Iron
Iron
TL;DR
Iron corrects a deficiency and does nothing otherwise — and excess is harmful. Test ferritin before supplementing. Every-other-day dosing absorbs better than daily because it avoids the hepcidin block.
Ideal For
- Menstruating women with heavy periods
- Anyone with ferritin below 30 ug/L
- Pregnant women, following antenatal guidance
- Vegetarians and vegans, whose non-haem iron absorbs less efficiently
- Endurance athletes, particularly female distance runners
- People with restless legs syndrome and ferritin below 75 ug/L
- Blood donors donating frequently
- Post-bariatric surgery patients
Avoid If
- Haemochromatosis or any iron-overload disorder
- Ferritin already in the normal range without symptoms
- Thalassaemia or sideroblastic anaemia unless deficiency is confirmed
- Active infection — iron withholding is part of the immune response
- Repeated blood transfusion history
- Taking iron blindly as a general energy supplement
Frequently Asked Questions
Overview
Iron is the most common nutrient deficiency worldwide and the supplement most often taken without testing, which is unfortunate in both directions: people who need it frequently go untreated, and people who do not need it take it anyway and accumulate a mineral the body has no mechanism to excrete.
The essential point for interpretation is that ferritin, not haemoglobin, is the marker that matters. Iron deficiency progresses through stages: stores deplete first, then transport iron falls, and only at the end does haemoglobin drop into the anaemic range. Symptoms — fatigue, poor concentration, hair shedding, restless legs, breathlessness on exertion, cold intolerance — can appear at the store-depletion stage with a completely normal full blood count. A ferritin below 30 ug/L indicates deficiency, and many clinicians treat symptomatic patients below 50, particularly with restless legs syndrome, where a threshold of 75 is often used. The complication is that ferritin is an acute-phase reactant and rises with inflammation, so it should be read alongside CRP.
Dosing changed meaningfully in the last decade. Iron absorption is regulated by hepcidin, which rises for around 24 hours after an iron dose and blocks further absorption. The Swiss studies by Moretti and Stoffel showed that alternate-day single doses achieve higher fractional absorption than daily or twice-daily dosing — more total iron absorbed, from fewer tablets, with fewer side effects. Guidelines have been slow to catch up, but the physiology is not in dispute.
Repletion is slow. Haemoglobin responds within two to four weeks, but refilling stores takes three to six months of continued treatment after haemoglobin normalises. Stopping early is the most common reason deficiency recurs.
Critically, iron deficiency in an adult male or a postmenopausal woman requires investigation for a source of blood loss, particularly gastrointestinal. Supplementing without finding the cause can delay a cancer diagnosis.
How It Works
- Incorporated into haemoglobin for oxygen transport and myoglobin for muscle oxygen storage
- Essential cofactor in mitochondrial electron transport, underpinning cellular energy production
- Required by tyrosine hydroxylase for dopamine synthesis — the link to restless legs and attention
- Supports myelination and neurotransmitter function during brain development
- Absorbed in the duodenum; haem iron uses a separate, far more efficient transporter than non-haem iron
- Absorption controlled by hepcidin, which rises after a dose and blocks uptake for around 24 hours
Quick Facts
- Essential for oxygen transport and energy production
- Key component of hemoglobin and myoglobin
- Iron deficiency is the most common nutrient deficiency
- Particularly important for women and growing children
- Absorption enhanced by vitamin C, inhibited by calcium
Benefits & Outcomes
VO2 Max Improvement
Correcting iron deficiency can produce large aerobic gains; supplementing when replete does not.
Cold Tolerance
Correcting iron deficiency restores cold tolerance in people who feel cold constantly, because iron is required for thyroid hormone activation and heat production.
Pregnancy Support
Iron deficiency in pregnancy is common and consequential; screening and repletion are standard care.
Nail Strength
Observational only: brittle nails and koilonychia are classic iron-deficiency signs that resolve with repletion.
Physical Stamina Increase
If your ferritin is low, iron repletion transforms stamina more than any other intervention on this page.
Chronic Fatigue Relief
Correcting iron deficiency is the single highest-yield intervention for unexplained fatigue in menstruating women.
Hair Growth
Iron repletion supports hair growth when ferritin is low — the single most impactful nutritional fix for diffuse shedding in premenopausal women.
Post-Illness Recovery
If illness or inflammation left you iron deficient, repletion is the intervention that resolves the fatigue.
