Does Iron Cause Constipation? How to Manage It | Pharmacist Advice
Updated: Sep 8
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The answer is yes!
Iron supplements can cause constipation, but the type of iron, the amount of elemental iron, and how the product releases that iron can all affect how well you tolerate it.
What Is Iron Needed For?
Iron is an important mineral your body needs to make hemoglobin, the part of red blood cells that carries oxygen throughout your body.
Low iron can lead to iron deficiency or iron-deficiency anemia, which may leave you feeling tired, weak, short of breath, or less able to concentrate. That is why, if you have been told to take iron, the goal is usually not to stop it because of constipation, but to find a form and dose that you can tolerate.
Why Does Iron Cause Constipation?
Iron can cause constipation because not all of the iron you take is absorbed. The unabsorbed iron stays in the digestive tract, where it can irritate the gut and affect normal bowel movement and stool consistency.
Higher elemental iron doses may be more likely to cause constipation and other stomach-related side effects. But before adding a laxative, I would first look at the iron product itself. Sometimes switching formulations is enough to make a noticeable difference. Let’s break down the different forms to help you choose.
Ferrous Sulfate: Effective, but Often Harder on the Stomach
Ferrous sulfate (Iron 65mg) is one of the most commonly used forms of iron. It is inexpensive and effective, but ferrous sulfate can also cause constipation, nausea, stomach discomfort, and dark stools in some people.
Ferrous Fumarate: More Elemental Iron per Amount of Iron Salt
Ferrous fumarate contains a higher percentage of elemental iron than ferrous sulfate, so a relatively small tablet can provide a substantial amount of iron.
That can be useful when you need more iron, but a higher elemental iron dose may also be harder to tolerate if you are already sensitive to constipation or stomach upset.
Ferrous Gluconate: Often a Lower-Dose Option
Ferrous gluconate usually provides less elemental iron per tablet than many ferrous sulfate or ferrous fumarate products. For someone who becomes constipated with a higher-dose iron product, the lower elemental iron amount may sometimes be easier to tolerate.
Iron Bisglycinate: A Gentler Alternative for Some People
Iron bisglycinate is a chelated form of iron in which iron is bound to the amino acid glycine. This formulation may improve absorption, which can reduce the amount of unabsorbed iron left in the intestine.
Slow-Release Iron: Less Iron Released at Once
Slow Fe®, for example, uses a controlled-release formulation and provides 45 mg of elemental iron per tablet.
The idea is simple: instead of releasing the entire amount of iron at once, the iron is released more gradually, which some people may find easier on the stomach.
What Does Vitamin C Help With?
Vitamin C can help your body absorb iron. The more iron that is absorbed, the less unabsorbed iron remains in the intestine, the less constipation expected.
What If Changing the Iron Still Does Not Fix the Constipation?
If you need to continue your iron and your stool becomes hard and difficult to pass, a stool softener may be a reasonable next step.
A product such as Colace® (docusate) is designed to soften stool and may be helpful when hard stool is the main problem. For mild iron-related constipation, this may be enough.
What If the Stool Softener Is Not Effective Enough?
If the stool softener is not helping enough, consider MiraLAX® (polyethylene glycol 3350). MiraLAX® works differently. It helps keep more water in the stool, making it softer and easier to pass.
For someone who needs to continue iron but still has constipation despite a stool softener, MiraLAX® may be the more useful next step.
For more constipation relief solutions, visit our Constipation Relief page
Quick Recap
If your iron is causing constipation, do not immediately assume you need to stop taking it. First, compare the formulation and elemental iron dose.
If changing the iron does not solve the problem, consider a stool softener such as Colace® when hard stool is the main issue.
If that is not effective enough, MiraLAX® may provide more relief while allowing you to continue your iron treatment.
Pharmacist Tip
Iron-related constipation can depend on both the dose and the formulation.
If switching the iron formulation does not improve the constipation, consider a stool softener for mild hard stools or MiraLAX® when more help is needed.
Pharmacist Reviewed: the information in this article was clinically reviewed by Samer Altameemi, a licensed pharmacist. It is intended for educational purposes, not to substitute for a doctor's professional medical advice.
