- Gastritis: inflammation of the stomach lining, which can erode enough to cause slow bleeding
- Tears in the esophageal lining: sometimes caused by severe vomiting or retching
Any of these conditions can produce enough blood to darken stool significantly. The bleeding may be slow and chronic, causing dark stool over days or weeks, or sudden and heavy.
How to Tell Harmless Dark Stool From Melena
This is the practical question most people searching this topic really need answered. Melena looks and behaves differently from stool that’s simply been stained by food or iron pills.
Classic melena is jet black with a tarry, sticky consistency. It clings to the toilet bowl and is difficult to flush. Most distinctively, it has a particularly strong, offensive odor that’s noticeably different from normal stool. That smell is a byproduct of blood being broken down and digested in the GI tract. Stool that’s been darkened by iron, bismuth, or blueberries won’t have that same sticky texture or foul smell. It looks dark, but its consistency is otherwise normal.
If you’re taking iron supplements and notice dark stool, the simplest test is to stop the iron for a few days (if your doctor is okay with that) and see if the color returns to normal. If it does, the iron was the cause.
Warning Signs That Need Urgent Attention
Dark stool on its own can be harmless, but certain accompanying symptoms point to active bleeding that needs emergency care. If you’re passing black, tarry stool and also feel faint, dizzy, or lightheaded, that combination suggests significant blood loss. Other red flags include vomiting blood or material that looks like coffee grounds, rapid heartbeat, cold or clammy skin, and severe abdominal pain.
Even without those dramatic symptoms, persistent dark stool that you can’t explain by diet or medications is worth investigating. Slow, chronic bleeding doesn’t always cause obvious symptoms right away, but over time it leads to iron-deficiency anemia, leaving you fatigued, pale, and short of breath.
How Hidden Blood Is Detected
When stool is dark but the cause isn’t obvious, a simple stool test can check for hidden blood. The most common version is the fecal immunochemical test (FIT), which specifically detects human blood proteins. It’s more accurate than older guaiac-based tests, which had sensitivity for detecting serious conditions ranging widely from 51% to 100%. Newer FIT tests perform more consistently and are now the preferred screening method.
A positive result doesn’t automatically mean something serious. It means blood is present and further investigation, typically an upper endoscopy (a thin camera passed down the throat to examine the esophagus and stomach), is needed to find the source. If the test is negative, bleeding is effectively ruled out and the dark color is almost certainly dietary or supplement-related.