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Research · Supplements

Iron

In adults who were iron deficient without anemia, iron reduced self-reported fatigue in two meta-analyses. VO2 max findings disagree across reviews. In non-anemic pregnant women, iron halved anemia risk. Ferrous sulfate doubled the odds of gut side effects. Iron overdose is a cause of child poisoning deaths, and in children iron may slightly raise malaria where malaria services are lacking.

Add to Bio HackChecked 2026-11-09
Status
Dietary supplement and over-the-counter product in the US. Prescription forms also exist.
Also called
ferrous sulfate, ferrous fumarate, ferrous gluconate, ferrous bisglycinate, iron bisglycinate, polysaccharide iron, heme iron, iron pills
Taken for
fatigue, energy, endurance, VO2 max, anemia, low ferritin, pregnancy, athletic performance, focus, memory, restless legs, hair loss

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Shown to help5

Reported risks5

What people take it for, and what was found

Shown to help5

Strong

Raising iron stores

Shown to raise ferritin in iron-deficient adults and athletes without anemia, most in those starting with very low ferritin.

The numbers and sources
Effect
In iron-deficient non-anemic endurance athletes, iron raised serum ferritin (Hedges' g 1.088, 95% CI 0.914 to 1.263) and hemoglobin (g 0.695) [1]. In 13 trials of athletes, oral iron raised ferritin (SMD 1.27, 95% CI 0.44 to 2.10) and did not significantly change hemoglobin (SMD 1.31, 95% CI -0.29 to 2.93) [2]. In non-anemic iron-deficient adults, ferritin rose by 9.23 (95% CI 6.48 to 11.97) and hemoglobin by 4.01 g/L [3].
Population
17 studies of endurance athletes [1]; 449 athletes in 13 RCTs [2]; 1,170 patients in 18 trials [3].
Sources
[1], [2], [3]
Caveats
In [2], the ferritin rise was evident in athletes starting at 12 µg/L or below, with minimal effect at higher starting ferritin.
Strong

Fatigue

Shown to reduce fatigue in iron-deficient adults without anemia. In one trial, only women with ferritin at or below 50 µg/L improved.

The numbers and sources
Effect
In iron-deficient adults without anemia, iron reduced self-reported fatigue (SMD -0.38, 95% CI -0.52 to -0.23, I² 0%, 4 trials, 714 participants) [3]. In 144 non-anemic women with unexplained fatigue, 80 mg a day of elemental iron for 4 weeks lowered fatigue by 29% versus 13% on placebo (difference 0.95 points on a 10-point scale, 95% CI 0.32 to 1.62). Only women with ferritin at or below 50 µg/L improved [4]. A 2025 meta-analysis of non-anemic participants found less fatigue in RCTs (d 0.34), and no effect when participants without iron deficiency were analyzed alone [5].
Population
Iron-deficient non-anemic adults [3]; Swiss women aged 18 to 55 in primary care [4]; 1,408 participants in 18 studies, 1,176 of them menstruating adults and 204 aged 5 to 17 [5].
Sources
[3], [4], [5]
Caveats
Fatigue is self-reported. Reference [4] is one of the trials pooled in [3], and the two reviews may share other trials [3][5]. The 2025 abstract does not give confidence intervals [5].
ModerateStudied in pregnancy

Anemia in pregnancy

Shown to halve anemia risk in non-anemic pregnant women, with no change in birth weight or preterm birth.

The numbers and sources
Effect
In 23 studies of 4,492 non-anemic pregnant women, oral iron raised hemoglobin by 6.95 g/L (95% CI 4.81 to 9.09) and ferritin by 12.22 ng/mL, and halved the risk of anemia (RR 0.50, 95% CI 0.34 to 0.74, high certainty, number needed to treat 10). Birth weight, preterm birth and cesarean section did not differ [7].
Population
Non-anemic pregnant women in RCTs and observational studies [7].
Sources
[7]
Caveats
Heterogeneity was high for hemoglobin (I² 91%) and ferritin (I² 87%). Reporting of harms was inconsistent, and the authors found too little evidence to judge them [7].
Moderate

Mood and thinking

Shown to improve anxiety, memory and intelligence scores modestly in iron-deficient children and menstruating adults, with no effect in those without deficiency.

