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Iron for runners: what's being used and what the research says

We report what runners are using and what the research says. We don't recommend taking any supplement. Talk to your GP or a registered sports dietitian before trying one.

Quick answer

Research suggests it can when iron is low, but not otherwise. In iron-deficient female athletes, a systematic review found endurance gains of 2% to 20%[1]. In distance runners whose iron was not low, intravenous iron raised iron stores but not 3,000 m speed[2]. Sports bodies say low iron should be confirmed with tests first[3][4].

Research
Emerging (proposed)Some gains when iron is low, mostly shown in women. No benefit shown when iron stores are normal.
Proposed by our editor; not yet confirmed by an expert reviewer.
WADA status
Permitted
Distances in the research
800 m–5K: limited research10K–half: not studiedMarathon: not studiedUltra: not studied

What is iron?

Iron is a mineral the body needs to make red blood cells, which carry oxygen around the body[5]. The body can't make its own iron, so it relies on food and supplements; almost two-thirds of its iron is in haemoglobin in red blood cells, with smaller amounts in ferritin and myoglobin[4].

The NHS lists liver (but not during pregnancy), red meat, beans such as red kidney beans, edamame and chickpeas, nuts, dried fruit such as apricots, fortified breakfast cereals and soy bean flour as good sources[5]. Iron from animal foods is called haem iron, and iron from plants non-haem iron[4]. Less is absorbed from plant-based diets: in people with no iron stores, estimated absorption is 14 to 18% from mixed diets and 5 to 12% from vegetarian diets[6].

Iron tablets come in several types, including ferrous sulfate, ferrous fumarate and ferrous gluconate, and the NHS says there is little difference in how well they work at the same dose of iron[7]. Doses are written in 2 ways, as the weight of the iron salt or as the elemental iron it contains: one meta-analysis equates 650 mg of iron sulphate with 130 mg of elemental iron[8]. The AIS advises checking the elemental iron content, and lists slow-release forms for athletes who don't tolerate ferrous salts[4].

The Australian Institute of Sport lists iron in Group A of its framework as a medical supplement rather than a performance supplement[4]. It says iron supplements should only be taken under medical supervision, as part of a plan that includes a dietary assessment[4].

Also called: iron tablets, ferrous sulfate, ferrous fumarate, ferrous gluconate, iron supplements.

Why is iron getting attention?

Low iron is common in endurance athletes, particularly women, according to the international athletics federation's consensus[9]. Reported rates are about 15 to 35% of female athletes and 3 to 11% of males[8]. The AIS lists female athletes, endurance athletes (including those training at altitude), vegetarian and vegan athletes, and athletes with low energy availability as groups at higher risk[4].

At the Zurich marathon, 28% of female runners had depleted iron stores, compared with under 2% of men[10]. In blood-test records from 2009 to 2015 for 38 elite runners and triathletes, 55.6% of the female runners and 31.3% of the male runners had at least one episode of low iron, even with monitoring in place[11].

Of 1,073 women surveyed at the 2015 London Marathon, 45% said they had taken iron at some point, including in multivitamins[12]. Among US collegiate endurance runners, iron and multivitamins were the most used supplements (46.7% each), and iron was taken by 61.4% of women and 30.8% of men[13]. More than 42% of 38 US high-school female distance runners were taking iron[14].

The claims, checked against the research

  • Faster times when iron is lowMixed

    A systematic review of iron-deficient female athletes found endurance gains of 2% to 20% with about 100 mg of elemental iron a day[1]. Runner trials are split: 8 weeks of oral iron didn't improve endurance more than a placebo in 18 women[15], while in a trial of 14 adolescent runners, a month of iron improved treadmill endurance compared with placebo[16].

  • Faster times when iron is normalNot supported

    In 14 distance runners whose iron was not clinically low, intravenous iron doubled iron stores but did not improve 3,000 m time[2]. A meta-analysis of oral iron trials found no benefit on blood or performance measures in athletes with normal ferritin[8].

  • Higher VO2maxMixed

    A 2015 meta-analysis found a moderate VO2max gain in endurance athletes with low iron but normal haemoglobin[17]. A 2024 meta-analysis of randomised oral iron trials found only a trend that was not significant[8], and one intravenous dose didn't change VO2max in 15 iron-deficient elite runners[18].

