Iron
An essential mineral, taken above all for tiredness and low iron.
- Tiredness and energy
- Low iron and anaemia
- Pregnancy
- Exercise and sport
- Thinking and learning
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
On this page
What is iron?
Iron is a mineral the body cannot do without: several hundred of its proteins are built around an iron atom, running things as basic as how cells turn oxygen into energy and how DNA is mended. Going short of it is one of the biggest contributors to ill health worldwide, falling hardest on young children, on women before the menopause and on people in lower-income countries. Anaemia is the best-known consequence, though the effects can be felt while a blood count still reads normal. As a supplement it is always bound to a partner - sulphate, fumarate, gluconate or glycine - which decides how much of each tablet is actually iron.
Why do people take iron?
Tiredness and energy
The thinking. Feeling flat and washed out is the commonest reason people reach for iron, and often what sends them for a blood test. Stores can run low well before a blood count looks abnormal, and reviews of iron deficiency make the same point.
What research has found. One review pooled 18 randomised trials in 1,170 adults who were iron deficient but not anaemic. Tiredness scores came out lower on iron, a modest but consistent difference; measured physical capacity did not change. A fair body of trials has looked at this, and most point the same way.
Low iron and anaemia
The thinking. Plenty of people start iron because a blood test came back low, or because they are losing more than they take in - heavy periods being the everyday example. The thinking is straightforward: put back what is missing and the blood follows.
What research has found. A review of 67 trials in 8,506 menstruating women found those on daily iron clearly less likely to be anaemic by the end, with higher haemoglobin and fuller iron stores. A second review, in 42,306 young children, found the same fall in anaemia. This is one of the best-studied uses, and several reviews of trials broadly agree.
Pregnancy
The thinking. Iron is offered as a matter of routine in antenatal care in much of the world, often on a midwife’s word or after a test showed levels drifting down. The thinking is to keep maternal stores from running down, so anaemia has not set in by the birth.
What research has found. One review of 43,274 women found anaemia at term about 70 per cent less common on daily iron, and iron deficiency down by well over half; a later review of 24,023 women found much the same. Birthweight and preterm birth showed no clear differences. Several reviews of trials have measured this, and they broadly agree.
Exercise and sport
The thinking. Iron sits inside the machinery cells use to turn oxygen into energy, so people who train reach for it when easy sessions feel hard. It is a common reason among runners and cyclists.
What research has found. The trials that measured exercise capacity were in adults with heart failure and low iron. One gave high-dose iron by mouth for sixteen weeks and found no difference against a dummy tablet; a smaller one found no change in peak oxygen uptake. Research here is still young: a few studies so far, mostly pointing the same way.
Thinking and learning
The thinking. Iron has a hand in how the brain develops, which is why it comes up for children slow to reach milestones and teenagers heading into exams. The studies ask whether topping it up changes how a child thinks, remembers and does at school.
What research has found. An umbrella review of seventeen earlier reviews found modest gains in intelligence, memory and attention among anaemic children, and little or nothing in those who were not short of iron. A review in adolescents found better school performance and attention but not memory; a third, on early development, found no clear relationship. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
What does iron do in the body?
Iron is a working part, not a fuel. Several hundred proteins have an iron atom built into them, and the body draws on its stores to make two particular pieces - iron-sulphur clusters and haem - that let cells breathe, read their genes and mend DNA.
Iron is also reactive: in an oxygen-rich body it readily makes the unstable molecules that damage cells. So rather than absorbing more whenever supplies run low, the body holds it on a tight rein, with a hormone called hepcidin as the main control, deciding how much iron leaves the gut and the stores for the bloodstream. That is why swallowing a big dose does not mean absorbing one: iron taken by mouth pushes hepcidin up, which caps how efficiently a high dose is taken up, and how much gets through during inflammation.
What do people say about taking iron?
These are anecdotes people have shared, not evidence.
What people like
- Very commonly, energy coming back and a mental fog lifting; people who were genuinely low describe by far the clearest change.
- Often it is slow: three to six weeks before anything is felt, and three months before a repeat test looks right.
- Some women say their periods grew lighter or their cycle lengthened as ferritin came up, and several mention mood steadying too.
- People who train describe tiredness and fog lifting once stores come up, and follow the ferritin number closely between tests.
Common complaints
- Stomach trouble dominates every thread: constipation above all, then nausea, cramping, bloating and wind, with a minority getting the opposite problem.
- Stronger sulphate tablets draw the worst reports by some distance, and people very commonly change form or dose rather than stop.
- Some find a repeat test has barely moved, or has fallen further, after months of taking something they thought was working.
- Black stools come up constantly and alarm people the first time, though posters take them as expected rather than a problem.
