Do Iron Supplements Change the Color of Your Poo?

January 10, 2026 •

4 min read

According to a 2021 review, oral iron supplementation is associated with significant gastrointestinal side effects, including a noticeable change in stool color. It is very common and completely normal for people taking iron supplements to observe their stool turning a darker shade, often black or dark green. This change occurs due to unabsorbed iron passing through the digestive system and is not a cause for concern in most cases.

Quick Summary

Taking oral iron supplements frequently results in dark green or black stools. This is a common and harmless side effect caused by unabsorbed iron reacting in the digestive tract. It is important to distinguish this from the black, tarry stool that can indicate gastrointestinal bleeding, which may have additional symptoms like abdominal pain and dizziness.

Key Points

Normal and Harmless: Iron supplements, especially in oral form, commonly cause stools to turn a dark green or black color, which is a benign side effect.
Chemical Reaction: The color change happens when unabsorbed iron from the supplement reacts with sulfur in the digestive tract, forming black iron sulfide.
Distinguish from Melena: It is important to distinguish this effect from melena, which is black, tarry, and foul-smelling stool caused by digested blood in the GI tract.
Other Side Effects: Iron supplements can also cause other gastrointestinal issues like constipation, nausea, and stomach pain.
Managing Discoloration: Strategies to minimize the effect include adjusting your dosage, taking supplements with food, or switching to a different type of iron.
When to See a Doctor: Seek immediate medical attention if black stools are tarry, have a foul odor, or are accompanied by severe abdominal pain, dizziness, or vomiting.

Why Iron Supplements Change Stool Color

It's a common and often startling experience to find that your stool has turned a very dark green or black after starting iron supplements. The reason behind this phenomenon is a simple and harmless chemical reaction within the digestive system. The body only absorbs a small percentage of the iron ingested from a supplement, and the rest must be eliminated. As this unabsorbed iron travels through the intestines, it reacts with sulfur compounds to form iron sulfide, a substance that is black in color. The more unabsorbed iron there is, the darker the stool will appear.

The Role of Absorption and Dosing

The extent of this color change is often dose-dependent, meaning higher doses are more likely to result in darker stools. Different types of iron supplements also have varying absorption rates and side effect profiles. For example, ferrous sulfate, a commonly prescribed form, is known to cause more gastrointestinal distress and color changes than some other forms, such as iron bisglycinate. Taking supplements on an empty stomach, which enhances absorption, can also increase the concentration of unabsorbed iron that reaches the lower digestive tract, contributing to the darkening effect.

Other Potential Digestive Side Effects

Beyond just changing the color of your stool, iron supplements are also known to cause other digestive issues. A study published in the journal Nutrients notes that up to 60% of people taking oral iron report gastrointestinal side effects. These can significantly impact a person's willingness to adhere to their treatment plan. Common side effects include:

Constipation: A very frequent side effect, where iron slows down bowel movements and can lead to harder, more difficult-to-pass stools. Staying hydrated and increasing dietary fiber can help.
Nausea and Stomach Pain: Iron can irritate the stomach lining, which may lead to discomfort, especially if taken on an empty stomach.
Diarrhea: While less common than constipation, some individuals may experience the opposite effect.
Bloating and Gas: Excess iron in the gut can alter the gut microbiota, potentially leading to an increase in gas-producing bacteria.

Distinguishing Iron-Induced Stool from Medical Issues

While black stool from iron supplements is typically not a concern, it is crucial to differentiate it from a more serious condition known as melena. Melena is black, tarry stool caused by digested blood from the upper gastrointestinal (GI) tract. The key is to pay attention to the consistency, odor, and accompanying symptoms.

Comparison Table: Iron-Induced Stool vs. Melena


Feature	Iron-Induced Black Stool	Melena (Blood in Stool)
Appearance	Dark green to black, often firm consistency.	Jet black, shiny, and sticky or tarry consistency.
Odor	Generally no foul or distinct odor.	Distinctly foul, offensive odor due to digested blood.
Associated Symptoms	Often accompanied by constipation, nausea, or stomach cramps.	Can be accompanied by dizziness, weakness, abdominal pain, or vomiting.
Chemical Test	Fecal occult blood test is negative.	Fecal occult blood test is positive for blood.
Onset	Occurs shortly after starting iron supplementation.	Can appear at any time and persist until the bleeding source is addressed.

If you are taking iron supplements and your black stools are not tarry, do not have a bad odor, and are not accompanied by other worrying symptoms, the cause is likely the supplement. However, if you have any doubts, it is always best to consult a healthcare provider.

Conclusion

In summary, observing a change in stool color to a dark green or black shade while taking iron supplements is a normal and expected side effect. It is a direct result of unabsorbed iron combining with sulfur in the digestive system. Although startling, this color change is typically harmless and indicates that the supplement is passing through your system. The key is to recognize the difference between this benign effect and the signs of a serious gastrointestinal bleed. By being mindful of the physical characteristics of your stool and any accompanying symptoms, you can easily distinguish between the two. If you have concerns, experiencing additional symptoms like tarry stools, foul odor, or abdominal pain, it's crucial to seek medical advice for proper diagnosis and peace of mind. For more detailed information on managing side effects, you can visit a reliable source on the topic, such as the Mega Lifesciences guide on managing iron supplementation side effects.

How to Manage Iron-Induced Stool Color Change

Consider adjusting your intake by taking smaller, spaced-out doses, which can sometimes reduce the amount of unabsorbed iron passing through the gut.
Taking the supplement with a small amount of food can also reduce stomach irritation and potentially moderate the color change, though it may slightly impact absorption.
Ensure you are adequately hydrated and maintain a high-fiber diet to counteract common iron-related constipation.
Discuss switching to an alternative form of iron, like iron bisglycinate or ferrous gluconate, with your doctor, as these may be gentler on the digestive system.

When to Consult a Doctor

If you experience black, tarry stools with a noticeably foul odor, seek medical attention immediately.
Consult your doctor if you have severe abdominal pain, vomiting, or persistent symptoms in addition to the color change.
Do not stop taking a prescribed iron supplement without speaking to your healthcare provider first.

Frequently Asked Questions

Yes, it is very normal and common for iron supplements to cause a change in your stool color, often making it appear dark green or black. This is a harmless side effect and indicates that the iron is passing through your system.

Black stools from iron are typically not sticky or tarry and do not have a particularly foul odor. Black, tarry, and foul-smelling stools, known as melena, often indicate internal bleeding and may be accompanied by symptoms like dizziness or abdominal pain.

The change in color is a result of a chemical reaction. When unabsorbed iron travels through your intestines, it reacts with sulfur compounds to create black iron sulfide, which is then excreted in your stool.

Besides changing stool color, other common gastrointestinal side effects of iron supplements include constipation, nausea, stomach cramps, and diarrhea.

You can try adjusting your dose, taking the supplement with a small amount of food, or asking your doctor about a different, potentially gentler form of iron, such as ferrous gluconate or iron bisglycinate.

No, you should never stop a prescribed iron supplement without consulting your doctor first. Stopping can lead to the return of iron deficiency symptoms. Your doctor can help you find a management strategy or an alternative.

Oral iron supplements are most likely to cause this side effect. While the effect varies by dose and formulation, unabsorbed oral iron is what triggers the reaction in the gut. Alternative options like intravenous iron do not cause this.