  • Bigger iron storesSupported

    Oral iron raised ferritin in a meta-analysis of 13 randomised trials, mainly in athletes starting at 12 µg/L or below[8]. Intravenous iron raised ferritin in runners with and without low iron[2][18].

  • Less tirednessMixed

    In 14 distance runners without clinical iron deficiency, intravenous iron improved self-reported fatigue and mood scores, but not performance[2]. The NHS notes tiredness may have other causes, and says to talk to a doctor or pharmacist before taking iron without a diagnosis[7].

  • More efficient runningMixed

    In non-anaemic female rowers with low iron stores, 6 weeks of iron improved energetic efficiency during exercise compared with placebo[19]. In iron-deficient elite runners, one intravenous dose did not change running economy[18].

How does iron work?

Iron carries oxygen in the blood, so it is important in events that rely on aerobic energy, from middle distance upwards[20]. The AIS says low iron is typically linked to lethargy and fatigue, and in athletes may also show up as poorer training and performance, or a reduced ability to adapt to training[4].

Low iron develops in stages. First, iron stores run down, seen as low ferritin in a blood test, while haemoglobin stays normal; the AIS calls this iron deficiency without anaemia. In the most severe stage, iron deficiency anaemia, both iron stores and haemoglobin are low[4]. The NHS says iron deficiency anaemia is caused by lack of iron, often because of blood loss or pregnancy[21].

Iron can run low for reasons linked to training, such as red blood cells damaged by foot strike and iron lost in sweat, urine and the gut, and for other reasons, such as menstrual losses and low intake[3]. In 10 male triathletes, the rise in free haemoglobin in the blood, a marker of red cells breaking down, was 4 times greater after an hour of running than after an hour of cycling at the same effort; the authors conclude foot strike is the main cause[22].

After exercise, hepcidin, the hormone that regulates iron, rises for several hours, which alters how iron is absorbed[23][20]. A pooled analysis of individual data found exercise raised hepcidin 1.5 to 2.5-fold, with session length explaining about 44% of the variation in the rise; the authors note that hepcidin peaks 3 to 6 hours after exercise[23]. In 8 moderately trained athletes, urinary hepcidin was 1.7 to 3.1 times its pre-run level 3, 6 and 24 hours after a 60-minute run[24].

Does iron improve running performance?

Sometimes, when iron is low. In endurance athletes with low iron but normal haemoglobin, a meta-analysis of 17 studies found iron improved iron status and VO2max[17]. A later meta-analysis of randomised oral iron trials found iron raised ferritin mainly in athletes who started very low and did not significantly improve VO2max; its authors conclude there is no benefit when ferritin is normal[8]. A systematic review of 12 studies in iron-deficient athletes without anaemia found performance gains in 6 and none in the other 6, and judged the evidence equivocal[25].

800 m to 5K (Limited research)

In 14 distance runners whose iron was not clinically low, intravenous iron raised iron stores but did not improve 3,000 m time[2]. A systematic review lists an older trial in 12 iron-deficient female runners, in which the 6 given oral iron improved 3,000 m running measures by about 4%[1]. Other runner trials used tests to exhaustion rather than races, and we found no trial with a 5 km race or time-trial result.

10K to half marathon (Not studied)

We found no iron trials with a 10 km or half-marathon result. Lab studies in distance runners were mixed: in iron-depleted women, 8 weeks of oral iron raised iron stores but not endurance[15], while in runners with the lowest iron, intravenous iron raised VO2max and time to exhaustion[26].

Marathon (Not studied)

We found no iron trials with a marathon result. At the Zurich marathon, 28% of female runners had depleted iron stores, while one in six men had signs of too much iron[10].

Ultra (Not studied)

We found no iron trials in ultra runners. The rise in hepcidin after exercise grows with the length of the session[23].

