Key takeaways
- Iron reduces tiredness only if you are deficient. Get a ferritin blood test first: below 30 µg/l confirms iron deficiency (NICE), and below 15 µg/l iron stores are empty (WHO).
- In a trial of 198 tired women without anaemia but with low ferritin, iron cut fatigue scores by 48%, against 29% with placebo.
- The NHS says 17 mg or less a day of iron from supplements is unlikely to cause harm, unless your GP prescribes more. EFSA considers up to 40 mg a day in total safe; Germany's BfR recommends no more than 6 mg per daily dose in over-the-counter supplements.
- Taking iron on alternate days may improve absorption. Keep it away from coffee, tea and calcium, and take it with vitamin C.
The short answer
If you feel tired for no clear reason, iron only helps if you are low in it. Before buying anything, ask your GP for a ferritin test: it is a routine blood test and it is how you find out. If you are deficient, iron has some of the clearest evidence of any supplement for tiredness. If you are not, it adds nothing, and the excess builds up in your body.
Who is most at risk
According to the NIH, the groups most at risk of iron deficiency are:
- women with heavy periods;
- pregnant women;
- frequent blood donors;
- people with gut conditions (such as coeliac disease) or gut surgery;
- people with heart failure or cancer.
Vegetarian and vegan diets provide only non-haem iron, which is less well absorbed, so they need planning.
What your ferritin result means
Ferritin is the protein that stores iron, and the amount in your blood reflects how full your stores are. It is the single most useful test for iron deficiency, but the number needs context.
| Ferritin (µg/l) | What it usually means | Who says so |
|---|---|---|
| Below 15 | Iron stores are empty, in an otherwise healthy adult | WHO (2020) |
| Below 30 | Iron deficiency confirmed, with or without anaemia | NICE (guidance for UK GPs) |
| 30 to 50 | Grey zone: deficiency is less likely, but a doctor may still consider it if you have heavy periods or other risk factors. The fatigue trial below included women up to 50 µg/l | CMAJ trial, 2012 |
| Below 70 during an infection or inflammation | Can still be iron deficiency, because inflammation pushes ferritin up | WHO (2020) |
| Above 150 (women who menstruate) or 200 (men and other women) | Possible iron overload in an otherwise healthy person: it needs more tests, not iron | WHO (2020) |
- “Within range” is not the same as fine. Each lab prints its own reference range, and its lower limit can sit below 30 µg/l, the level NICE uses to confirm deficiency.
- Ferritin rises with inflammation, infections and liver disease, so a normal or high result can hide a deficiency. Your GP may add CRP (a marker of inflammation) or transferrin saturation.
- A high result is not a diagnosis on its own. The WHO says ferritin alone should not be used to identify iron overload; it needs further tests. Don’t take iron until it has been checked.
Your GP reads ferritin together with haemoglobin. You can be iron deficient without being anaemic: haemoglobin is normal, but stores are low.
How much it really helps
In a trial published in CMAJ in 2012, 198 women aged 18–53 with unexplained fatigue, no anaemia and ferritin below 50 µg/l took 80 mg of iron a day or a placebo for 12 weeks. Fatigue scores fell 47.7% with iron and 28.8% with placebo. A real effect — in people with low stores, at a high dose, under trial conditions.
UK and EU law allow iron to claim that it contributes to the reduction of tiredness and fatigue, normal formation of red blood cells and haemoglobin, and normal oxygen transport. No claim says it gives energy to people whose stores are fine.
Why not take it “just in case”
- NHS (UK): taking 17 mg or less a day of iron supplements is unlikely to cause harm; side effects such as constipation, nausea and stomach pain become more common above 20 mg. Continue a higher dose if your GP has advised it. The NHS puts daily needs at 8.7 mg for men and women over 50, and 14.8 mg for women aged 19 to 49.
- EFSA could not set a tolerable upper level for iron and in 2024 set a safe level of 40 mg a day of total iron for adults.
- Germany’s BfR recommends no more than 6 mg of iron per daily dose in over-the-counter supplements, and that men, postmenopausal women and pregnant women only take them after medical advice.
