Iron supplements: forms, absorption and side effects

By the DieticiansWorldwide.com editorial team · 17 October 2026 · Supplements & industry

Who actually needs an iron supplement?

Iron deficiency is the most common nutritional cause of anaemia worldwide. The World Health Organization estimates that 30% of women aged 15 to 49, 37% of pregnant women and 40% of children aged 6 to 59 months are affected by anaemia, and that dietary iron deficiency is among its largest causes (WHO, anaemia).

That does not mean everyone should take iron. The US National Institutes of Health lists the groups most likely to run short: pregnant women, infants and young children, women with heavy periods, frequent blood donors, and people with cancer, gastrointestinal disorders or heart failure (NIH Office of Dietary Supplements). People who eat no meat or fish are also at higher risk, because iron from plants is absorbed less well.

Most other adults get enough iron from food. The NHS says you should be able to get all the iron you need from your daily diet, and that women with heavy periods are the main group who may need supplements (NHS, iron).

Why you should test before you take iron

Tiredness is the reason most people reach for iron, and tiredness has dozens of causes. Taking iron without a test risks two problems.

First, iron deficiency is a symptom, not a diagnosis. The British Society of Gastroenterology (BSG) notes that roughly a third of men and postmenopausal women with iron deficiency anaemia have an underlying abnormality, most often in the gut, and that bowel and stomach cancers can present this way (BSG guidelines, Gut 2021). It also found coeliac disease in 3% to 5% of cases. Treating the anaemia with a supplement and never asking why can delay a diagnosis that matters. If coeliac disease turns out to be the cause, our guide to the coeliac diet explains the next steps.

Second, some people store too much iron. Haemochromatosis is an inherited condition in which iron builds up slowly over many years, and if it is not treated it can damage the liver, joints, pancreas and heart. The NHS says it most often affects people of white northern European background, particularly those with Irish, Scottish or Welsh ancestry, and that people being treated are usually advised to avoid iron and vitamin C supplements (NHS, haemochromatosis).

A blood test usually includes a full blood count and ferritin, which reflects iron stores. NIH cautions that ferritin can be raised by inflammation, so a "normal" result does not always rule out deficiency; doctors often use more than one measure.

How much iron do you need?

Recommended intakes differ by country and life stage. The US figures below are Recommended Dietary Allowances from the National Academies, published by NIH; the UK figures are from the NHS.

Two details are worth knowing. NIH says the requirement is 1.8 times higher for people who follow vegetarian diets, because non-haem iron from plants is less bioavailable. And it estimates that the body absorbs about 14% to 18% of iron from mixed diets that include meat, seafood and vitamin C, compared with 5% to 12% from vegetarian diets. The upper limits apply to healthy people taking iron for prevention; treatment doses prescribed for diagnosed deficiency are higher and should be supervised. For the US picture in more detail, including how dietetic care is organised, see our United States country guide.

Forms of iron supplement compared

The iron in a tablet comes attached to another molecule, and the form decides how much actual iron, called elemental iron, you get. NIH gives the elemental iron content as 33% for ferrous fumarate, 20% for ferrous sulphate and 12% for ferrous gluconate. A 200 mg tablet of ferrous sulphate therefore supplies about 40 mg of elemental iron. Supplement Facts panels list elemental iron, so you rarely need to do the sum yourself.

Ferrous sulphate versus bisglycinate

This is the comparison most people search for. A 2023 systematic review and meta-analysis of 17 randomised trials compared ferrous bisglycinate with other iron supplements (Fischer and colleagues, Nutrition Reviews 2023). In pregnant women, 4 to 20 weeks of bisglycinate gave higher haemoglobin, with a standardised mean difference of 0.54, and 64% fewer reported gastrointestinal side effects (incidence rate ratio 0.36). Ferritin showed only a non-significant trend, and there were no differences in children. Only four small trials covered men and non-pregnant women, too few to pool.

So bisglycinate looks promising, particularly if you have struggled with side effects, but the evidence outside pregnancy is thin. The BSG still recommends starting with ferrous sulphate, fumarate or gluconate, and moving to other preparations if those are not tolerated.

How to take iron so it works

Once a day, or every other day?

For years, the standard advice was to split iron into two or three doses a day. Research on hepcidin, the hormone that controls iron absorption, has changed that.

In two randomised trials in iron-depleted women in Switzerland, a 60 mg dose taken daily raised hepcidin and reduced absorption from the next day's dose (Stoffel and colleagues, Lancet Haematology 2017). Fourteen doses taken on alternate days over 28 days delivered 175.3 mg of absorbed iron, against 131.0 mg from fourteen doses on consecutive days over 14 days; fractional absorption was 21.8% versus 16.3%. Splitting a 120 mg dose into two 60 mg doses raised hepcidin and gave no extra absorption.

Whether this improves outcomes in people with anaemia is less clear. In a double-blind trial of 200 adults with iron deficiency anaemia in India, 120 mg every other day and 60 mg daily for eight weeks raised haemoglobin by similar amounts, +1.05 g/dL and +1.36 g/dL, with no significant difference (Pasupathy and colleagues, Scientific Reports 2023). The BSG reviewed a similar trial in which twice-daily dosing raised haemoglobin faster, but alternate-day dosing caught up once the same total dose had been given, with less nausea.

The practical reading: a single daily dose is a sound default, and there is no case for splitting doses. Alternate-day dosing is a good option if side effects are stopping you taking iron, and the BSG explicitly suggests "a reduced dose of one tablet every other day" in that situation. Do not change a prescribed regimen without checking with whoever prescribed it.

With food, orange juice or on an empty stomach?

Here official advice differs, and it is worth understanding why.