Study by study

The main studies on this page. What they took is what the study gave, not advice for any individual.
StudyWho took partWhat they tookWhat was measuredWhat they found
Burden 2015 [17]Meta-analysisEndurance athletes with low iron but normal haemoglobin, in 17 studies (meta-analysis)Iron treatment by mouth or injection, pooled across studiesFerritin, serum iron, transferrin saturation, haemoglobin and VO2maxLarge effect on ferritin (Hedges' g 1.09) and a moderate effect on VO2max (g 0.61). Treatments lasting beyond 80 days had the least effect on ferritin.
Šmid 2024 [8]Meta-analysis449 active adults and athletes (432 women) in 13 randomised trials (meta-analysis)Oral iron against placebo or control; doses that improved iron status ranged from 16 to 100 mg of elemental iron a day for 6 to 8 weeksFerritin, haemoglobin and VO2maxFerritin rose (SMD 1.27), mainly from 12 µg/L or below. VO2max showed a small trend that was not significant (SMD 0.49, p = 0.086). Evidence moderate to low quality.
Pengelly 2025 [1]Systematic review669 female athletes in 23 studies (systematic review)About 100 mg of elemental iron a day by mouth for up to 56 days, or iron by injection over 8 to 10 days, in the trials with endurance gainsEndurance performance and maximal aerobic capacityIron deficiency was linked to 3-4% lower endurance. In deficient athletes, iron improved endurance by 2-20%; maximal aerobic capacity rose 6-15% in studies using ferritin under 16 µg/L, with little effect above about 25 µg/L.
Rubeor 2018 [25]Systematic review283 iron-deficient athletes without anaemia in 12 studies (systematic review)Iron supplements, in studies that reported a performance outcomePerformance outcomesPerformance improved in 6 studies (146 people) and not in the other 6 (137). All 6 positive studies used a ferritin cut-off of 20 µg/L or less and oral iron. Evidence judged equivocal.
Klingshirn 1992 [15]Randomised controlled trial18 iron-depleted women distance runners aged 22 to 39 (ferritin under 20 µg/L, normal haemoglobin)Oral iron or placebo for 8 weeks; pairs matched on endurance time, then randomisedVO2max and a run to exhaustionIron status improved, but endurance capacity did not differ from placebo.
Rowland 1988 [16]Randomised controlled trial14 adolescent runners with ferritin under 20 µg/L and no anaemiaFerrous sulfate (975 mg a day, the salt weight) or placebo for 1 month after a 4-week control period, during a competitive season; double-blind, with random assignment within ferritin strataTreadmill endurance time, oxygen uptake and heart rateFerritin rose from 8.7 to 26.6 µg/L with iron. Endurance time improved in 6 of 7 on iron and fell in all 7 on placebo. Gas exchange and heart rate measures did not favour iron.
Woods 2014 [2]Randomised controlled trial14 trained distance runners (6 men, 8 women) with ferritin of 30 to 100 µg/L3 intravenous injections of 100 mg of ferric carboxymaltose or saline over 4 weeks, randomised and blinded3,000 m time trial, a 10 × 400 m training session, oxygen transport measures, and fatigue and mood questionnairesFerritin roughly doubled (62.8 to 128.1 µg/L). 3,000 m time did not improve in either group; average 10 × 400 m session time was slightly faster at week 2. Self-reported fatigue and mood scores improved.
Burden 2015 [18]Randomised controlled trial15 national and international standard runners with iron deficiency but no anaemiaOne 500 mg intravenous iron injection or placeboVO2max, running economy, haemoglobin mass and time to exhaustion, within 24 hours and at 4 weeksIron status improved. No difference from placebo in haemoglobin mass, VO2max, running economy or time to exhaustion.
Garvican 2014 [26]Non-randomised trial27 highly trained distance runners with low or borderline ferritinIntravenous iron (ferric carboxymaltose) or oral iron for 6 weeks; no placebo groupHaemoglobin mass, VO2max and run time to exhaustionIn the lowest-iron intravenous group, haemoglobin mass rose 4.9%, VO2max 3.3% and time to exhaustion 9.3%.
DellaValle 2014 [19]Randomised controlled trial40 non-anaemic female rowers at the start of a training season (31 finished)100 mg of ferrous sulfate a day or placebo for 6 weeks, randomised and double-blind4 km time trial, VO2peak, energetic efficiency and blood lactateThe iron group had a slower lactate rise early in the time trial and larger gains in energetic efficiency; time-trial times did not improve significantly. The ferritin gain was not significant (p = 0.07).
von 2023 [27]Randomised controlled trial34 iron-depleted women (median ferritin 19.4 µg/L)100 mg iron doses taken with water in the morning, with 80 or 500 mg of vitamin C, with coffee, with breakfast, or with water in the afternoon; each woman had all 6, in random orderIron absorption (stable isotopes) and hepcidin80 mg of vitamin C: 30% more absorbed; 500 mg: no significant further gain. Coffee: 54% less; breakfast with coffee: 66% less. Afternoon: 37% less, not significant (p = 0.059).

