Iron
Also known as Ferrous sulfate, Ferrous bisglycinate
Essential mineral forming the oxygen-binding core of haemoglobin and myoglobin; supplementation corrects iron-deficiency anaemia, a common cause of fatigue, particularly in menstruating women.
Reported side effects:Gastrointestinal effects - constipation, feeling or being sick, and stomach pain with high-dose supplements (NHS: doses above 20 mg/day; NIH ODS: high doses of supplements)
Be careful if: certain ages, pregnant, breastfeeding
Ask first if you take: thyroid medicines, other medicines
Suspect a side effect? Report it to the MHRA Yellow Card scheme ↗
What the research actually shows
We link to relevant studies and leave interpretation to you and your healthcare professional.
Safety and considerations
What sources report about side effects, who should be careful, and medicines to ask about first.
Reported side effects
| Reported effect | How often | Basis | Sources |
|---|---|---|---|
| Gastrointestinal effects - constipation, feeling or being sick, and stomach pain with high-dose supplements (NHS: doses above 20 mg/day; NIH ODS: high doses of supplements) | unknown | Listed by NHS (conditions/medicines pages) | Iron - NHS vitamins and mineralsIron (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12; sheet updated 2025-09-04) |
Serious effects reported
| Reported effect | How often | Basis | Sources |
|---|---|---|---|
| Severe poisoning with extremely high (overdose) intakes - corrosive gut injury, multisystem organ failure and death (NIH ODS); NHS notes very high doses can be fatal, particularly in children | unknown | Listed by NIH ODS fact sheets | Iron (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12; sheet updated 2025-09-04)Iron - NHS vitamins and minerals |
Be careful if you…
| Applies to | What sources say | Sources |
|---|---|---|
| certain ages | Keep iron supplements out of children's reach: NHS states very high doses can be fatal, particularly in children. | Iron - NHS vitamins and minerals |
| pregnant | No pregnancy-specific caution found in the sources checked (NHS iron page lists none; NHS advises 17 mg/day or less from supplements is unlikely to cause harm). | Iron - NHS vitamins and minerals |
| breastfeeding | No breastfeeding-specific caution found in the sources checked (NHS iron page lists none). | Iron - NHS vitamins and minerals |
Medicines to ask about
| Severity | Medicine | Sources |
|---|---|---|
| caution | thyroid medicines — Iron supplements may interact with levothyroxine (NIH ODS lists levothyroxine among interacting medicines; ferrous sulfate has been reported to reduce thyroxine effect). Ask a clinician or pharmacist about spacing doses. | Iron (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12; sheet updated 2025-09-04) |
| caution | other medicines — Iron supplements may interact with levodopa (NIH ODS). | Iron (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12; sheet updated 2025-09-04) |
| listed | other medicines — Proton pump inhibitors can potentially reduce iron absorption (NIH ODS). | Iron (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12; sheet updated 2025-09-04) |
How much is too much
Sources
Iron - NHS vitamins and mineralsIron (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12; sheet updated 2025-09-04)EFSA NDA Panel (2024): Scientific opinion on the tolerable upper intake level for iron, EFSA Journal 22(4):8819Stoffel NU et al., Lancet Haematol 2017;4(11):e524 - iron absorption from supplements on consecutive vs alternate days (PMID 29032957)
Suggested uses (unproven)
Where people report using it and what trials suggest
- Strong evidenceIron-deficiency anaemia
- Moderate evidenceFatigue in deficient but non-anaemic women
Where the evidence is negative
Common claims the trials do not support
- No benefit (and potential harm) for fatigue in iron-replete people.
- No cognitive benefit without deficiency.
- Restless-legs benefit limited to people with low ferritin.
Essential mineral with high-supplement risk if not deficient — testing before supplementing matters more than with most supplements.
Evidence grades reflect study quality and consistency, not marketing claims.
Ordered by strength of evidence, then result direction.
01Psychiatric and cognitive outcomes of iron supplementation in non-anemic children, adolescents, and menstruating adults: A meta-analysis and systematic reviewFiani D, et al. · Neuroscience and biobehavioral reviews · 2025PMID 40945632 ↗02The Role of Iron and Zinc in the Treatment of ADHD among Children and Adolescents: A Systematic Review of Randomized Clinical TrialsGranero R, et al. · Nutrients · 2021PMID 34836314 ↗03Peripheral iron levels in children with attention-deficit hyperactivity disorder: a systematic review and meta-analysisTseng PT, et al. · Scientific reports · 2018PMID 29335588 ↗04Iron Status in Attention-Deficit/Hyperactivity Disorder: A Systematic Review and Meta-AnalysisWang Y, et al. · PloS one · 2017PMID 28046016 ↗05Daily iron supplementation for improving anaemia, iron status and health in menstruating womenLow MS, et al. · The Cochrane database of systematic reviews · 2016PMID 27087396 ↗06Association Between Trace Mineral Concentrations and Oxidative Stress in Children with ADHD Supplemented with MultinutrientsRobinette LM, et al. · Biological trace element research · 2026PMID 41776068 ↗Show all 13 studies
Published research summaries are unavailable.
Why people look into Iron — and what they report
Associated conditions and community-reported outcomes, wrapped together on one page. Aggregates appear only once enough reports arrive.
Where to get Iron (UK)
How this compound is sold in the UK, and where our readers typically find it.
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