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Strong evidenceGeneral health

Iron

Iron is one of the few supplements with unambiguous, large benefits — but only if you are actually deficient. Taking it without a confirmed low ferritin is at best pointless and at worst dangerous, and how you dose it matters more than most people realise.

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What the science says

  • Prevents anaemia and iron deficiency in pregnancy Strong evidence

    A Cochrane review of 61 trials (over 43,000 women) found daily oral iron reduced maternal anaemia at term by 70% (RR 0.30, 95% CI 0.19-0.46) and iron deficiency at term by 57% (RR 0.43, 95% CI 0.27-0.66), with fewer low-birthweight babies (8.4% versus 10.3%, RR 0.84). This is the strongest evidence base for any iron indication.

  • Reduces fatigue in non-anaemic women with low ferritin Moderate evidence

    A randomised trial in 198 menstruating women with unexplained fatigue and ferritin below 50 mcg/L found 80 mg/day ferrous sulfate for 12 weeks reduced fatigue by 47.7% versus 28.8% on placebo (difference -18.9%, 95% CI -34.5 to -3.2, P=0.02). There was no effect on quality of life, depression or anxiety, and the result applies only to women with genuinely low ferritin — not to tired people in general.

  • Alternate-day single doses absorb better than daily split doses Moderate evidence

    A dose of iron raises hepcidin for 24 hours or more, blunting absorption of the next dose. In two randomised absorption studies in iron-depleted women, alternate-day dosing gave higher fractional absorption (21.8% versus 16.3%) and more total absorbed iron (175.3 mg versus 131.0 mg, P=0.0010) than consecutive-day dosing at the same total intake, and twice-daily splitting was worse still. Alternate-day dosing also halves the GI side-effect exposure.

Dosage & safety

Studied doseFor documented deficiency: 60-120 mg elemental iron, given as a single morning dose on alternate days rather than split through the day. Ferrous sulfate 325 mg provides 65 mg elemental; ferrous gluconate 325 mg provides 38 mg; ferrous bisglycinate is gentler on the gut at comparable absorption. Vitamin C modestly aids uptake; coffee, tea, calcium, dairy and phytate inhibit it. Recheck ferritin at 3 months — refilling stores typically takes 3-6 months beyond haemoglobin normalisation.
SafetyConstipation, black stools, nausea and abdominal pain are common and dose-related, and are the main reason people abandon treatment. Acute overdose is a leading cause of fatal poisoning in young children — keep it locked away. Do not supplement without a documented low ferritin: people with hereditary haemochromatosis, thalassaemia or other iron-loading disorders can be seriously harmed, and excess iron may feed gut pathogens. Iron reduces absorption of levothyroxine, tetracyclines, quinolones, levodopa and bisphosphonates — separate by 2-4 hours. Proton pump inhibitors reduce iron absorption. In malaria-endemic regions, routine iron supplementation of children should be paired with malaria surveillance and treatment.

How to take it

TimingA single morning dose on alternate days — hepcidin rises after a dose, so every-other-day dosing absorbs more total iron than splitting it daily.
With food?Best on an empty stomach; if that causes nausea, take it with food and accept slower repletion — the dose you keep taking beats the dose you abandon.
Worth knowingKeep coffee and tea at least an hour away from the dose; the polyphenols cut absorption sharply and this is the most common self-sabotage in iron therapy.

Interactions

With medications

  • levothyroxine

    Iron binds thyroid hormone in the gut and substantially reduces absorption. separate by at least 4 hours

  • tetracycline and fluoroquinolone antibiotics

    Mutual chelation reduces absorption of both drug and iron. take the antibiotic at least 2 hours before or 4-6 hours after iron

  • levodopa and methyldopa

    Iron forms complexes with these drugs and can noticeably reduce their effect. separate by at least 2-4 hours

  • bisphosphonates

    Absorption of the bisphosphonate is reduced. take the bisphosphonate first on an empty stomach, iron at least 2 hours later

  • proton pump inhibitors and H2 blockers

    Less stomach acid means less iron is solubilised and absorbed. take iron with vitamin C, and expect repletion to take longer

With other supplements

  • calcium

    Calcium competes directly with iron for absorption. take them at different meals

  • zinc

    Zinc and iron compete for the same transporter at supplemental doses. separate by at least 2 hours

  • vitamin C

    Vitamin C converts iron to the better-absorbed form and modestly increases uptake. take them together deliberately, or simply take iron with a glass of orange juice

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Should I take iron if I feel tired?

Only if blood tests show low ferritin. The evidence is moderate that 80 mg/day of iron for 12 weeks reduced fatigue in women with ferritin below 50 mcg/L, but iron without a confirmed deficiency is pointless at best and can harm people with iron-loading disorders.

Is it better to take iron every other day?

Yes, with moderate evidence: a single morning iron dose on alternate days absorbed more total iron than daily dosing, because each dose raises hepcidin for 24 hours or more. Alternate-day iron also halves exposure to stomach side effects.

What should I avoid when taking iron?

Keep coffee and tea at least an hour away from your iron dose, and take calcium and dairy at a different meal. Vitamin C, on the other hand, modestly boosts iron absorption.

How long do iron supplements take to work?

Recheck ferritin after 3 months of iron; refilling iron stores typically takes 3–6 months beyond the point where haemoglobin normalises.

Scientific references

Where to buy

California Gold Nutrition, Iron, as Ferrous Bisglycinate Chelate and Yellow Dock Root, 4 fl oz (118 ml)

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