Why does iron upset my stomach?

Ferrous salts (sulfate, fumarate, gluconate) release a high local concentration of free ferrous iron in the gut, which generates reactive oxygen species and irritates the gastric mucosa. This causes nausea, abdominal pain, and altered bowel habit in around 30–50% of users. Options to reduce this: switch to a different ferrous salt, try iron bisglycinate or Sucrosomial iron (better tolerability profiles in trials), reduce the dose, switch to alternate-day dosing, or take with a small low-dairy meal.

Is bisglycinate worth the extra cost?

If you tolerate ferrous sulfate well, no — sulfate is cheap and effective. If you don't tolerate sulfate, bisglycinate is one of the better-evidenced alternatives. The Pineda & Ashmead 2001 trial and subsequent reviews show comparable efficacy with fewer GI side effects. Realistic 2026 UK pricing puts bisglycinate at £10–£25/month versus £3–£8/month for prescribed sulfate, so the premium is meaningful but not extreme.

Should I take iron daily or alternate days?

The Stoffel 2017 and 2020 trials in The Lancet Haematology suggest alternate-day dosing produces equal or better total absorbed iron with fewer side effects, because daily dosing raises hepcidin and blunts absorption from subsequent doses. NICE and BNF acknowledge alternate-day dosing as a reasonable option. If you are currently on a daily NHS-prescribed regimen, do not change without speaking to your GP — but alternate-day dosing is worth discussing.

Do I need a blood test before starting iron?

Yes. The NHS strongly recommends a serum ferritin test before iron supplementation. There are three reasons: hereditary haemochromatosis (around 1 in 200 in UK ancestry) makes supplementation actively harmful; iron deficiency in adult men and post-menopausal women can be the first sign of bleeding from the gut and must be investigated; and other anaemias mimic iron deficiency but need different treatment. Self-supplementing without a ferritin test is one of the riskiest things you can do in the supplement aisle.

How long until ferritin recovers?

Haemoglobin typically rises within 2 weeks and normalises within 2–3 months on effective oral iron. Ferritin recovers more slowly. NICE and the British Society of Gastroenterology recommend continuing supplementation for at least 3 months after haemoglobin has normalised, with a target ferritin of at least 30–50 micrograms/L. Total typical treatment course is 3–6 months, after which you should retest rather than continuing indefinitely.

Can I take iron with my coffee?

No. Polyphenols in coffee, black tea, and red wine bind iron in the gut and reduce absorption substantially — Hurrell et al. 1999 reported reductions of up to 60% with tea. Wait at least 1–2 hours either side of an iron dose before drinking tea or coffee. Calcium also reduces absorption modestly, so avoid taking iron at the same time as a glass of milk or a calcium supplement. Vitamin C, by contrast, modestly enhances non-heme iron absorption.

Iron Supplements UK: How to Pick the Right Form Without Wrecking Your Gut

Nutripedia Research Team4 April 2026

Iron is one of the most-needed and worst-tolerated supplements on the UK market. This is what the evidence says about ferrous sulfate vs bisglycinate, why alternate-day dosing may beat once-daily, and why you should never start iron without a ferritin test first.

Not medical advice

Nutripedia summarises published peer-reviewed research. This content is for informational purposes only and is not a substitute for professional medical advice. Product mentions are not endorsements.

Before You Read This

Nutripedia is a research librarian, not a doctor. Nothing on this page is medical advice. Consult a UK GP, NHS pharmacist, or registered dietitian before starting any supplement, especially if you are pregnant, breastfeeding, on medication, or managing a chronic condition. Iron is unusual among supplements: oversupplementation is genuinely harmful, and undiagnosed iron-deficiency anaemia in adults — particularly men and post-menopausal women — can be the first sign of bleeding from the gut, including from cancer. Do not start iron without first confirming a deficiency through blood testing. The NHS strongly recommends a GP investigation of any new iron deficiency in adult men and post-menopausal women before treatment.

Our research is based on 67 peer-reviewed studies. View the full evidence database

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Nutripedia is an educational resource. Content is sourced from peer-reviewed studies and does not constitute medical advice. Product mentions are not endorsements. Consult a healthcare professional before starting any supplement.

Reviewed by

Archie Roberts

Founder, Nutripedia — ALDR Ltd

This page summarises published research from PubMed, NHS, EFSA, and SACN. It does not constitute medical advice; consult a qualified healthcare professional before changing any supplement regimen.

Last reviewed: 04 Apr 2026Methodology