Reviewed
Reviewed on 2026-09-19 against the evidence rules: every claim carries a grade, every claim and every safety flag carries a source, and every citation was re-resolved against the record it names. The read was delegated by the owner on 2026-09-18 and signed by the Lead on his behalf (D-295) — it is a delegated review, not the owner's own reading of the page. Every claim on it carries an evidence grade and a named weakest link. It is educational and is not medical advice.
Last reviewed
One good trial in women whose iron was actually low, one Cochrane review saying the cognitive evidence is too conflicting to pool, and a gastrointestinal harm that is better evidenced than the benefit. Nothing here applies without a blood test.
Safety
Who should not
Never start iron without a ferritin or haemoglobin result. Iron is stored rather than excreted, and in haemochromatosis — which is common enough to be worth ruling out — supplementing accelerates the accumulation that causes the harm.
Adverse effects
In the Cochrane review, iron raised gastrointestinal side effects (risk ratio 1.99), loose stools or diarrhoea (2.13, high-quality evidence) and constipation (2.07, high-quality evidence). The harm evidence in this review is graded higher than the cognitive benefit evidence in the same review.
No data in your group
The cognitive trial is in women aged 18 to 35, and the pooled review is in menstruating women aged 12 to 50. There is no comparable evidence base for men, and men have a much lower prior probability of deficiency to begin with.
What the evidence says, and how strong it is
A grade belongs to one claim, in one population, for one preparation — never to a compound, a food or a page. Open a grade to see the four links between the evidence and the thing you would actually do, and which of them is doing the least work.
In a blinded placebo-controlled trial of 149 women aged 18 to 35 across the range of iron status, the iron-sufficient women performed better and faster at baseline than the women with iron-deficiency anaemia, with the deficient but non-anaemic women in between. After 16 weeks, improvement in serum ferritin tracked a five- to sevenfold improvement in cognitive performance.
Evidence grade B: May supportIn whom: Women aged 18–35 with iron deficiency, with and without anaemia
What form: Oral iron supplementation for 16 weeks, against placebo
Measured as: Accuracy and speed on eight cognitive performance tasks
Why this grade
- Weakest link
- Study quality — design, size, blinding or replication. One blinded trial of 149 women, never replicated; the pattern across three iron-status groups is what makes it hard to explain away, and it is still one trial.
- What was actually studied
- Women aged 18–35 of varied iron status; 149 at baseline, 113 after 16 weeks (n = 113, retested after 16 weeks of treatment, of 149 measured at baseline)
- Detail worth knowing
- At baseline the iron-sufficient women (n = 42) performed better and faster than the women with iron-deficiency anaemia (n = 34), with the iron-deficient non-anaemic women (n = 73) in between. After treatment, improvement in serum ferritin tracked a five- to sevenfold improvement in cognitive performance.
Cochrane pooled 67 trials and 8,506 women and reported that the evidence iron supplementation improves cognitive performance is uncertain: the studies could not be meta-analysed and individual studies reported conflicting results. The same review found high-quality evidence that iron raises haemoglobin and reduces anaemia.
Evidence grade D: Human evidence insufficientIn whom: Menstruating women aged 12–50
What form: Daily oral iron, at least five days a week, any dose
Measured as: Cognitive performance
Why this grade
- Weakest link
- Outcome — what was measured is not what you would notice. The cognitive outcome specifically could not be meta-analysed and the individual studies conflict, which the reviewers record as uncertainty rather than as a null.
- What was actually studied
- Menstruating women aged 12–50, 67 trials (n = 8506)
- Detail worth knowing
- Anaemia risk ratio 0.39 (0.25–0.60); haemoglobin +5.30 g/L (high-quality evidence). "Evidence that iron supplementation improves cognitive performance in women is uncertain, as studies could not be meta-analysed and individual studies reported conflicting results." Gastrointestinal side effects RR 1.99 (1.26–3.12); loose stools RR 2.13; constipation RR 2.07.
Across the same 67 trials, daily iron reduced the risk of anaemia (risk ratio 0.39, 95% confidence interval 0.25 to 0.60), raised haemoglobin by 5.30 g/L on high-quality evidence, reduced the risk of iron deficiency, improved exercise performance and appeared to reduce symptomatic fatigue.
Evidence grade B: May supportIn whom: Menstruating women aged 12–50
What form: Daily oral iron, at least five days a week
Measured as: Anaemia, haemoglobin, iron status and symptomatic fatigue
Why this grade
- Weakest link
- Outcome — what was measured is not what you would notice. Haemoglobin, anaemia and fatigue are what was measured, and none of them is cognition — the distance between this claim and this page's subject is what holds it down.
- What was actually studied
- Menstruating women aged 12–50, 67 trials (n = 8506)
- Detail worth knowing
- Anaemia risk ratio 0.39 (0.25–0.60); haemoglobin +5.30 g/L (high-quality evidence). "Evidence that iron supplementation improves cognitive performance in women is uncertain, as studies could not be meta-analysed and individual studies reported conflicting results." Gastrointestinal side effects RR 1.99 (1.26–3.12); loose stools RR 2.13; constipation RR 2.07.
Every trial above recruited on iron status or in a population where deficiency is common. Nothing here supports taking iron with a normal ferritin, and the body has no route to excrete a surplus — searched on 2026-08-23.
Evidence grade D: Human evidence insufficientIn whom: Adults with normal iron status
What form: Iron supplementation without a measured deficiency
Measured as: Any cognitive outcome
Why this grade
- Weakest link
- Population — the people studied are not the people reading. Every trial here recruited on iron status or in a group where shortfall is common, and an adult with a normal ferritin is in none of them.
- What was actually studied
- Women aged 18–35 of varied iron status; 149 at baseline, 113 after 16 weeks (n = 113, retested after 16 weeks of treatment, of 149 measured at baseline); Menstruating women aged 12–50, 67 trials (n = 8506)
- Detail worth knowing
- At baseline the iron-sufficient women (n = 42) performed better and faster than the women with iron-deficiency anaemia (n = 34), with the iron-deficient non-anaemic women (n = 73) in between. After treatment, improvement in serum ferritin tracked a five- to sevenfold improvement in cognitive performance.
- Anaemia risk ratio 0.39 (0.25–0.60); haemoglobin +5.30 g/L (high-quality evidence). "Evidence that iron supplementation improves cognitive performance in women is uncertain, as studies could not be meta-analysed and individual studies reported conflicting results." Gastrointestinal side effects RR 1.99 (1.26–3.12); loose stools RR 2.13; constipation RR 2.07.
Everything above, and where it comes from
- Murray-Kolb LE, Beard JL (2007). Iron treatment normalizes cognitive functioning in young women. American Journal of Clinical Nutrition 85(3):778–787
- Low MS, Speedy J, Styles CE, De-Regil LM, Pasricha SR (2016). Daily iron supplementation for improving anaemia, iron status and health in menstruating women. Cochrane Database of Systematic Reviews 2016(4):CD009747.pub2
The one thing worth doing
If you are tired and suspect iron, ask for a ferritin test rather than buying a supplement — the test is the part that tells you anything, and taking iron first makes the result harder to read.
Regulatory note — this note has not yet been reviewed by a lawyer. In the United States, supplements are sold under DSHEA: a structure/function claim is not reviewed by the FDA before it appears, and the label must carry the words "This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease." In the EU and the UK, only health claims on the authorised register may be used in advertising, and a food or supplement advertisement may not say a product prevents, treats or cures a disease. Whether an evidence page that carried a purchase link would itself count as advertising under those rules is an open question that counsel, not this page, will answer; there are no purchase links here. Nothing on this page is medical advice.