Oral iron is one of the most commonly supplied medicines in UK community pharmacy during pregnancy, but the choice of salt is rarely explained at the counter. Ferrous sulfate is the cheapest, most widely stocked and most often prescribed. It is also the most likely to cause nausea and constipation. Ferrous fumarate and ferrous gluconate are alternatives that deliver a similar amount of elemental iron and often prove easier to stay on.

This is the comparison a UK pharmacy is usually making on behalf of its customers — whether prompted or not.

Why elemental iron is the number that matters

The labels say different things — 200mg ferrous sulfate, 210mg ferrous fumarate, 300mg ferrous gluconate — but the quantity that affects haemoglobin is the elemental iron inside each tablet. NICE CKS and the BNF both frame doses this way.

For treatment of iron-deficiency anaemia, UK guidance recommends 100–200 mg elemental iron daily. In practice that tends to be one 200 mg ferrous sulfate twice daily, one 210 mg ferrous fumarate twice daily, or two to three 300 mg ferrous gluconate tablets twice daily. Prophylactic doses (e.g. for women on multiple pregnancies or with known low stores) are lower.

Tolerability — where the real choice is

All three salts cause the same family of side effects: nausea, abdominal discomfort, constipation, black stools, and occasional diarrhoea. Randomised data is mixed on whether fumarate and gluconate are objectively better tolerated than sulfate, but UK clinical practice has long accepted that individual patients often do notably better on one than another.

Ferrous sulfate is usually tried first. It is cheapest on the NHS drug tariff and most widely stocked. Side effects tend to be most pronounced in the first week and partially settle.

Ferrous fumarate delivers a broadly similar elemental iron dose with a frequently better GI tolerability profile. It is a common switch when sulfate is causing constipation or nausea severe enough to threaten adherence.

Ferrous gluconate contains less elemental iron per tablet, so treatment doses involve more tablets per day. It is often the best-tolerated option, making it a reasonable third choice when both sulfate and fumarate have caused problems.

Practical tips for staying on iron in pregnancy

The pharmacy counter can offer a surprisingly high-impact set of small adjustments:

• Take with orange juice or a vitamin C-containing drink — ascorbate improves absorption and is widely recommended by NHS patient information.
• Separate from tea, coffee and dairy by at least an hour — tannins and calcium reduce absorption.
• Take with a meal if nausea is the main problem. Absorption is slightly reduced but completion of the course matters more.
• Every other day dosing — a more recent body of evidence, reflected in NICE CKS commentary, suggests alternate-day dosing may achieve similar haemoglobin rises with fewer side effects. Worth asking the prescriber about if adherence is the barrier.
• Expect black stools. Stools will turn dark; this is harmless and not internal bleeding.

What the pharmacy should not do

A UK community pharmacy will not unilaterally switch a prescribed iron salt without consulting the prescriber, because the prescription specifies what to dispense. But where iron is bought over-the-counter for prophylaxis or mild iron deficiency — and the woman's GP or midwife has not specified a formulation — the pharmacist can offer an alternative.

Intravenous iron — not a pharmacy decision

For pregnant women who cannot tolerate oral iron, cannot absorb it adequately, or present late in pregnancy with severe anaemia, intravenous iron preparations (ferric carboxymaltose, ferric derisomaltose) are options in secondary care. These are hospital or specialist-led and not something the community pharmacy supplies.

When to refer

• Haemoglobin not rising after 2–4 weeks of good adherence
• Severe side effects forcing stopping
• Pica, unusual cravings, breathlessness, or palpitations suggesting severe anaemia
• Rectal bleeding, unexplained weight loss or suspicion of blood loss from another source
• Coeliac disease or other suspected malabsorption

Summary

Iron in pregnancy is a tolerability problem more than a pharmacology problem. Sulfate → fumarate → gluconate is a reasonable ladder, with every-other-day dosing and vitamin C co-administration as counter-level adjustments. For many women, the pharmacy's quietest useful intervention is pointing out that a different iron salt exists.

See our constipation in pregnancy guide for the related management.

Sources

• NICE CKS — Iron-deficiency anaemia
• British National Formulary — Iron-deficiency anaemia treatment summary
• NHS — Ferrous sulfate patient information

General information for UK pharmacy customers. Prescribing decisions and switches in pregnancy should be confirmed with the GP, midwife or pharmacist.