The key studies, in order

  1. 1988Trial14 adolescent runners with low ferritin: a month of iron raised ferritin and improved treadmill endurance time against placebo. [16]
  2. 1992Trial18 iron-depleted female distance runners: 8 weeks of oral iron raised iron stores, but time to exhaustion did not differ from a placebo. [15]
  3. 2003Trial10 male triathletes, random order: the rise in plasma free haemoglobin, a marker of red cell breakdown, was fourfold greater after running than cycling. [22]
  4. 2009Trial8 athletes, randomised crossover: urinary hepcidin was 1.7 to 3.1 times its pre-run level 3 to 24 hours after a 60-minute run. [24]
  5. 2010Expert reviewAbsorption estimated at 14-18% from mixed diets and 5-12% from vegetarian diets; single enhancers and inhibitors matter less in varied diets. [6]
  6. 2010Observational study170 Zurich marathon runners: depleted iron stores in 28% of women and under 2% of men; signs of iron overload in one in six men. [10]
  7. 2014TrialNon-anaemic female rowers (31 finished): 6 weeks of iron improved energetic efficiency during exercise compared with placebo. [19]
  8. 2014Non-randomised trial27 highly trained distance runners, no placebo group: in the lowest-iron group, intravenous iron raised haemoglobin mass, VO2max (3.3%) and time to exhaustion (9.3%). [26]
  9. 2014Trial14 distance runners without clinical iron deficiency: intravenous iron doubled iron stores but did not improve 3,000 m time. Self-reported fatigue and mood scores improved. [2]
  10. 2015Meta-analysis17 studies of endurance athletes with low iron but normal haemoglobin: iron treatment improved iron status and had a moderate effect on VO2max. [17]
  11. 2015Trial15 elite runners with low iron: one 500 mg intravenous dose improved iron status, but not VO2max, running economy or time to exhaustion. [18]
  12. 2015Expert reviewSwiss review: in athletes over 15, ferritin under 15 µg/L means empty stores and 15-30 µg/L low stores; suggests 30 µg/L as the cut-off. [29]
  13. 2015TrialStudies in 54 young women with low ferritin: 24 hours after 60 mg or more of iron, hepcidin was up and absorption from the next dose 35-45% lower. [34]
  14. 2015Meta-analysis43 randomised trials, 6,831 adults: ferrous sulfate raised the odds of gut side effects (OR 2.32 against placebo), with no clear link to dose. [38]
  15. 2016Survey1,073 women surveyed at the 2015 London Marathon: 45% said they had taken iron at some point, including in multivitamins. [12]
  16. 2017Observational studyRecords of 38 elite runners and triathletes: at least one episode of low iron in 55.6% of female and 31.3% of male runners. [11]
  17. 2017TrialTwo trials in iron-depleted women: in 40, 60 mg on alternate days led to more iron absorbed in total than daily doses (175 v 131 mg); in 20, split doses did not help. [35]
  18. 2018ConsensusIdentify deficiencies before supplementing; iron taken when stores are adequate can cause harm. Vitamin and mineral products have rarely contained banned substances. [3]
  19. 2018Systematic review12 studies, 283 athletes: performance improved in 6 and not in 6; all positive studies used ferritin cut-offs of 20 µg/L or less. Evidence equivocal. [25]
  20. 2019ConsensusLow iron status is common in endurance athletes, particularly women. Recommends routine screening; injected iron only in a medical setting under a physician's supervision. [9]
  21. 2019Expert reviewIron is important for events from middle distance up. A rise in hepcidin after exercise alters iron absorption and recycling. [20]
  22. 2019Trial16 endurance-trained runners, crossover: more iron was absorbed from breakfast 30 minutes after a morning run than at rest, despite a hepcidin rise. [32]
  23. 2019Observational study81 recreational runners: fewer than 30% of omnivores, vegetarians or vegans had depleted iron stores. [44]
  24. 2020Expert reviewReview: proposes morning iron, ideally within 30 minutes of morning exercise, and alternate days or lower doses for athletes with gut symptoms. [36]
  25. 2020Observational study38 US high-school female distance runners: more than 42% took iron, and female athlete triad risk factors were common. [14]
  26. 2021Survey135 US collegiate endurance runners: 46.7% used iron, including 61.4% of women and 30.8% of men. [13]
  27. 2022Non-randomised trial28 collegiate cross-country runners: after a run of about 21 km, hepcidin was 51% higher and iron absorption 36% lower than after rest. [31]
  28. 2023Meta-analysisExercise raised hepcidin 1.5 to 2.5-fold; session length was the main factor in the size of the rise. The authors note hepcidin peaks 3 to 6 hours after exercise. [23]
  29. 2023ConsensusLow iron is 5-7 times more common in female than male athletes. Oral contraceptive users are advised not to take iron without medical advice. [28]
  30. 2023Trial34 iron-depleted women, randomised crossover, 100 mg doses: 80 mg of vitamin C raised absorption 30% (500 mg: no significant further gain); coffee cut it 54%, coffee with breakfast 66%; afternoon doses 37% lower (not significant). [27]
  31. 2024Meta-analysis13 randomised trials, 449 athletes (432 women): oral iron raised ferritin, mainly from 12 µg/L or below, but did not significantly improve VO2max. No benefit when ferritin was normal. [8]
  32. 2025Systematic review23 studies, 669 female athletes: iron deficiency cut endurance performance by 3-4%; in deficient athletes, about 100 mg of iron a day improved it by 2-20%. [1]