The numbers and sources
Effect
In RCTs of non-anemic children, adolescents and menstruating adults, iron improved anxiety symptoms (d 0.34), physical well-being (d 0.42), cognitive intelligence scores (d 0.46) and short-term memory (d 0.53), and did not change attention or depression [5]. An umbrella review of 17 systematic reviews in children found modest gains in intelligence, memory and attention in anemic children, negligible or uncertain effects in children without deficiency, and no consistent change in broader mental development or school achievement [8].
Population
1,408 participants in 18 studies [5]. Children in 17 systematic reviews, 3 rated high confidence [8].
Sources
[5], [8]
Caveats
The two reviews studied different groups. The 2025 abstract gives effect sizes without confidence intervals, and the effects were absent in participants without iron deficiency [5].
Moderate

Restless legs

Shown to lower restless legs scores about 5 points and double the share who improved, pooling oral and intravenous iron.

The numbers and sources
Effect
In 12 RCTs (511 patients), iron lowered the International RLS score by 5.28 points against placebo (95% CI -7.66 to -2.90), doubled the number of patients whose score improved (RR 2.06, 95% CI 1.49 to 2.84), and improved RLS quality of life (MD 7.42) and a sleep scale (MD -24.83) [9].
Population
Patients with restless legs syndrome [9].
Sources
[9]
Caveats
Oral and intravenous iron were pooled, and the authors single out intravenous ferric carboxymaltose. Overall adverse events were twice as common on iron (RR 2.04, 95% CI 1.46 to 2.85), with no difference in serious events [9].

Mixed results2

Moderate

VO2 max and endurance performance

Mixed results: some reviews found higher VO2 max in women and iron-deficient athletes, while others found no change in iron-deficient adults.

The numbers and sources
Effect
In women of reproductive age (22 trials), iron raised relative VO2 max by 2.35 mL/kg/min (95% CI 0.82 to 3.88) and lowered heart rate at set workloads by 4.05 beats per minute [6]. In iron-deficient non-anemic endurance athletes, VO2 max rose (g 0.610, 95% CI 0.399 to 0.821) [1]. In non-anemic iron-deficient adults, VO2 max did not change (SMD 0.11, 95% CI -0.15 to 0.37, 9 trials, 235 participants) [3]. In athletes, the VO2 max trend did not reach significance (SMD 0.49, 95% CI -0.09 to 1.07) [2].
Population
Women of reproductive age [6]; endurance athletes [1][2]; non-anemic iron-deficient adults [3].
Sources
[1], [2], [3], [6]
Caveats
The reviews disagree. Only 3 of 24 studies in [6] were at low risk of bias, and its abstract does not state participants' anemia or iron status.
LimitedStudied in women

Hair loss

Linked to low iron stores: women with non-scarring hair loss had lower ferritin than women without it. No controlled trial has tested iron supplements for hair loss.

The numbers and sources
Effect
In a meta-analysis of 36 studies with 10,029 participants, women with non-scarring hair loss had ferritin 18.51 ng/dL lower than women without it (95% CI -25.85 to -11.16), and 21% had ferritin of 10 to 15 or below [16]. A 2010 review found only small studies of differing designs and no randomized treatment trial, and called for a trial of iron in iron-deficient women with hair loss [17]. In mice, iron deficiency from before birth caused hair loss in the pups, which regrew when iron was restored to the diet [18].
Population
Women with and without non-scarring hair loss in 36 studies [16]. A literature review [17]. C57BL/6J mice [18].
Sources
[16], [17], [18]
Caveats
The human evidence is observational, so low ferritin may travel with hair loss without causing it [16]. No trial shows that taking iron regrows hair in people [17]. The abstract reports ferritin in ng/dL, though ferritin is usually reported in ng/mL [16].

Doses studied

The fatigue trial used 80 mg a day of elemental iron as ferrous sulfate for 4 weeks [4]. In athletes, 16 to 100 mg of elemental iron a day for 6 to 8 weeks changed blood markers, and shorter protocols did not [2]. In iron-depleted young women, 60 mg on alternate days for 28 days gave higher fractional absorption (21.8% versus 16.3%) and total absorption (175.3 mg versus 131.0 mg) than 60 mg daily for 14 days. Splitting 120 mg into two daily doses did not change absorption and raised hepcidin [10].