- Excess builds up: the body has no efficient way to get rid of iron. People with haemochromatosis, an inherited condition, must not take it.
- Children: according to the NIH, accidental overdose of iron products is a leading cause of fatal poisoning in children under 6. Keep it out of reach.
An example from our database: Solgar Gentle Iron provides 25 mg of iron (bisglycinate) per capsule. That is a dose for treating deficiency: above the NHS figure of 17 mg and four times the BfR’s recommendation for a freely sold supplement. Use it only with a test result and medical advice.
How to take it if advised to
- On an empty stomach or with a light meal if you tolerate it; with food if it upsets you.
- With vitamin C (for example orange juice), which improves absorption of non-haem iron.
- Away from calcium, coffee and tea by at least a couple of hours.
- On alternate days, if your GP agrees: in a study of women with low iron stores, absorption was about a third higher than with daily dosing.
- Re-test after a few months to check ferritin is rising and avoid taking it longer than needed.
Common side effects: constipation, nausea and dark stools. Black stools are normal on iron, but abdominal pain or black stools when you are not taking iron need a doctor. Very high doses of iron can be fatal for children, so keep iron supplements out of their reach.
Interactions: ask your GP or pharmacist if you take levothyroxine, levodopa or proton pump inhibitors such as omeprazole.
Our goal finder shows other ingredients with tiredness claims, such as vitamin B12 and magnesium, and when they make sense.
FAQ
How do I know if I'm low in iron?
With a blood test for ferritin, read together with haemoglobin. NICE, which guides UK GPs, treats a ferritin below 30 µg/l as confirmed iron deficiency; the WHO uses below 15 µg/l to define empty iron stores in otherwise healthy adults.
What ferritin level is too low?
For an otherwise healthy adult, the WHO defines empty iron stores as a ferritin below 15 µg/l, and NICE treats below 30 µg/l as confirmed iron deficiency. During an infection or inflammation, ferritin rises, so the WHO uses below 70 µg/l instead. Many labs print a lower reference range, so "within range" does not always mean your stores are fine.
What does a high ferritin result mean?
According to the WHO, a ferritin above 150 µg/l in women who menstruate, or above 200 µg/l in men and other women, may point to iron overload in otherwise healthy people. Ferritin alone can't diagnose it, because inflammation, infection and liver disease also raise it. Don't take iron supplements until your GP has checked it.
Who is most at risk of iron deficiency?
Women with heavy periods, pregnant women, frequent blood donors, people with gut conditions or gut surgery, and people on poorly planned vegetarian or vegan diets.
Can I take iron without a prescription?
Yes, low-dose iron is sold over the counter in the UK. The NHS says 17 mg or less a day is unlikely to cause harm; treatment doses for iron deficiency anaemia are higher and are prescribed by a GP. Germany's BfR recommends that men, postmenopausal women and pregnant women only take iron after medical advice. Excess iron builds up and can damage the liver.
Which iron is gentlest on the stomach?
According to the NIH, amino-acid chelates (such as bisglycinate) and some other forms may cause fewer digestive side effects than ferrous sulfate. Side effects rise with the dose.
What should I not take with iron?
Keep it apart from calcium, coffee and tea. If you take levothyroxine, levodopa or proton pump inhibitors, ask your GP or pharmacist, as they interact with iron.
Sources
- WHO (2020) — Guideline on use of ferritin concentrations to assess iron status in individuals and populations
- WHO eLENA — Ferritin concentrations to assess iron status (cut-offs for deficiency and overload)
- NICE Clinical Knowledge Summaries — Anaemia, iron deficiency
- NHS — Iron
- DHSC — Great Britain nutrition and health claims (NHC) register
- NIH Office of Dietary Supplements — Iron fact sheet for health professionals
- EFSA (2024) — Scientific opinion on the tolerable upper intake level for iron (safe level 40 mg/day)
- Verbraucherzentrale — Iron, quality not quantity (BfR 6 mg recommendation; 2026, German)
- Vaucher P et al. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ, 2012
- Stoffel NU 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. Lancet Haematol, 2017
- European Commission — EU Register of nutrition and health claims