Iron fixes tiredness

In trials, iron modestly reduced tiredness in women who were tired and had low iron stores, and the trials have not shown it doing so in people whose iron is already normal. Fatigue is a recognised symptom of iron deficiency anaemia, and daily iron reduces anaemia. In women who were tired but not anaemic and had low iron stores, a 2018 pooled analysis of four placebo-controlled trials (714 participants) found iron lowered self-reported fatigue by a modest amount, a standardised difference of 0.38, with the evidence graded moderate. A 2025 pooled analysis of randomized trials in non-anaemic menstruating adults, with a smaller group of children and adolescents, found a similar effect on fatigue (0.34), and no effect in analyses that left out participants with iron deficiency.

Where a benefit appeared, it was in people with low iron stores, and low stores alone did not guarantee one. In a 2003 trial the benefit appeared only in women whose ferritin, the blood marker of stored iron, was 50 micrograms per litre or lower. A 2020 trial of 405 blood donors with low stores found that intravenous iron raised ferritin and haemoglobin but left fatigue unchanged, including in donors who reported more fatigue at the start. Tiredness and iron deficiency both have causes of their own: iron deficiency is associated with blood loss and with conditions that impair absorption, and iron replaces the missing iron without addressing either. People with hereditary haemochromatosis accumulate iron, and accidental overdose of iron products is a leading cause of fatal poisoning in children under six.

The two oral trials used 80 mg of elemental iron a day as ferrous sulfate. In 198 French women with unexplained fatigue and ferritin below 50 micrograms per litre, fatigue scores fell 47.7% on iron and 28.8% on placebo over 12 weeks, with no difference in quality of life, depression or anxiety. In 144 Swiss women over four weeks, fatigue fell 29% on iron and 13% on placebo. Both trials were sponsored by pharmaceutical companies: Pierre Fabre, which supplied the tablet tested in the French trial, and Robapharm in the Swiss trial. The 2018 review also found that iron did not improve objective measures of physical capacity, including maximal oxygen uptake across nine trials. A 2025 pooled analysis of 15 controlled trials in 380 women with iron deficiency, with or without anaemia, found no clear change in time to exhaustion or in time trial performance, and a gain in maximal oxygen uptake that was no longer significant once women with anaemia were left out.

A 2016 Cochrane review of 67 trials in menstruating women found that daily iron cut the risk of anaemia (risk ratio 0.39) and raised haemoglobin. Eight of those trials reported fatigue in forms too different to pool; the reviewers read them as showing that iron may improve symptoms in women who are fatigued, with effects in women without symptoms less evident. Iron roughly doubled the rate of constipation (risk ratio 2.07) and of loose stools (2.13).

The US tolerable upper intake level for adults is 45 mg of iron a day, set from gastrointestinal effects, and doctors sometimes prescribe more to treat deficiency. A common over-the-counter ferrous sulfate tablet supplies 65 mg of elemental iron, and its label lists one tablet two to three times daily for adults. Iron reduces the absorption of levothyroxine and levodopa, and the tablet label warns against taking it within two hours of tetracycline antibiotics. At usual doses iron has little or no effect on the liver; in high doses and overdoses it causes serious toxicity, including acute liver damage. None of the fatigue trials here studied pregnancy, and the label advises pregnant or nursing women to seek advice from a health professional before use.

The World Health Organization's 2020 guideline on iron status describes ferritin, read alongside clinical and laboratory evaluation, as the measure for detecting both iron deficiency and iron overload. It lists fatigue, impaired physical performance and lower work productivity among the effects of iron deficiency in adults.