Why Do Iron Supplements Cause Constipation?
Last updated: August 2026 · Reviewed by the FactoWiki Editorial Team for clarity and source accuracy
Because most of the iron is not absorbed, and the unabsorbed fraction irritates the gut lining and disrupts the microbiome. Ferrous sulphate is the worst for this. Iron bisglycinate causes far less, and alternate-day dosing reduces it further while absorbing proportionally better.
Key takeaways
- Unabsorbed iron in the gut is what causes the constipation, not the absorbed part.
- Bisglycinate is chelated and leaves much less free iron behind.
- Alternate-day dosing means fewer side effects and better fractional absorption.
The mechanism
Only a small fraction of an oral iron dose is absorbed. The rest travels through the gut as free ferrous iron, where it generates reactive oxygen species, irritates the intestinal lining and alters the microbiome — shifting it in ways associated with constipation and discomfort. The black stools people notice are the same unabsorbed iron.
Why bisglycinate is gentler
Iron chelated to two glycine molecules is absorbed through amino acid transport routes rather than the standard ferrous iron pathway. Less free iron reaches the colon, so there is less irritation and less microbiome disruption at comparable absorption. It is the form worth trying if standard iron has defeated you.
The dosing fix
A dose of iron raises hepcidin, which blocks absorption for roughly 24 hours — so a second dose the same day, or one the next morning, absorbs poorly while still delivering the full side-effect load to your gut. Alternate-day dosing improves fractional absorption and halves the exposure. Lower doses have also performed as well as higher ones with fewer complaints.
Key ingredients to understand
The ingredients most relevant to this question, with what the evidence actually supports:
- Iron Bisglycinate — Iron bisglycinate is chelated iron with markedly better tolerability than ferrous sulphate — far less constipation and nausea — at comparable…
- Ferrous Sulfate — Ferrous sulphate is the cheapest and most-studied iron supplement and the standard first-line treatment for iron deficiency anaemia. It also…
- Iron — Iron is an essential mineral the body uses to carry oxygen in the blood. Supplements clearly help when you are iron-deficient — improving anemia…
- Psyllium Husk — Psyllium is a soluble fibre with strong, well-established evidence — it relieves constipation, modestly lowers LDL cholesterol, and can help…
Frequently asked questions
Should I take it with food?
It reduces absorption but improves tolerance. A tolerated dose with food beats an abandoned one without.
What helps the constipation itself?
Fluid, fibre and movement. Do not add a stimulant laxative long term; address the iron form and schedule instead.
Are black stools a problem?
No, that is expected. Black tarry stools with a foul smell are different and can indicate bleeding — see a doctor.
Related on FactoWiki
- All health answers
- Ingredient evidence guides
- Iron Bisglycinate — ingredient guide
- Ferrous Sulfate — ingredient guide
- How we review
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