Who has the research been done in?

Most of the evidence comes from women[8], and the review of female athletes says its findings apply only to high-level female athletes[1]. The runner trials were small, with 14 to 27 people each[2][26], and several involved highly trained or elite runners[18].

Men, masters and recreational runners are thinly covered. Among recreational marathon runners, low iron was rare in men but signs of too much iron were not[10], so results from iron-deficient women may not apply to them. The ISSN notes that in post-menopausal women, hepcidin was slower to return to its resting level after exercise, for up to 24 hours[28].

For teenagers, one small trial in adolescent runners with low ferritin found better treadmill endurance after a month of iron[16]. A Swiss expert review uses lower ferritin cut-offs for children and young teenagers than for adults: 15 µg/L for ages 6 to 12 and 20 µg/L for ages 12 to 15, against 30 µg/L over 15[29]. The NHS warns that very high doses of iron can be fatal, particularly for children[5].

What we know about women

Low iron is 5 to 7 times more common in female athletes than in males, which the ISSN links to diet, training-related losses and the menstrual cycle[28]. The NHS says women who lose a lot of blood during their periods are at higher risk of iron deficiency anaemia and may need to take iron supplements[5]. In the London Marathon survey, more than a third of the women (36%) reported heavy menstrual bleeding[12].

Hepcidin varies across the menstrual cycle, and the ISSN suggests iron may be more effective in the follicular phase than the luteal phase, while saying more research is needed[28]. It adds that hormonal contraception protects against iron deficiency, and says oral contraceptive users should not take iron without medical advice[28]. In pregnancy, the NHS says a doctor or midwife may suggest iron if blood tests show low levels, and that constipation is common when taking it[30].

What the research can't tell us yet

Most participants were women: in the meta-analysis of oral iron trials, 432 of 449 participants were female[8].

The studies were small, there is no agreed ferritin cut-off for low iron, and the quality of evidence ranged from moderate to low[8]. Of the runner trials here, the one with a positive result on VO2max compared intravenous with oral iron and had no placebo group[26].

