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Iron — The Complete Supplement Guide

The world's most common deficiency — and a mineral you should never megadose blindly. Forms, absorption, deficiency anaemia, correct dosing and the real overload risks.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden)Last reviewed July 2026Supplement guide

SupplementsMineralsIron

The most common nutrient deficiency in the world — and one of the few nutrients that is genuinely harmful in excess, so more is not better. For the short dietary version, see the Minerals page.

Read first. Only supplement iron if you are actually low (ideally confirmed by a blood test) — and always look for the cause. Iron overdose is a leading cause of poisoning death in children; keep supplements locked away. People with haemochromatosis (genetic iron overload) must not supplement. Information, not a prescription.

Key takeaways

  • The most common deficiency — but never megadose blindly; test first.
  • Adults need women 15–18 mg / men 8–9 mg/day.
  • Upper limit 45 mg/day (US); excess is genuinely dangerous.

Function

Iron sits at the centre of oxygen transport and energy:

  • Oxygen transport: the core of haemoglobin in red blood cells and myoglobin in muscle — it binds and delivers oxygen to every tissue.
  • Energy production: essential to the electron-transport chain in mitochondria that generates ATP.
  • DNA synthesis and cell division rely on iron-dependent enzymes.
  • Brain & cognition: needed for neurotransmitter synthesis and myelin; deficiency impairs concentration and, in children, development.
  • Immune function: supports normal defence against infection.
  • Detox & metabolism: a cofactor for many enzymes (including the liver's cytochrome P450 system).

Different forms — and which to choose

Ferrous saltsThe absorbable form. Ferrous sulfate is standard and cheap but often causes nausea/constipation; ferrous fumarate and gluconate are alternatives.
Ferrous bisglycinateChelated — better tolerated and well absorbed at lower elemental doses; a good choice if sulfate upsets your stomach.
Ferric / iron oxidePoorly absorbed (ferric maltol is a newer better-absorbed prescription option).
Heme vs non-hemeFood iron from meat/fish (heme) is far better absorbed than plant (non-heme) iron — relevant for vegetarians.

Absorption in the body

How muchVery variable: heme iron ~15–35% absorbed, non-heme ~2–20%. The hormone hepcidin regulates uptake; the body absorbs more when stores are low.
HelpsVitamin C in the same meal; taking on an empty stomach; alternate-day dosing actually improves fractional absorption vs daily.
HindersTea/coffee polyphenols, calcium, phytates (whole grains) and PPIs (low stomach acid) — separate iron from these.

Deficiency symptoms

SymptomsIron-deficiency anaemia: fatigue, pallor, breathlessness on exertion, cold hands, brittle nails, hair shedding, poor concentration, restless legs, and pica (craving ice/non-foods).
CausesBlood loss (heavy periods, GI bleeding), pregnancy, low dietary intake, and malabsorption (coeliac).
Who's at riskMenstruating and pregnant women, vegetarians/vegans, frequent blood donors, endurance athletes, and people with GI disease. Always investigate the cause, especially new anaemia in men or post-menopausal women.

Recommended intake

Women 15–18 mg · Men 8–9 mg/day

Reference intakes differ greatly by sex and life stage:

  • Men & post-menopausal women: ~8–9 mg/day
  • Menstruating women: ~15–18 mg/day
  • Pregnancy: ~27 mg/day
  • Vegetarians: aim higher (non-heme iron is less absorbed).

Best sources: red meat, liver, shellfish (heme); lentils, beans, tofu, spinach, fortified cereals (non-heme) — pair plant iron with vitamin C.12

Therapeutic use

Iron-deficiency anaemiaThe main use: typically ferrous sulfate/bisglycinate to replenish stores over 3+ months, guided by blood tests.
Malabsorption / intoleranceIV iron under medical care when oral iron fails or isn't tolerated.
Not a tonicIron is not an energy booster for people who aren't deficient — supplementing without deficiency is useless and potentially harmful.

Therapeutic doses

Typical: 40–100 mg elemental/day (or alternate-day)
RepletionCommonly 40–100 mg elemental iron/day; growing evidence that a single daily dose or alternate-day dosing absorbs as well with fewer side effects.
How to takeWith vitamin C, away from tea/coffee/calcium; expect darker stools (harmless).
RecheckConfirm response and stop when stores are refilled — don't continue indefinitely.

Toxicity & upper limit

UL: 45 mg/day (US) · EFSA sets none
Upper limit45 mg/day for adults (US), largely to limit GI upset; EFSA set no numerical UL but advises caution with supplements.1
Acute overdoseA medical emergency, especially in children — iron poisoning is potentially fatal. Store safely.
OverloadHaemochromatosis (genetic) and repeated transfusions cause iron overload damaging liver, heart and pancreas — never supplement iron in these.
InteractionsReduces absorption of thyroid medicine, some antibiotics and is itself reduced by antacids/calcium — separate by 2–4 hours.

Bottom line: iron is essential and deficiency is common — but supplement only when you're genuinely low, find the cause, use a tolerable form (bisglycinate if sulfate upsets you) with vitamin C, and stop once refilled. Never take iron "for energy" without a deficiency, and keep it away from children.

← All supplementsShort dietary version

References

Reference and upper-limit values are general adult figures from official bodies; individual needs vary. Therapeutic doses describe clinical/research use, not prescriptions.

  1. US NIH Office of Dietary Supplements. Iron — Health Professional Fact Sheet. ods.od.nih.gov
  2. EFSA Panel (NDA). Dietary Reference Values for iron. EFSA Journal 2015;13(10):4254. efsa.onlinelibrary.wiley.com

This article summarizes nutrition science from official health authorities and peer-reviewed sources for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, in kidney disease, or alongside medication.

✓
Bodil Isaksson, Licensed Biomedical Scientist (Sweden)

At Food Health Lab we explain what the science says in plain language, and we cite our sources so you can check for yourself.

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