How to Choose an Iron Supplement: Elemental Iron, Forms and Strengths

Choose an iron supplement by three things, in order: the elemental iron per daily portion printed on the label, read against the NHS figure for your group (14.8 mg a day for women aged 19 to 49, 8.7 mg for men and for women aged 50 and over); whether a blood test has shown you are low, because treatment-strength iron is a GP decision; and only then the form, since no permitted iron compound has been shown to beat another at a matched elemental dose.^{1, 6, 10} The NHS line for anyone not under a GP is that 17 mg or less a day from supplements "is unlikely to cause any harm", and above 20 mg a day every label carries an advisory statement.^{1, 5} The 200 mg ferrous sulphate and 210 mg ferrous fumarate tablets are medicines, prescribed or sold from pharmacies, not food supplements. Part of the vitamins and minerals guide.

Evidence at a glance

• Label law: a food supplement states elemental iron per daily portion with its %NRV; the NRV is 14 mg.^{2, 3}
• Prescription tablets: 200 mg ferrous sulphate holds 65 mg of iron, 210 mg ferrous fumarate 65 to 70 mg, 300 mg ferrous gluconate 37 mg (BSG guideline, 2021).⁶
• Forms compared: the bisglycinate trial most often quoted was retracted in 2026; the surviving trial set 25 mg as bisglycinate against 50 mg as sulphate, not a matched dose.^{10, 11}

Sources checked on 30 October 2026.

Elemental iron: the number that counts

Iron in a supplement is bound to a sulphate, a fumarate, a gluconate or two glycine molecules, and the compound weighs far more than the iron inside it. UK law settles which number the label leads with: a food supplement must state the amount of each mineral per daily portion, as elemental iron in milligrams, with its percentage of the nutrient reference value.³ So "14 mg iron (as ferrous fumarate)" means 14 mg of iron, 100% NRV, and the fumarate weight is not shown. Prescription tablets are named the other way round, by compound weight; the table below gives the iron inside each.^{6, 9} How to read a supplement label covers the other lines.

The 20 mg ferrous bisglycinate question. If a label reads "20 mg iron (as ferrous bisglycinate)", you are getting 20 mg of elemental iron, 143% of the NRV. If someone means 20 mg of the compound, the iron is about 5.5 mg, 39% NRV, because ferrous bisglycinate, iron (55.8) bound to two glycines, formula weight 204, is 27% iron. A UK label must give the first figure; read the %NRV column and the ambiguity disappears.³

How much iron: the NRV, the NHS figures and the 17 mg line

The %NRV on the label is of 14 mg, so 100% is about 95% of a woman's need before 50 and about 160% of the 8.7 mg a man or a post-menopausal woman needs.^{1, 2} The NHS gives one supplement figure: "Taking 17mg or less a day of iron supplements is unlikely to cause any harm", with more only on a GP's advice.¹ Above 20 mg a day the agreed label statement is "this amount of iron may cause mild stomach upset in sensitive individuals".⁵ Iron contributes to normal formation of red blood cells and haemoglobin and to the reduction of tiredness and fatigue, the authorised wordings, where low iron is the cause: in 18 trials of 1,170 non-anaemic iron-deficient adults, iron improved fatigue, and every trial recruited by ferritin.^{16, 17} Multivitamin with iron or without sets them out by group.

Which form of iron? The forms as facts

Fourteen iron sources are permitted in UK food supplements; four account for nearly every product.⁴ The table says what each is, not which is best: no trial has shown one beating another at a matched elemental dose, and UK guidance treats sulphate, fumarate and gluconate as interchangeable first choices.^{6, 10}

Food supplement, pharmacy medicine or prescription: three strengths

Three kinds of iron product sit on three shelves. A food supplement is on open sale, states its iron per daily portion against the 14 mg NRV, and carries the statutory warnings, keep out of reach of young children among them.³ Ferrous sulphate 200 mg and ferrous fumarate 210 mg are medicines: the NHS says ferrous fumarate is "used to treat or prevent symptoms of iron deficiency anaemia" and "you can get it on prescription or buy it from a pharmacy".^{7, 8} Each tablet carries 65 to 70 mg of iron, four to five times the NRV; whether to take them, at what dose and for how long is the GP's or pharmacist's call after a blood test, not a shelf decision.⁶ Iron supplement side effects covers alternate-day dosing.

Check first: ferritin and a full blood count

The trials recruited by ferritin, so ask your GP for a full blood count and a ferritin before buying anything stronger than a multivitamin dose.¹⁷ UK guidance treats a ferritin below 30 micrograms per litre as low iron stores and below 15 as absent, and you can be low with a normal haemoglobin, because the store empties first.⁶ A man or a post-menopausal woman with low iron needs the cause found: "approximately a third of men and postmenopausal women presenting with IDA have an underlying pathological abnormality, most commonly in the GI tract".⁶ Iron and heavy periods covers the commonest cause in women.