Studies defined low iron differently: ferritin under 20 µg/L in older runner trials[15][16], under 35 µg/L with low transferrin saturation in another[26], and 30 to 100 µg/L in runners without clinical deficiency[2]. In one systematic review, all 6 studies that found performance gains used a ferritin cut-off of 20 µg/L or less[25].

Most runner trials measured laboratory outcomes such as VO2max and time to exhaustion rather than races[15][18][26], and 18 of the 23 studies in the review of female athletes had groups of 20 or fewer[1]. The research on dose timing measured iron absorption, not performance, and its 2 studies of absorption after running point in different directions[31][32].

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How and when is iron used in studies?

The NHS says men need 8.7 mg of iron a day and women aged 19 to 49 need 14.8 mg, and most people can get enough from a varied diet. It says 17 mg or less a day from supplements is unlikely to cause any harm, and that a higher dose should be continued if a GP has advised it[5].

In a meta-analysis of oral iron trials in athletes, doses that improved iron status ranged from 16 to 100 mg of elemental iron a day, taken for 6 to 8 weeks[8]. For athletes with diagnosed low iron, the AIS framework starts with diet, then about 100 mg of elemental iron each morning, moving to alternate mornings, a lower dose or a slow-release form if gut symptoms persist[4].

The NHS says to take ferrous sulfate on an empty stomach if possible, 30 minutes before or 2 hours after eating, but with or after food if it upsets the stomach[33]. The AIS adds that calcium and the tannins in tea and coffee should be avoided for an hour either side of a dose[4].

Injected iron is a medical treatment: the international athletics federation says it should only be given in a medical setting under a doctor's supervision[9], and the AIS says intravenous iron should only be considered in consultation with a sports physician[4].

When studies gave it

In athlete trials, oral iron courses that improved iron status lasted 6 to 8 weeks, and shorter courses were ineffective[8]. The AIS fact sheet describes 8 to 12 weeks, followed by a blood test to confirm the change[4].

Research on timing centres on hepcidin. In 41 young women with low iron stores, hepcidin was still raised 24 hours after doses of 60 mg or more, and absorption from the next day's dose was 35 to 45% lower[34]. In a randomised trial in 40 iron-depleted women, 60 mg on alternate days for 28 days led to more iron absorbed in total than the same 14 doses on consecutive days (about 175 mg against 131 mg); in a second trial, in 20 women, twice-daily split doses raised hepcidin without raising absorption[35]. In 34 iron-depleted women, absorption from a 100 mg dose was 37% lower in the afternoon than in the morning, a difference that was not significant (p = 0.059), while hepcidin was higher in the afternoon[27]. None of these studies measured performance.

Exercise adds a second hepcidin rise[23]. In 28 collegiate cross-country runners with low stores, iron absorbed from a meal 2 hours after a run of about 21 km was 36% lower than after rest[31]. But in 16 endurance-trained runners, more iron was absorbed from breakfast 30 minutes after a 90-minute morning run than after rest[32]. A 2020 review suggests morning iron, ideally within 30 minutes of morning exercise, and alternate days or lower doses for athletes with gut symptoms[36].

What studies used. These are research protocols, not advice for any individual.
What studies usedWhenRef.
16 to 100 mg of elemental iron a day, by mouth6 to 8 weeks, in trials of athletes (meta-analysis)[8]
About 100 mg of elemental iron a day, by mouthUp to 56 days, in iron-deficient female athletes[1]
3 intravenous injections of 100 mg over 4 weeksDistance runners without clinical iron deficiency[2]
60 mg of iron on alternate days, by mouth28 days, in iron-depleted women (an absorption study)[35]
One 500 mg intravenous doseSingle dose, in iron-deficient elite runners[18]
100 mg of ferrous sulfate a day, by mouth6 weeks, in non-anaemic female rowers[19]

What are the side effects, and who should be careful?

The NHS lists constipation, feeling or being sick and stomach pain as side effects of high doses (over 20 mg), and warns that very high doses can be fatal, particularly for children[5]. For ferrous sulfate, it lists feeling or being sick, stomach discomfort or heartburn, loss of appetite, constipation and diarrhoea as common side effects[37].

