Iron supplements for women: who needs them and how
Iron is the supplement women most often need and most often take wrong. Who benefits, which forms are gentlest, and how to take it for absorption.
Reviewed by Emma Carter, Senior Health & Nutrition Editor
Iron is the supplement women most often need, and the one most often taken without a plan. Get it right and it can lift the fatigue and hair shedding of low iron. Get it wrong and you deal with side effects for no benefit, or you top up iron you did not need in the first place. Here is how to take it well.
Should you be taking iron at all?
Iron is different from most supplements: more is not better. Take it if you have confirmed low iron or iron deficiency, usually from heavy periods, pregnancy or a plant-based diet. If you are not sure, test first. A ferritin test shows your iron stores, and our guide to iron deficiency explains the full picture. Taking iron long term without knowing you are low offers no benefit and, over time, can cause harm, because the body has no easy way to get rid of the excess.
How much you need
| Life stage | Iron a day |
|---|---|
| Women 19 to 50 | About 18 mg |
| Pregnancy | 27 mg |
| After 50 | 8 mg |
| Vegetarians | Roughly 1.8 times more, since plant iron absorbs less well |
| Upper limit from supplements | 45 mg, unless your doctor advises more |
Food first, and the heme versus non-heme divide
For many women, better food and fixing the underlying cause do most of the work. Iron comes in two forms. Heme iron, from red meat, fish and shellfish, is absorbed well. Non-heme iron, from beans, lentils, tofu, fortified cereals and leafy greens, is absorbed less easily, which is why pairing it with a source of vitamin C, and keeping tea and coffee away from the meal, makes a real difference. A supplement is a top-up for a genuine gap, not a substitute for a varied diet. Our guide to iron-rich foods covers the best sources and how to build meals that maximize absorption.
Choosing a form
Iron in supplements comes as ferrous sulfate, ferrous gluconate, ferrous bisglycinate and others, and they differ in how much actual (elemental) iron they deliver and how easily they sit with you. Ferrous sulfate works well but can cause constipation, nausea and dark stools. Gentler forms such as ferrous bisglycinate are often better tolerated. This matters more than it sounds, because the best iron supplement is simply the one you can keep taking long enough to refill your stores.
How to take iron for best results
- Pair it with vitamin C, a glass of orange juice or some fruit, to boost absorption
- Keep it away from tea, coffee, dairy and calcium, which all reduce absorption
- Separate it from thyroid medication by several hours, since iron blocks levothyroxine
- Consider every-other-day dosing. In iron-depleted women, alternate-day dosing raised the fraction of iron absorbed compared with daily dosing, because a daily dose lifts hepcidin and blunts the next one (Lancet Haematology, 2017)
- Take it with food if it upsets your stomach, and do not be alarmed by darker stools
How long, and rechecking
Refilling iron stores is slow. Symptoms may lift within a few weeks, but the stores behind them take months to rebuild, so most women continue for several months, often about three months after their levels return to normal, before rechecking ferritin with their doctor. Stopping the moment you feel better is a common reason iron dips again. And because periods keep draining iron, treating the cause, such as heavy bleeding, is as important as the supplement.
A safety note
Iron supplements are a leading cause of accidental poisoning in young children, so always keep them well out of reach. And because the body cannot easily remove excess iron, do not take high doses long term without medical guidance.
When to see a doctor
If you have symptoms of low iron such as fatigue, breathlessness or hair shedding, ask for a blood test before supplementing. Treating a confirmed deficiency, and finding its cause, matters more than the supplement itself.
Frequently asked questions
Who should take an iron supplement?
Women with confirmed low iron or iron deficiency, often from heavy periods, pregnancy, or a plant-based diet. Iron is one of the few supplements worth testing for first, because taking it when you do not need it offers no benefit and, over time, too much iron can cause harm.
How much iron do women need?
Women aged 19 to 50 need about 18 mg a day, rising to 27 mg in pregnancy, and 8 mg after 50. Vegetarians need roughly 1.8 times more because plant (non-heme) iron is absorbed less well. The upper limit from supplements is 45 mg unless a doctor advises otherwise.
Which iron supplement is gentlest?
Ferrous sulfate is effective but can cause constipation and nausea for some. Gentler options like ferrous bisglycinate are often better tolerated. Taking iron every other day, or with food, can also reduce side effects, and the best supplement is the one you can actually keep taking.
How long should I take iron for?
It takes months to refill your stores, so most people continue for several months, often around three months after blood levels return to normal, then recheck. Do not stop the moment you feel better, or your stores can run low again. Treating the cause matters as much as the supplement.
How do I get the best absorption from iron?
Take it with a source of vitamin C, and away from tea, coffee, dairy and calcium supplements, which all reduce absorption. Iron also blocks thyroid medication, so separate them by several hours.
References
- Iron absorption from oral iron supplements given on consecutive versus alternate days in iron-depleted women · Stoffel NU et al, Lancet Haematology, 2017
- Iron: Fact Sheet for Consumers · NIH Office of Dietary Supplements
- Effectiveness of dietary interventions to treat iron-deficiency anaemia in women · Nutrients, 2022