Taking iron supplements often results in dark or black feces. This common and typically harmless side effect is a normal physiological response and usually does not indicate a serious health issue.
How Iron Changes Stool Color
Your body absorbs only a portion of the iron from a supplement. The unabsorbed iron travels through the digestive tract to the colon, where it reacts with hydrogen sulfide gas, naturally produced by gut bacteria. This reaction forms iron sulfide, a black compound responsible for the dark stool color. The more unabsorbed iron present, the darker the stool can be. This effect is particularly common with traditional, high-dose iron salts like ferrous sulfate, which leave more unabsorbed iron.
Other Reasons for Dark Stool
While iron supplements frequently cause dark stools, other factors can also lead to this change. Certain dark-pigmented foods can temporarily alter stool color, such as black licorice, blueberries, blood sausage, dark chocolate, and dark leafy vegetables. These dietary items can result in a dark brown to black stool, which is generally not a cause for concern.
Beyond diet, some medications also cause dark or black stools. Bismuth subsalicylate, found in some over-the-counter stomach remedies, reacts with sulfur in the digestive tract to create bismuth sulfide, a black compound. Activated charcoal, sometimes used to treat poisoning, is another medication that leads to black stool because it is not absorbed and passes through, carrying its dark color.
When to Consult a Doctor
While black stool from iron supplements is usually benign, it is important to distinguish it from melena. Melena is the medical term for black, tarry stools caused by bleeding in the upper gastrointestinal (GI) tract, such as the esophagus or stomach. This bleeding causes the blood to become digested, resulting in a dark, sticky, and often foul-smelling appearance.
If black stool is accompanied by other symptoms, it warrants immediate medical attention. Signs of melena include a tarry, sticky consistency, a very strong and foul odor, dizziness, weakness, lightheadedness, severe abdominal pain, or vomiting blood. While iron supplements rarely cause stomach damage leading to bleeding, if you experience these concerning symptoms, or if black stool persists after discontinuing supplements or dietary changes, seek prompt medical evaluation to rule out internal bleeding or other serious conditions.