What not to take iron with

Tea and coffee: the NHS says to avoid them "during or after" a meal that supplies iron,.¹² Large separate doses of calcium, zinc and magnesium compete with iron, so a high-dose calcium tablet goes at a different meal. Two medicines carry an NHS note: levothyroxine, where "calcium and iron supplements" may not mix well with it, and doxycycline, where "medicines or supplements that contain iron or zinc" are listed; ask a pharmacist about the gap.^{13, 14} Vitamin C increases iron absorption, an authorised wording.¹⁶

Who should check first

Anyone with haemochromatosis or a family history of it, who the NHS says is "usually advised to avoid" iron supplements; anyone with thalassaemia or sickle cell disease, on their specialist's advice; anyone pregnant, whose iron goes through the midwife; and every household with children, because the NHS warns that "very high doses of iron can be fatal, particularly if taken by children".^{1, 9, 13, 14, 15}

What we don't know

• Whether bisglycinate is better tolerated than ferrous sulphate at a matched dose. The trial most cited was retracted in 2026.^{10, 11}
• Whether 14 mg a day changes anything measurable in an adult whose iron is normal. The trials used treatment doses in people with low ferritin.¹⁷
• The best drug, dose and timing for mild deficiency. The guideline says they "are not clearly defined".⁶

Frequently asked questions

What is the best form of iron supplement?

None has earned the word. Ferrous sulphate has the trial record, ferrous fumarate and gluconate are equivalent first choices in UK guidance, and bisglycinate's one surviving trial ran at half the sulphate dose; choose by the elemental iron per daily portion, then by the form you find easiest to take.^{6, 10, 11}

Can I buy iron tablets over the counter?

Food supplements with iron at or around the 14 mg NRV are on open sale. Ferrous sulphate 200 mg and ferrous fumarate 210 mg are medicines that the NHS says you can get on prescription or buy from a pharmacy, where the pharmacist checks they suit you; a treatment course for a diagnosed deficiency follows a GP's blood test.^{7, 8}

What is ferrous fumarate used for?

It is an iron salt, about a third iron by weight, licensed in 210 mg tablets for "prophylaxis and treatment of iron deficiency states" at 65 to 70 mg of iron a tablet, and a permitted source in food supplements at far lower doses. Whether you need it is a GP or pharmacist question.^{4, 7, 9}

How much elemental iron is in 20 mg ferrous bisglycinate?

If the label reads "20 mg iron (as ferrous bisglycinate)", 20 mg, because UK labels state the elemental amount. If it means 20 mg of the compound, about 5.5 mg, because the compound is 27% iron by formula weight.³

Sources

1. NHS. Vitamins and minerals: iron. Reviewed 3 August 2020. nhs.uk.
2. Regulation (EU) No 1169/2011 as retained in UK law, Annex XIII, nutrient reference values. legislation.gov.uk.
3. The Food Supplements (England) Regulations 2003, regulation 6 (labelling). legislation.gov.uk.
4. Commission Regulation (EC) No 1170/2009, Annex II, permitted vitamin and mineral sources, as retained in UK law. legislation.gov.uk.
5. Department of Health. Food supplements: label advisory statements. Agreed May 2004; PDF dated 31 May 2013. gov.uk.
6. Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut, 2021. Clinical guideline; some authors declare advisory work for iron manufacturers. PubMed 34497146.
7. NHS. Ferrous fumarate. Reviewed 13 March 2026. nhs.uk.
8. NHS. Ferrous sulfate. Reviewed 9 February 2023. nhs.uk.
9. Ferrous Fumarate 210 mg Tablets, Summary of Product Characteristics, certified November 2025. medicines.org.uk.
10. Retraction statement: efficacy of ferrous bis-glycinate versus ferrous glycine sulfate in the treatment of iron deficiency anemia with pregnancy. Journal of Maternal-Fetal and Neonatal Medicine, 2026, retracting a 2019 trial of 187 pregnant women (PubMed 29843553). PubMed 41883305.
11. Milman N, et al. Ferrous bisglycinate 25 mg iron is as effective as ferrous sulfate 50 mg iron in the prophylaxis of iron deficiency and anemia during pregnancy in a randomized trial. Journal of Perinatal Medicine, 2014. Randomised double-blind trial, 80 women; funding not stated on PubMed. PubMed 24152889.
12. NHS. The vegan diet. Reviewed 5 June 2026. nhs.uk.
13. NHS. Levothyroxine. Reviewed 6 July 2026. nhs.uk.
14. NHS. Doxycycline. Reviewed 5 September 2025. nhs.uk.
15. NHS. Haemochromatosis. Reviewed 29 March 2023. nhs.uk.
16. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Last updated 19 May 2026. gov.uk.
17. Houston BL, et al. Iron supplementation, fatigue and physical capacity in non-anaemic iron-deficient adults: systematic review. BMJ Open, 2018. 18 trials, 1,170 adults; no specific grant. PubMed 29626044.

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General information, not medical advice. Take treatment-strength iron only after a blood test shows you need it and at the dose your GP or pharmacist sets. Keep every iron product out of children's reach.