Oxygen Utilization Improvement
Correcting iron deficiency produces the largest single supplement-driven improvement in oxygen utilisation — but only if you are actually deficient.
Hair Follicle Stimulation
The single highest-yield thing to check in diffuse hair shedding.
Increased Energy Production
If ferritin is low, iron repletion has one of the best evidence-to-benefit ratios in all of supplementation.
Altitude Adaptation
Iron sufficiency is required for the haemoglobin response to hypoxia, so correcting low ferritin measurably supports acclimatization.
Afternoon Slump Prevention
Iron repletion reliably reduces daytime fatigue in people with low ferritin — and does nothing for those already replete.
Immune Function
Correct deficiency for immune support; do not supplement speculatively — excess iron can feed pathogens.
Cognitive Function
Test ferritin first. If low, correcting it is among the most impactful things you can do for brain fog.
Enhanced Athletic Performance
Essential for deficient athletes; pointless (and risky) at normal ferritin.
Health Concerns Addressed
Morning Fatigue
One of the few supplements with randomised evidence for fatigue — but only in iron-deficient people.
Lethargy
Iron reliably improves lethargy where ferritin is low, and importantly it works even without anaemia. Where iron status is normal it does nothing and carries real risk — testing first is essential.
Uterine Fibroids
Iron doesn't treat fibroids themselves, but it addresses one of their most common consequences: iron-deficiency anemia from heavy menstrual bleeding. Anyone with fibroids and heavy periods should have ferritin and hemoglobin checked.
Cold Sensitivity
Genuine physiology and good evidence — but only when ferritin is low. Test first, replete, and find the source; pointless and potentially harmful when iron status is normal.
Pale Skin
Iron is the correct treatment when pallor comes from confirmed iron-deficiency anaemia — but test before supplementing, because pallor has other causes and unexamined iron deficiency can hide an important source of blood loss.
Restless Legs Syndrome
Iron is not just a supplement option for RLS — guidelines make it first-line treatment when ferritin is low, with trials showing genuine symptom reduction even when standard blood counts are normal.
Cold Hands & Feet
Iron deficiency is a genuine and commonly missed cause of cold hands and feet, and repletion resolves it when that's the cause. Testing ferritin first is essential — iron supplements taken without deficiency carry real risks.
Excessive Sleepiness
Genuinely effective in the specific case of iron-deficient restless legs disrupting sleep. Useless for sleepiness from apnea, insufficient sleep or medication.
Chronic Fatigue
Where fatigue is caused by iron deficiency, iron works — and it works even when haemoglobin is normal. The Verdon trial showed reduced fatigue in non-anaemic women with ferritin under 50 ug/L. Test first; iron does nothing for fatigue in people who are replete.
Shortness of Breath
Correcting iron deficiency reliably improves exertional breathlessness and exercise capacity, including in heart failure patients who are not anaemic. Test ferritin before supplementing.
Difficulty Breathing
Where breathlessness is driven by iron deficiency, correcting iron reliably improves exercise capacity — but only in that specific group.
Celiac Disease
Iron deficiency is the most common nutritional consequence of celiac disease, and replacement corrects it reliably once the gut begins healing on a gluten-free diet.
Exercise Intolerance
Clear benefit in iron deficiency (ferritin under ~30); no benefit, and potential harm, when iron status is normal.
Hair Thinning & Loss
Iron helps hair shedding when ferritin is low, and only then. Telogen effluvium is strongly associated with low ferritin, and most trichologists treat toward 70 ug/L rather than the standard lab cut-off — but regrowth is slow and requires the deficiency to be genuinely corrected.
Fainting
Corrects the anaemic contribution to syncope; irrelevant to the vasovagal reflex behind most faints.
ADHD (Attention Deficit Hyperactivity Disorder)
Iron is not an ADHD treatment, but iron deficiency worsens ADHD symptoms through the dopamine pathway, and low ferritin is more common in children with ADHD. Correcting a measured deficiency is worthwhile; supplementing a replete child is not.
Supporting Research37 studies
Daily iron supplementation for improving anaemia, iron status and health in menstruating women
Low MSY, Speedy J, Styles CE +2 more
Daily iron supplementation raised haemoglobin and ferritin and roughly halved the risk of anaemia and iron deficiency.