How people take it
- Almost everyone’s first suggestion is vitamin C or orange juice with the dose, and keeping tea, coffee and milk hours away.
- Taking it every other day rather than daily is very commonly suggested, both for the stomach and because people believe more is absorbed.
- Evening or bedtime dosing is the usual fix for daytime nausea; some take it on an empty stomach, others only with food.
- The gentler forms - glycinate, fumarate, gluconate, liquids - are where people go when sulphate proves too harsh, often at lower doses.
Read from these public discussions:
- Has anyone felt better after taking iron supplements at home? (Mumsnet, 2021)
- Supplement suggestions for low Iron/ferritin (Mumsnet, 2023)
- Low ferritin - fatigue (Slowtwitch triathlon forum, 2024)
- Iron supplements and stomach issues (Mumsnet, 2024)
- To want to find an iron supplement that doesn't cause digestive issues (Mumsnet, 2024)
- AIBU to ask about ferritin, short cycles and ferrous fumarate side effects? (Mumsnet, 2026)
What forms of iron are there?
| Form | What’s different | Elemental iron by weight | Per serving, on products we list |
|---|---|---|---|
| Bisglycinate (gentle iron) | Iron bound to two molecules of the amino acid glycine. | stated on the label | not in our price tables yet |
| Fumarate | Iron bound to fumaric acid. | about 32.9 % | 20 mg |
| Sulphate | Iron bound to sulphate, the form most commonly prescribed. | about 20.1 % | not in our price tables yet |
| Gluconate | Iron bound to gluconic acid. | about 11.6 % | not in our price tables yet |
| Other forms | Citrate, pyrophosphate, a liquid preparation, or a form the label does not name. | stated on the label | 14 mg |
What else is worth knowing about iron?
- Labels give the elemental iron per serving, and a serving may be more than one capsule; the partner the iron is bound to decides how much of a tablet’s weight is iron.
- Gut symptoms are what the trials report most: the review in menstruating women found constipation and loose stools both more common on iron, and the trials in young children reported more vomiting and fever, with slightly less gain in length and weight.
- Reviews of iron deficiency are firm that it always has a cause worth finding: coeliac screening is routine, and in men and postmenopausal women bleeding in the gut is looked for.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (19 studies)
- Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. (2018, systematic review)
- Interventions to Treat Pediatric Iron Deficiency Anemia: A Systematic Review and Meta-Analysis. (2026, meta-analysis)
- Safety and efficacy of liposomal iron for iron deficiency anemia in pediatric population: A systematic review and meta-analysis of randomized controlled trials. (2026, meta-analysis)
- Daily iron supplementation for improving anaemia, iron status and health in menstruating women. (2016, meta-analysis)
- Effect of daily iron supplementation on health in children aged 4-23 months: a systematic review and meta-analysis of randomised controlled trials. (2013, meta-analysis)
- Effect of Low-Dose Ferrous Sulfate vs Iron Polysaccharide Complex on Hemoglobin Concentration in Young Children With Nutritional Iron-Deficiency Anemia: A Randomized Clinical Trial. (2017, randomised trial)
- Screening and Supplementation for Iron Deficiency and Iron Deficiency Anemia During Pregnancy: Updated Evidence Report and Systematic Review for the US Preventive Services Task Force. (2024, systematic review)
- Daily oral iron supplementation during pregnancy. (2015, meta-analysis)
- Effect of Oral Iron Repletion on Exercise Capacity in Patients With Heart Failure With Reduced Ejection Fraction and Iron Deficiency: The IRONOUT HF Randomized Clinical Trial. (2017, randomised trial)
- IRON-HF study: a randomized trial to assess the effects of iron in heart failure patients with anemia. (2013, randomised trial)
- Effect of iron supplements on cognitive development in children: an umbrella review. (2026, systematic review)
- Iron Status, Anemia, and Iron Interventions and Their Associations with Cognitive and Academic Performance in Adolescents: A Systematic Review. (2022, systematic review)
- The Role of Iron in Brain Development: A Systematic Review. (2020, systematic review)
- Targeting Ferroptosis to Iron Out Cancer. (2019, review)
- The molecular and metabolic landscape of iron and ferroptosis in cardiovascular disease. (2023, review)
- Mechanisms controlling cellular and systemic iron homeostasis. (2024, review)
- Lipid Peroxidation and Iron Metabolism: Two Corner Stones in the Homeostasis Control of Ferroptosis. (2022, review)
- The role of iron and reactive oxygen species in cell death. (2014, review)
- Iron deficiency. (2021, review)
How much does iron cost?
That depends on the form and the pack, so we work out the price of the same dose of every iron product we can find and rank them.