Why does iron change stool color? The science explained

January 6, 2026 •

4 min read

According to the National Institutes of Health, taking iron supplements can lead to a change in stool color. This common side effect, which often results in black or dark green feces, is caused by unabsorbed iron reacting with other substances in your digestive tract and is generally considered harmless.

Quick Summary

Unabsorbed iron, typically from supplements, combines with sulfur compounds in the colon to produce a black substance called iron sulfide. This chemical reaction is the reason for darkened stools, which can range from dark green to black, and is a normal, benign occurrence during iron supplementation. It does not indicate gastrointestinal bleeding.

Key Points

Chemical Reaction: Unabsorbed iron from supplements reacts with sulfur in the colon to form black-colored iron sulfide.
Harmless Side Effect: Darkening of stool to a black or dark green color is a normal and harmless side effect of oral iron supplementation.
Distinguish from Melena: Iron-induced dark stools are different from melena (digested blood), which is tarry, sticky, and has a foul odor.
Dose Dependent: Higher doses of iron supplements can increase the amount of unabsorbed iron, leading to more pronounced stool discoloration.
Not a Sign of Bleeding: The change in stool color due to iron is not indicative of gastrointestinal bleeding.
Improve Absorption: Taking iron with vitamin C can enhance absorption and potentially reduce unabsorbed iron.

The Science Behind the Stool Color Change

When you ingest an iron supplement, your body's small intestine attempts to absorb the mineral. However, only a portion of the iron is actually absorbed, especially from high-dose supplements like ferrous sulfate. The unabsorbed iron continues its journey through the digestive system until it reaches the large intestine, or colon.

Inside the colon, the remaining unabsorbed iron comes into contact with naturally occurring sulfur compounds. These sulfur compounds are produced by bacteria that aid in digestion. The chemical reaction between the unabsorbed iron and the hydrogen sulfide gas produced by these bacteria creates a black-colored substance called iron sulfide. It's the presence of this new compound that directly changes the color of your stool to dark green or black.

Factors Influencing the Color

The intensity of the color change can vary depending on several factors, including the type of iron supplement and the dosage. Higher doses of iron supplements generally lead to more unabsorbed iron in the colon, resulting in a darker, more pronounced black color. Some individuals may also experience dark green stool, which is also a normal variation of this side effect.

Certain forms of iron are known to be more or less reactive in the gut. For instance, traditional iron salts like ferrous sulfate are more likely to cause significant stool discoloration due to lower absorption rates compared to newer, chelated forms like iron bisglycinate. The speed at which your digestive system processes food can also play a role, with faster transit times sometimes leading to greener stools.

Is Black Stool from Iron Dangerous?

For individuals who have recently started taking iron supplements, black or dark green stool is an expected and harmless side effect. It is not a sign of internal bleeding, which is a common but incorrect concern. However, it is crucial to differentiate between iron-induced black stools and a more serious condition known as melena, which is caused by digested blood in the stool.

Comparison Table: Iron-Induced Black Stool vs. Melena


Feature	Iron-Induced Black Stool	Melena (Blood in Stool)
Appearance	Dark brown to black; dense and often well-formed.	Sticky, tarry, and shiny black.
Odor	May have a mild, metallic, or slightly sulfurous smell.	Characterized by a distinctively strong, foul odor.
Associated Symptoms	Typically no other symptoms aside from potential GI discomfort.	Can be accompanied by dizziness, weakness, and abdominal pain.
Hemoccult Test	Negative (no hidden blood detected).	Positive (indicates presence of occult blood).

It is always wise to contact a healthcare provider if you are concerned, especially if your black stool is tar-like, has a foul smell, or if you experience other symptoms like severe abdominal pain or fatigue.

Other Common Gastrointestinal Side Effects

Beyond stool discoloration, iron supplements can cause other digestive issues that may affect your gut and stool consistency.

Constipation: This is one of the most common side effects of oral iron therapy. Staying hydrated and increasing fiber intake can help manage it.
Nausea: Many people experience nausea, which can sometimes be reduced by taking the supplement with a small amount of food.
Diarrhea: While less common than constipation, diarrhea can also occur.
Abdominal Pain: Stomach cramps and general gastrointestinal discomfort can be side effects, often due to the unabsorbed iron.

How to Mitigate Digestive Issues

There are several strategies to help reduce the digestive impact of iron supplements, including managing the resulting constipation and minimizing the unabsorbed iron. Taking the supplement with vitamin C can enhance absorption, potentially reducing the amount of unabsorbed iron that reaches the colon. Avoiding certain foods and beverages, such as milk, coffee, and tea, around the time of supplementation can also improve absorption. If side effects are persistent, a healthcare provider might suggest a different type of iron or a lower, more frequent dosing schedule.

Conclusion

In summary, the color change in your stool while taking iron supplements is a normal and expected chemical reaction. Unabsorbed iron combines with sulfur compounds in the gut, forming harmless iron sulfide that appears black or dark green. It is a benign side effect that demonstrates the iron is moving through your system and is not typically a sign of serious health issues like internal bleeding. By understanding the causes and recognizing the signs that may require medical attention, you can manage this common side effect with confidence. If you have any concerns or experience other severe symptoms, always consult your doctor.

For more information on iron deficiency and treatment options, the World Health Organization provides global health guidelines and resources. More info

Frequently Asked Questions

Black stool indicates that the iron is passing through your system, but it doesn't necessarily confirm that your body is effectively absorbing it. Clinical signs like increased energy and rising hemoglobin levels are better indicators of efficacy.

While iron supplements are the main cause, large amounts of iron-rich foods, especially those from animal sources, can also lead to darker stools, though this is less common.

The discoloration will continue as long as you are taking the iron supplement. Your stool color should return to normal within a few days of stopping the supplement.

It is common for children on iron supplements to have dark stools. However, it is always best to check with a pediatrician, especially if the stool appears very tarry or is accompanied by other symptoms.

Yes, other causes include certain foods (like black licorice or blueberries) and medications (such as bismuth subsalicylate, found in Pepto-Bismol). A serious cause is bleeding in the upper gastrointestinal tract.

Black stools from iron are dark but not tarry or sticky. Tarry stool, known as melena, is sticky, foul-smelling, and can be a sign of internal bleeding.

To reduce side effects like constipation and nausea, you can try taking the supplement with a small amount of food, increasing fiber intake, and staying hydrated. Some people also benefit from switching to a more gentle form of iron.