Iron deficiency, supplementation, and sports performance in female athletes: A systematic review
Pengelly M, Pumpa K, Pyne DB +1 more
Maximal aerobic capacity improved by 6%-15% following 16 mg/day-100 mg/day of elemental iron for 36-126 days
Psychiatric and cognitive outcomes of iron supplementation in non-anemic children, adolescents, and menstruating adults: A meta-analysis and systematic review
Fiani D, et al.
This suggests that, before anemia emerges, ID may impact brain function, potentially requiring identification and treatment.
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
Stoffel NU, Cercamondi CI, Brittenham G
Providing iron supplements on alternate days and in single doses optimises iron absorption and might be a preferable dosing regimen.
Iron for the treatment of restless legs syndrome
Trotti LM, Becker LA
Iron therapy probably improves restlessness and RLS severity in comparison to placebo.
Ferric Carboxymaltose in Patients with Heart Failure and Iron Deficiency
Anker SD
Treatment with intravenous ferric carboxymaltose in patients with chronic heart failure and iron deficiency, with or without anemia, improves symptoms, functional capacity, and quality of life.
Intermittent iron supplementation for reducing anaemia and its associated impairments in adolescent and adult menstruating women
Fernandez-Gaxiola AC, et al.
Intermittent iron supplementation may reduce anaemia and may improve iron stores among menstruating women in populations with different anaemia and malaria backgrounds. In comparison with daily supplementation, the provision of iron supplements intermittently is probably as effective in preventing or controlling anaemia.
Iron supplementation and aerobic capacity in iron-depleted women: a systematic review and meta-analysis
Pasricha SR, Low M, Thompson J +2 more
Iron supplementation improved maximal oxygen uptake and exercise efficiency in iron-deficient participants
Daily oral iron supplementation during pregnancy
Pena-Rosas JP, De-Regil LM, Garcia-Casal MN +1 more
Daily iron reduced maternal anaemia and low birthweight, with more side effects at higher doses
Dietary supplements for preventing postnatal depression
Miller BJ, Murray L, Beckmann MM +1 more
To assess the benefits of dietary supplements for preventing postnatal depression either in the antenatal period, postnatal period, or both.
Impaired thermoregulation and thyroid function in iron-deficiency anemia
Beard JL, Borel MJ, Derr J
Iron-deficient women maintained lower core temperature during cold exposure with blunted thyroid hormone responses.
Factors Associated with Premature Hair Graying in a Young Indian Population
Chakrabarty S
PHG is associated with low serum ferritin, Vitamin B12, and HDL-C levels in Indian patients aged <25 years.
Intravenous versus oral iron for the treatment of anemia in inflammatory bowel disease: a systematic review and meta-analysis
Bonovas S, Fiorino G, Allocca M
Intravenous iron was more effective and better tolerated than oral iron for the treatment of anemia in inflammatory bowel disease.
Iron for restless legs syndrome
Trotti LM, Becker LA
Iron therapy reduced restless legs syndrome severity scores compared with placebo.
The Role of Vitamins and Minerals in Hair Loss: A Review
Almohanna HM, Ahmed AA, Tsatalis JP +1 more
Deficiency of such micronutrients may represent a modifiable risk factor associated with the development, prevention, and treatment of alopecia.
Vitamin and mineral deficiencies are highly prevalent in newly diagnosed celiac disease patients
Wierdsma NJ, van Bokhorst-de van der Schueren MA, Berkenpas M
Iron deficiency was present in 28% of newly diagnosed celiac patients.
Iron Supplementation in Management of Neurodevelopmental Disorders: Systematic Review, Meta-Analysis, and Qualitative Synthesis
Iron supplementation benefited children with neurodevelopmental disorders primarily when iron deficiency was present.
Impact of intravenous iron or exogenous erythropoietin on hemoglobin mass, exercise performance, and acute mountain sickness during altitude exposure
Intravenous iron increased hemoglobin mass but did not meaningfully reduce acute mountain sickness scores at altitude.
Sleep disturbances in children and adolescents with iron deficiency: Questionnaire-based and actigraphic findings
Sleep Medicine study authors
Pediatric ID/IDA was associated with caregiver-reported and actigraphic sleep disruption, characterized by restless and fragmented sleep.
Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials
Houston BL, Hurrie D, Graham J +9 more
Iron supplementation significantly reduced fatigue scores in non-anaemic iron-deficient adults but did not consistently improve physical capacity.
