How to Take Iron: Dosage, Timing and Duration

Iron deficiency is one of the most common nutritional problems in sport, particularly among female and endurance athletes. Yet the way iron tablets are taken can make a large difference to how much is absorbed and how well they are tolerated. Our editorial team summarises what research says about dose, timing and duration.

First test, then supplement

Iron should not be taken "just in case". Unlike many vitamins, excess iron is not easily excreted, and supplements can cause harm in people with iron overload conditions such as haemochromatosis, which is relatively common in people of Northern European descent.

A blood test for ferritin, haemoglobin and often transferrin saturation helps distinguish iron deficiency without anaemia from iron deficiency anaemia. Ferritin is an acute-phase marker, so it can be falsely raised by inflammation or hard training, and tests are best taken when you are well and rested.

Iron deficiency has causes that need attention beyond supplements: heavy periods, gastrointestinal blood loss, coeliac disease and low dietary intake. In adults, especially men and post-menopausal women, unexplained iron deficiency requires medical investigation.

A review by Pasricha and colleagues in The Lancet stressed that iron deficiency causes fatigue and reduced exercise capacity even before anaemia develops, which is why athletes benefit from monitoring.

How much iron?

In the UK, the most commonly used preparation is ferrous sulphate 200 mg tablets, each providing about 65 mg of elemental iron. Ferrous fumarate and ferrous gluconate provide different elemental amounts per tablet, so always compare elemental iron, not the salt weight.

Historically, treatment involved two or three tablets a day. Research by Moretti and colleagues changed this thinking: a dose of iron raises the hormone hepcidin for about 24 hours, which blocks absorption of the next dose. Taking more frequent or larger doses therefore yields diminishing returns.

Many clinicians now recommend a single daily dose, and NHS guidance has moved towards once-daily treatment for many patients. Your GP will decide the dose based on your results, as treatment of anaemia differs from maintenance in athletes with low stores.

For those with only mildly low stores and good dietary habits, lower-dose supplements may be enough, but this should still be guided by testing.

Timing: alternate days, mornings and after exercise

A pair of trials by Stoffel and colleagues in iron-depleted women showed that giving iron on alternate days led to higher total absorption than giving the same dose on consecutive days, and that a single morning dose was better than splitting it into two doses.

Exercise also influences hepcidin. Peeling and colleagues reported that hepcidin rises around three to six hours after strenuous exercise, especially endurance sessions. A practical approach discussed in sports nutrition literature is to take iron in the morning, either before training or soon after it, rather than late in the day after a hard session.

Taking iron on an empty stomach improves absorption, but it also increases nausea and stomach pain. If side effects are a problem, taking it with a small meal is better than stopping altogether.

Vitamin C or a glass of orange juice can enhance absorption of non-haem iron. Conversely, tea, coffee, calcium supplements, dairy products and antacids reduce absorption and are best separated by at least one to two hours.

Side effects and how to manage them

Gastrointestinal side effects are the main reason people abandon iron. A meta-analysis by Tolkien and colleagues found that ferrous sulphate significantly increased nausea, constipation, diarrhoea and abdominal pain compared with placebo.

• Switching to alternate-day dosing often improves tolerance.
• Taking iron with a light meal reduces nausea at the cost of some absorption.
• Dark or black stools are expected and harmless.
• Different salts or liquid formulations may suit some people better.
• Persistent pain, black tarry stools with dizziness or vomiting blood require urgent medical care.

Keep iron tablets locked away from children. Accidental iron overdose is a well-recognised cause of serious poisoning in young children.

If oral iron fails or cannot be tolerated, doctors may consider intravenous iron. In sport, IV infusions above a certain volume are restricted under WADA rules unless given in a hospital setting or with a therapeutic use exemption, so athletes should check before any treatment.

How long to continue

Haemoglobin usually starts rising within two to four weeks if iron deficiency anaemia is being corrected. However, replenishing stores takes longer, and NHS guidance typically advises continuing treatment for about three months after haemoglobin returns to normal.

A follow-up blood test after around 8–12 weeks is commonly used to assess response. If levels do not improve, the reasons might include poor adherence, ongoing blood loss, malabsorption or an incorrect diagnosis.

Athletes with recurring low ferritin may benefit from periodic monitoring rather than continuous supplementation. A narrative review by Sim and colleagues highlighted that iron needs vary with training load, altitude exposure and menstrual losses.

Dietary strategies remain important: red meat, oily fish, pulses, fortified cereals and dark leafy greens, combined with vitamin C sources, help maintain stores once they are restored.

The editorial team's conclusions

Iron works best when taken once daily or on alternate days, in the morning, away from tea, coffee and calcium.

Athletes should consider training timing because of the post-exercise rise in hepcidin.

Treatment typically lasts several months, and the underlying cause of deficiency should always be identified.

We recommend our related articles on interpreting ferritin results, iron needs in female athletes, and vitamin C and absorption of minerals.

References

1. Moretti D, Goede JS, Zeder C, Jiskra M, Chatzinakou V, Tjalsma H, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981–1989.
2. Stoffel NU, Cercamondi CI, Brittenham G, Zeder C, Geurts-Moespot AJ, Swinkels DW, 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: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524–e533.
3. Peeling P, Dawson B, Goodman C, Landers G, Trinder D. Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. Eur J Appl Physiol. 2008;103(4):381–391.
4. Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AKA, Stellingwerff T, et al. Iron considerations for the athlete: a narrative review. Eur J Appl Physiol. 2019;119(7):1463–1478.
5. Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. Iron deficiency. Lancet. 2021;397(10270):233–248.
6. Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JR. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383.