Risks

Reported risks4

LimitedVery common

Interaction with thyroid hormone

One small trial without a control group found TSH rose from 1.6 to 5.4 mU/L when ferrous sulfate was taken at the same time as thyroxine.

The numbers and sources
How common
Very common: more than 1 in 10 people.
Effect
In 14 patients with hypothyroidism on stable thyroxine, taking 300 mg ferrous sulfate at the same time each day for 12 weeks raised TSH from 1.6 to 5.4 mU/L. Nine had more hypothyroid symptoms. Iron and thyroxine formed a poorly soluble complex in vitro [15].
Population
14 patients, uncontrolled trial [15].
Sources
[15]
Caveats
No control group. Free thyroxine index did not change significantly.
ModerateHow often is not known

Gut side effects

Shown to raise gut side effects with ferrous sulfate, about 2.3 times the odds on placebo, in adults.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Across 20 placebo-controlled trials, ferrous sulfate raised the odds of gastrointestinal side effects (OR 2.32, 95% CI 1.74 to 3.08, I² 53.6%). Against intravenous iron the OR was 3.05 (95% CI 2.07 to 4.48). In pregnant women the OR was 3.33 (95% CI 1.19 to 9.28). The odds did not track dose [11].
Population
43 RCTs, 6,831 adults [11].
Sources
[11]
Caveats
Ferrous sulfate only. Other iron salts were not analyzed.
LimitedStudied in childrenHow often is not known

Accidental poisoning in young children

Limited evidence: after the 1997 FDA unit-dose packaging rule, iron poisoning deaths in children under 6 fell from 29 in 10 years to 1 in 5 years.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
After the 1997 FDA unit-dose packaging rule, iron ingestion calls for children under 6 fell from 2.99 to 1.91 per 1,000 poison center calls, and iron poisoning deaths fell from 29 in the 10 years before to 1 in the 5 years after (OR 13.56, 95% CI 1.85 to 99.52) [12].
Population
US poison control data, children under 6 [12].
Sources
[12]
Caveats
A before and after comparison of national data, not a trial. It covers the 5 years after the 1997 rule and does not describe packaging rules in force today.
ModerateStudied in childrenHow often is not known

Slower growth in breastfed infants

Shown to slow weight and head growth slightly in healthy breastfed infants, while lowering anemia at 6 months.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In 8 trials (685 healthy exclusively breastfed infants), iron raised hemoglobin and lowered iron deficiency anemia at 6 months (RR 0.58, 95% CI 0.40 to 0.84), with no difference at 12 months. Weight gain (MD -0.04, 95% CI -0.07 to -0.01) and head circumference gain (MD -0.14, 95% CI -0.18 to -0.09) were smaller on iron. Mental development scores did not differ [14].
Population
Healthy, exclusively breastfed infants [14].
Sources
[14]
Caveats
The authors describe the evidence as limited, and the abstract does not state units for the growth differences [14].

Mixed results1

ModerateStudied in childrenHow often is not known

Malaria in children in endemic areas

Clinical malaria was about as common as without iron in children in endemic areas. Where malaria services were lacking, it may rise slightly.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Across 40 trials (33,785 children), oral iron made little or no difference to clinical malaria (RR 0.93, 95% CI 0.87 to 1.00, high certainty), slightly lowered severe malaria (RR 0.90, 95% CI 0.81 to 0.98) and may not change deaths (RR 1.15, 95% CI 0.76 to 1.74, low certainty). Where malaria prevention and treatment services were lacking, iron with or without folic acid may raise clinical malaria slightly (RR 1.15, 95% CI 1.02 to 1.30, 7 trials, 19,754 children). With services in place, the RR was 0.91 (95% CI 0.84 to 0.97) [13].
Population
Children under 18 in areas of intense, year-round malaria transmission [13].
Sources
[13]
Caveats
The results split by malaria services are low certainty [13]. They do not apply to areas without malaria.

Claims tested and not found

  1. That iron raises VO2 max in iron-deficient adults without anemia. VO2 max did not differ from control across 9 trials (SMD 0.11, 95% CI -0.15 to 0.37) [3].
  2. That splitting an iron dose improves absorption. Twice-daily split dosing did not change absorption compared with one morning dose [10].