Daily iron supplementation for improving anaemia, iron status and health in menstruating women
Low MSY, Speedy J, Styles CE +2 more
Daily iron supplementation effectively reduces the prevalence of anaemia and iron deficiency, raises haemoglobin and iron stores, improves exercise performance and reduces symptomatic fatigue.
Ferric carboxymaltose in patients with restless legs syndrome and nonanemic iron deficiency: A randomized trial
Trenkwalder C
In patients who responded to treatment, ferric carboxymaltose may require more time to stabilize restless legs syndrome than previously assumed.
Alternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled study
von Siebenthal HK, et al.
At equal total iron doses, compared to consecutive day dosing of iron, alternate day dosing did not result in higher serum ferritin but reduced iron deficiency at 6 months and triggered fewer gastrointestinal side effects.
Intravenous ferric derisomaltose in patients with heart failure and iron deficiency in the UK (IRONMAN): an investigator-initiated, prospective, randomised, open-label, blinded-endpoint trial
Kalra PR
Intravenous ferric derisomaltose administration was associated with a lower risk of hospital admissions for heart failure and cardiovascular death.
Demographic Characteristics and Association of Serum Vitamin B12, Ferritin and Thyroid Function with Premature Canities in Indian Patients
Sonthalia S
Anemia, serum ferritin, and fasting blood glucose did not show significant association with premature canities.
Hematologic manifestations of celiac disease
Halfdanarson TR, Litzow MR, Murray JA
Iron deficiency anaemia often resolves with a gluten-free diet alone in celiac disease.
Retrospective Review of 2851 Female Patients With Telogen Effluvium: A Single-Center Experience
Karakoyun O
Deficiency of laboratory parameters, especially ferritin and serum iron, is a common finding in female patients presenting with TE.
Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial
Vaucher P, Druais PL, Waldvogel S +1 more
Iron reduced fatigue by about 48% from baseline versus 29% with placebo in non-anaemic women with low ferritin.
The efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical trial
Li N, Zhao G, Wu W
Among patients with IDA, oral iron supplements alone were equivalent to oral iron supplements plus vitamin C in improving hemoglobin recovery and iron absorption.
Correcting a marginal riboflavin deficiency improves hematologic status in young women in the United Kingdom (RIBOFEM)
Powers HJ
Moderately poor riboflavin status can affect iron status: the lower the riboflavin status, the greater the hematologic benefits of improving status.
Iron supplementation benefits physical performance in women of reproductive age: a systematic review and meta-analysis
Pasricha SR, Low M, Thompson J +2 more
Iron supplementation improved maximal oxygen uptake and reduced exercise heart rate, with the largest gains in iron-deficient women.
Is a lower dose of more bioavailable iron (18 mg ferrous bisglycinate) noninferior to 60 mg ferrous sulfate in increasing ferritin concentrations in Cambodian women? A randomized controlled noninferiority trial
Fischer JAJ, Pei LX, Goldfarb DM
Ferrous bisglycinate (18-mg) was not as effective as ferrous sulfate (60-mg) in increasing ferritin concentrations and did not differentially influence biomarkers of gut health.
Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women
Stoffel NU, et al.
Alternate day dosing of oral iron supplements in anemic women may be preferable because it sharply increases FIA.
The effects of oral iron supplementation on cognition in older children and adults: a systematic review and meta-analysis
Falkingham M, Abdelhamid A, Curtis P +3 more
Iron improved attention and concentration, but improved IQ only in participants anaemic at baseline.
Intravenous versus oral iron supplementation for the treatment of anemia in CKD: an updated systematic review and meta-analysis
Shepshelovich D, Rozen-Zvi B, Avni T +2 more
Our results agree with current recommendations for IV iron replacement for patients with CKD stage 5D and support increased use of IV iron for patients with CKD stages 3 to 5.
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
Stoffel NU, Cercamondi CI, Brittenham G +5 more
Alternate-day single doses produced greater total fractional iron absorption than daily or twice-daily dosing.
Effects of iron supplementation on attention deficit hyperactivity disorder in children
Konofal E, Lecendreux M, Deron J +5 more
ADHD Rating Scale scores improved with iron in children with low ferritin but did not reach statistical significance versus placebo.