Darker poo is common and usually nothing to worry about, but the NHS says to talk to a doctor or call 111 straight away if it is black, tar-like or red[37].

Many athletes get gut side effects, which can put them off finishing a course of oral iron[36]. A meta-analysis of 43 randomised trials in adults found ferrous sulfate more than doubled the odds of gut side effects compared with placebo (odds ratio 2.32), with no clear link to dose[38]. In the review of female athletes, only 1 athlete dropped out because of stomach discomfort[1].

The IOC warns that iron taken when stores are already adequate can cause vomiting, diarrhoea and abdominal pain, and can go on to cause iron overload and liver failure[3]. The AIS says injected iron carries a risk of anaphylactic shock[4].

Medicines and health conditions

The NHS lists medicines that can interact with ferrous sulfate, and asks people to tell a pharmacist or doctor before starting it if they take any of them. The list includes antacids and indigestion remedies; antibiotics such as oxytetracycline, doxycycline, ciprofloxacin or chloramphenicol; levothyroxine; bisphosphonates for osteoporosis; laxatives containing magnesium; some HIV antivirals; and some medicines for Parkinson's disease. It suggests a gap between iron and other medicines, with a doctor or pharmacist advising how long[39].

Supplements and food interact too. The NHS says ferrous sulfate can stop zinc working as well as it should, calcium can reduce its effect, and some vitamin and mineral products already contain iron[39]. It advises a 2 hour gap between iron and tea, coffee, dairy products or eggs[40].

The NHS describes haemochromatosis as an inherited condition in which iron levels slowly build up over many years, particularly common where many people have a Celtic background, such as Ireland, Scotland and Wales[41]. It says people with it will usually be advised to avoid iron and vitamin C supplements[42].

Be careful if

  • You feel tired but haven't been diagnosed with iron deficiency anaemia: the NHS says there may be other reasons you lack energy, and to talk to a doctor or pharmacist before taking iron[7].
  • You have haemochromatosis, a condition in which iron builds up in the body: the NHS says you'll usually be advised to avoid iron and vitamin C supplements[42].
  • You take other medicines, such as antacids, some antibiotics or levothyroxine: they can interact with iron[39].
  • You use the contraceptive pill: the ISSN says oral contraceptive users should not take iron without medical advice[28].
  • You're pregnant or trying to get pregnant: the NHS says to tell your doctor, who may suggest iron if blood tests show low levels[30].

Is iron allowed in competition?

Iron is not on the World Anti-Doping Agency's Prohibited List. But intravenous infusions or injections of more than 100 mL in 12 hours are a prohibited method, unless legitimately received during hospital treatment, surgery or clinical investigations; otherwise they need a Therapeutic Use Exemption[43]. WADA's FAQ adds that intravenous iron dextran is not considered a prohibited plasma expander[43].

The IOC notes that even common vitamin and mineral products have, rarely, been found to contain prohibited substances, and athletes are responsible for anything found in their sample[3].

Independent quality-assurance schemes, often called batch testing, check supplement products for prohibited substances[3]. WADA says using a product from one of these schemes can reduce, but not eliminate, the risk of an anti-doping violation[43], and the IOC says no product can be guaranteed entirely safe[3].

Myth or evidence?

5 things runners hear about iron. Pick an answer to see what the research says.

  1. 1.Iron supplements make every runner faster.

    Show the answer

    Myth. In runners whose iron was not low, intravenous iron didn't improve 3,000 m time[2], and a meta-analysis of oral iron trials found no significant gain in VO2max[8].

  2. 2.Female runners are more likely to have low iron.

    Show the answer

    Evidence. At the Zurich marathon, 28% of women had depleted iron stores, against under 2% of men[10].

  3. 3.Hard running changes how the body handles iron.

    Show the answer

    Evidence. Hepcidin, the hormone that regulates iron, rises 1.5 to 2.5-fold after exercise[23].

  4. 4.Extra iron is harmless if you don't need it.

    Show the answer

    Myth. The IOC warns that iron taken when stores are adequate can cause harm[3]. In one study, one in six male marathon runners had signs of too much iron, and the authors suggest supplements only when tests show deficiency[10].