Sources

  1. Burden RJ, Morton K, Richards T, et al. Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. British Journal of Sports Medicine. 2015. https://doi.org/10.1136/bjsports-2014-093624
  2. Šmid AN, Golja P, Hadžić V, et al. Effects of Oral Iron Supplementation on Blood Iron Status in Athletes: A Systematic Review, Meta-Analysis and Meta-Regression of Randomized Controlled Trials. Sports Medicine. 2024. https://doi.org/10.1007/s40279-024-01992-8
  3. Houston BL, Hurrie D, Graham J, et al. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. BMJ Open. 2018. https://doi.org/10.1136/bmjopen-2017-019240
  4. Verdon F, Burnand B, Stubi CLF, et al. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial. BMJ. 2003. https://doi.org/10.1136/bmj.326.7399.1124
  5. Fiani D, Chahine S, Zaboube M, et al. Psychiatric and cognitive outcomes of iron supplementation in non-anemic children, adolescents, and menstruating adults: A meta-analysis and systematic review. Neuroscience & Biobehavioral Reviews. 2025. https://doi.org/10.1016/j.neubiorev.2025.106372
  6. Pasricha SR, Low M, Thompson J, et al. Iron Supplementation Benefits Physical Performance in Women of Reproductive Age: A Systematic Review and Meta-Analysis. Journal of Nutrition. 2014. https://doi.org/10.3945/jn.113.189589
  7. Watt A, Eaton H, Eastwick-Jones K, et al. The benefits and harms of oral iron supplementation in non-anaemic pregnant women: a systematic review and meta-analysis. Family Practice. 2025. https://doi.org/10.1093/fampra/cmae079
  8. Caballero-Apaza LM, Arbildo-Vega HI, Mamani-Cori V, et al. Effect of iron supplements on cognitive development in children: an umbrella review. Frontiers in Nutrition. 2026. https://doi.org/10.3389/fnut.2026.1718507
  9. Mahmoud A, Salamah HM, Alshaker H, et al. Efficacy and safety of iron supplements for restless leg syndrome, a systematic review, meta-analysis, meta-regression, and trial sequential analysis of randomized controlled trials. Baylor University Medical Center Proceedings. 2025. https://doi.org/10.1080/08998280.2025.2527549
  10. Stoffel NU, Cercamondi CI, Brittenham G, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematology. 2017. https://doi.org/10.1016/S2352-3026(17)30182-5
  11. Tolkien Z, Stecher L, Mander AP, et al. Ferrous Sulfate Supplementation Causes Significant Gastrointestinal Side-Effects in Adults: A Systematic Review and Meta-Analysis. PLOS ONE. 2015. https://doi.org/10.1371/journal.pone.0117383
  12. Tenenbein M. Unit-dose packaging of iron supplements and reduction of iron poisoning in young children. Archives of Pediatrics & Adolescent Medicine. 2005. https://doi.org/10.1001/archpedi.159.6.557
  13. Itzkovich M, Neuberger A, Harris I, et al. Oral iron supplements for children in malaria-endemic areas. Cochrane Database of Systematic Reviews. 2026. https://doi.org/10.1002/14651858.CD006589.pub5
  14. Tian K, Liu W, Huang Y, et al. Effect of iron supplementation in healthy exclusively breastfed infants: a systematic review and meta-analysis. Frontiers in Pediatrics. 2025. https://doi.org/10.3389/fped.2025.1587457
  15. Campbell NRC, Hasinoff BB, Stalts H, et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Annals of Internal Medicine. 1992. https://doi.org/10.7326/0003-4819-117-12-1010
  16. Treister-Goltzman Y, Yarza S, Peleg R. Iron deficiency and nonscarring alopecia in women: systematic review and meta-analysis. Skin Appendage Disorders. 2022. https://doi.org/10.1159/000519952
  17. St Pierre SA, Vercellotti GM, Donovan JC, Hordinsky MK. Iron deficiency and diffuse nonscarring scalp alopecia in women: more pieces to the puzzle. Journal of the American Academy of Dermatology. 2010. https://doi.org/10.1016/j.jaad.2009.05.054
  18. Kwon SA, Bu SY, Kim YH, et al. Iron deficiency-induced hair loss is associated with ROS-mediated disruption of Wnt/β-catenin signaling. Nutrients. 2026. https://doi.org/10.3390/nu18142321

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