Safety Information
Potential Side Effects
Constipation, nausea, abdominal cramping, dark stools and a metallic taste are common with ferrous sulfate and are dose-related. Alternate-day dosing, a lower elemental dose or a bisglycinate form resolves most cases. Dark stools are harmless and expected, but should not be confused with the black tarry stool of gastrointestinal bleeding. Iron overdose is a leading cause of poisoning death in young children — keep supplements out of reach and in child-resistant containers. Long-term unnecessary supplementation risks iron accumulation, which is associated with liver and cardiac damage.
Contraindications
Haemochromatosis, iron overload, and haemolytic anaemias with normal or high iron stores. Do not supplement iron in an adult male or postmenopausal woman without investigating the cause of deficiency, since gastrointestinal blood loss including malignancy must be excluded.
Drug Interactions
- Levothyroxine — separate by at least four hours; iron markedly reduces absorption
- Tetracycline and quinolone antibiotics — separate by two to four hours in both directions
- Proton pump inhibitors and H2 blockers — reduce iron absorption by raising gastric pH
- Calcium supplements and dairy — take at different times
- Methyldopa and levodopa — absorption reduced
- Bisphosphonates — separate by at least two hours
- Tea, coffee and red wine tannins — reduce non-haem iron absorption substantially
Pregnancy & Breastfeeding
Pregnancy: safe
Breastfeeding: safe
Dosage Guidelines
Dosage Used in Studies
18-50 mg elemental
Best Time to Take
On empty stomach or with vitamin C
Best Form
Ferrous bisglycinate (gentlest)
Bioavailability
Heme iron (from meat) absorbed better than non-heme. Vitamin C enhances absorption. Calcium, tannins (tea/coffee) reduce absorption. Take 2 hours apart from other supplements/medications.
Dosing by Goal
| Goal | Dose | Notes |
|---|---|---|
| Iron deficiency without anaemia (adults) | 60-100 mg elemental iron, every other day | Alternate-day dosing achieves higher fractional absorption than daily by avoiding the hepcidin block, with fewer side effects. Continue 3 months, then retest. |
| Iron deficiency anaemia | 100-200 mg elemental iron, every other day or daily | Recheck haemoglobin at 4 weeks; expect a rise of at least 10 g/L. Continue for 3 months after haemoglobin normalises to refill stores. |
| Pregnancy | 27-60 mg elemental iron daily, or as directed | Requirements rise sharply in the second and third trimesters. Follow antenatal guidance; ferritin should be checked rather than assumed. |
| Restless legs syndrome | 65 mg elemental iron with vitamin C, alternate days | Treat if ferritin is below 75 ug/L — a higher threshold than for general deficiency. Allow 3 months. |
| Maintenance in menstruating women with recurrent deficiency | Lower dose or intermittent weekly dosing | Weekly dosing is an option supported by WHO guidance where daily is not tolerated. |
Forms Compared
Ferrous sulfate
Best for Standard first-line repletion
65 mg elemental iron per 325 mg tablet. Cheapest and most studied. GI side effects are the main limitation.
Ferrous bisglycinate
Best for People who cannot tolerate sulfate
Chelated and considerably gentler on the gut, with good absorption at lower elemental doses (typically 25-28 mg).
Ferrous gluconate
Best for Milder alternative to sulfate
Lower elemental iron per tablet (around 35 mg), often better tolerated but requires more tablets.
Iron polysaccharide complex
Best for Tolerability
Well tolerated, though evidence suggests absorption may be lower than ferrous salts.
Intravenous iron (ferric carboxymaltose, iron sucrose)
Best for Malabsorption, intolerance, or rapid repletion needed
Corrects stores in one or two infusions. Requires medical administration. The right choice in IBD, post-bariatric surgery and heavy uterine bleeding with severe deficiency.
Heme iron polypeptide
Best for Sensitive stomachs
Absorbed via the haem transporter, so less affected by food inhibitors. Expensive per mg of iron.
Food & Timing
Take on an empty stomach, one hour before or two hours after food, with a source of vitamin C such as orange juice, which converts iron to the better-absorbed ferrous form. If that causes nausea, taking it with a small non-dairy snack is a reasonable compromise — some absorption beats none. Keep it well away from tea, coffee, calcium supplements, dairy and antacids, all of which substantially reduce absorption.
Medical Disclaimer
The information provided on this website is for educational and informational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or before starting any supplement regimen.
Individual results may vary. The statements on this website have not been evaluated by the Food and Drug Administration. Products and information are not intended to diagnose, treat, cure, or prevent any disease.