  5. 5.It doesn't matter what you drink with an iron tablet.

    Show the answer

    Myth. The NHS advises a 2 hour gap from tea, coffee and dairy, which can stop your body absorbing iron[40].

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Frequently asked questions

Does iron improve running performance?
Research suggests it can when iron is low: a meta-analysis found iron improved VO2max in endurance athletes with low iron[17]. In runners whose iron was not low, intravenous iron did not improve 3,000 m time[2].
Why are female runners more likely to have low iron?
Menstrual losses add to the ways running lowers iron, such as red blood cells damaged by foot strike and iron lost in sweat[3]. At the Zurich marathon, 28% of women had depleted iron stores, against under 2% of men[10].
How do I know if my iron is low?
Through a blood test. The AIS's minimum athlete screen is ferritin, haemoglobin and transferrin saturation, ideally taken in the morning, well hydrated, after 24 hours of low to moderate activity, with no muscle-damaging exercise in the 2 to 3 days before and no illness[4]. The NHS says a simple blood test will confirm whether someone is anaemic[7].
What side effects can iron tablets cause?
The NHS lists constipation, feeling or being sick and stomach pain at doses over 20 mg[5]. Darker poo is common, but the NHS says black, tar-like or red poo needs a doctor or a call to 111[37].
Can I take iron with coffee or breakfast?
The NHS advises leaving a 2 hour gap between iron and tea, coffee, dairy products or eggs, which can stop your body absorbing it[40]. In 34 iron-depleted women, taking iron with coffee cut absorption by 54%, and with a breakfast including coffee and orange juice by 66%[27].
Is iron allowed in sport? What about iron drips?
Iron is not on the WADA Prohibited List, but infusions or injections of more than 100 mL in 12 hours are a prohibited method unless legitimately received during hospital treatment, surgery or clinical investigations; otherwise they need a Therapeutic Use Exemption[43].
What ferritin level counts as low iron?
There is no single cut-off. A Swiss expert review treats ferritin under 15 µg/L as empty stores and 15 to 30 µg/L as low in athletes over 15[29]; the AIS framework uses under 35 µg/L for iron deficiency without anaemia[4]. In one systematic review, every study that found performance gains used a cut-off of 20 µg/L or less[25].
Is iron better absorbed every other day?
In iron-depleted women, yes: 60 mg on alternate days led to more iron absorbed in total than the same doses on consecutive days[35]. Hepcidin was still raised 24 hours after doses of 60 mg or more, and less iron was absorbed from the next day's dose[34]. Neither study measured performance.
Does it matter when I take iron around a run?
Possibly, but the evidence is thin and mixed. More iron was absorbed from breakfast 30 minutes after a morning run than at rest[32], but 36% less from a meal 2 hours after a long run[31]. The AIS says morning iron, close to exercise, may be absorbed better[4].
Do vegetarian and vegan runners have lower iron?
They absorb less iron from food, but many are not low. Estimated absorption is 5 to 12% from vegetarian diets against 14 to 18% from mixed diets[6]. In 81 recreational runners, fewer than 30% of the omnivores, vegetarians or vegans had depleted iron stores[44].
Does vitamin C help iron absorption?
The NHS says vitamin C is believed to increase the amount of iron absorbed, and a doctor or pharmacist may suggest taking iron with orange juice or a vitamin C supplement[33]. The AIS suggests about 50 to 100 mg of vitamin C with the supplement[4]. In 34 iron-depleted women, 80 mg of vitamin C raised absorption from a 100 mg iron dose by 30%, and 500 mg did not raise it significantly further[27].
Is an iron infusion better than tablets for runners?
The evidence is mixed. A single 500 mg infusion didn't improve VO2max or endurance in runners with low iron[18], while in a study without a placebo group, intravenous iron raised both in runners with the lowest iron[26]. The AIS says intravenous iron should only be considered with a sports physician[4].

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Who wrote and reviewed this

Written by Grant MacMillan, Editor, RunV. Not yet reviewed by an expert.

Last checked against its references . Next review due . How pages are reviewed · How we grade research

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