• Empty stomach. The BSG says oral iron is best absorbed fasting, first thing in the morning or between meals, and notes that the inhibitory effect of tea on absorption fades within about 60 minutes.
• With food. The NHS suggests taking tablets with or soon after food to reduce side effects, and NIH agrees that food can help minimise gut upset (NHS, iron deficiency anaemia). Absorption is a little lower, but a tablet you can tolerate beats a tablet you stop taking.
• Vitamin C. The NHS says a glass of orange juice after a tablet may help absorption. The BSG, by contrast, says routinely taking vitamin C with oral iron is not recommended, citing a large randomised trial that found it did not improve the haemoglobin response, iron loading or side effects.

A sensible compromise is to try iron on an empty stomach first, and move it to just after a light meal if it makes you feel sick. Vitamin C-rich foods remain useful for absorbing iron from meals, whatever you decide about the tablet.

What to keep apart from your iron

NIH lists these interactions. Experts recommend taking calcium and iron supplements at different times, and the labels of some levothyroxine products advise not taking it within 4 hours of iron. NIH also notes that iron supplements of 25 mg or more can reduce zinc absorption. The NHS adds that tea, coffee, milk and dairy, and foods high in phytic acid such as wholegrain cereals, make it harder to absorb iron from food and tablets. Our guide to supplement and drug interactions covers the wider picture.

Side effects and how to manage them

Side effects are the main reason people stop iron. The NHS lists constipation or diarrhoea, tummy pain, heartburn, feeling sick, and darker than usual poo. NIH notes that doses of 45 mg a day or more commonly cause nausea and constipation.

Ways to make iron easier to take:

1. Take one dose a day, not several. In trials, splitting the dose raised hepcidin and did not improve absorption.
2. Try alternate days if daily dosing is not tolerated, as the BSG suggests.
3. Take it with or after food if it upsets your stomach, accepting slightly lower absorption.
4. Try a different form, such as a lower-iron salt or a chelate, after discussing it with your GP or pharmacist.
5. Manage constipation with fluids and fibre. Our high-fibre foods list shows where to find it.

Dark or black stools are expected on iron. Black, tarry stools with tummy pain, vomiting blood, or feeling faint are different, and need urgent medical attention.

Overdose risk for children

Iron is one of the most dangerous supplements to leave within a child's reach. In the US, NIH notes that iron-containing products must carry the warning that accidental overdose "is a leading cause of fatal poisoning in children under 6", and that supplements with 250 mg or more of elemental iron per container must be in child-resistant packaging. The NHS gives the same warning: an overdose of iron in a young child can be fatal. If a child swallows iron tablets, seek emergency help immediately, even if they seem well.

How long does it take for iron to work?

You may feel better within a few weeks, but replacing stores takes months.

• First two to four weeks. The BSG recommends checking for a haemoglobin response in the first four weeks, and says a rise of at least 10 g/L within two weeks strongly suggests iron deficiency was the cause.
• Until haemoglobin is normal, then longer. The BSG advises continuing treatment for about three months after haemoglobin returns to normal, to refill the body's stores.
• Typical course. The NHS says you will need to take tablets for about six months, with repeat blood tests to check progress.

If your levels are not improving, do not simply keep going or double the dose. Poor response can mean you are not absorbing iron, are still losing blood, or have another cause of anaemia, such as low vitamin B12. Our guide to B12 deficiency covers that overlap.

When oral iron is not enough: IV iron

Intravenous (IV) iron is given through a drip in hospital or clinic. The BSG recommends considering it when oral iron is unsuitable, ineffective or not tolerated, and early on when oral iron is unlikely to work or the anaemia needs correcting quickly. It is a medical decision made with your doctor, not something to arrange privately on the strength of a ferritin result.

Iron in anaemia caused by chronic inflammatory disease is a separate, more complex area. NIH notes that iron supplementation in this setting remains controversial, so it should be managed by a specialist.

Food first or supplement first?

For prevention, food first is the right default. For a confirmed deficiency, the NHS recommends iron tablets alongside iron-rich food.

Good food sources, according to the NHS, include red meat, liver (but not in pregnancy), beans such as kidney beans, edamame and chickpeas, nuts, dried fruit such as apricots, fortified breakfast cereals and soy bean flour. The NHS also points to Department of Health advice to limit red and processed meat, which is one reason pulses, fortified cereals and greens matter. To improve absorption from plant sources, eat them with vitamin C-rich foods such as peppers, citrus or tomatoes, and keep tea and coffee away from iron-rich meals. Our healthy eating check gives a quick picture of where your diet stands.

Pregnancy is a special case, and guidance varies. NIH notes that the US Preventive Services Task Force found insufficient evidence to recommend routine screening and iron supplementation for pregnant women without symptoms, while other bodies do recommend supplementation. Follow your midwife's or obstetrician's advice.

Buying iron supplements: what labels guarantee

Iron supplements are sold freely, which makes them easy to misuse and hard to judge.

In the US, the FDA does not have the authority to approve dietary supplements for safety and effectiveness, or to approve their labelling, before they are sold (FDA 101: dietary supplements). Responsibility for safety sits with the manufacturer. NIH notes that iron-only supplements often provide 65 mg of iron, 360% of the Daily Value, well above what most people need for prevention.

Independent quality testing helps with one part of the problem. NIH explains that a seal from a testing organisation indicates the product was properly manufactured, contains the ingredients on the label and does not contain harmful levels of contaminants, but that "these seals do not guarantee that a product is safe or effective" (NIH, what you need to know). Our guide to third-party tested supplements explains how to read those seals, and the site's supplements hub links our other ingredient guides, including vitamin D supplements and omega-3 supplements.

When choosing, check:

• the elemental iron per tablet, not just the compound weight;
• whether the dose matches what your doctor recommended;
• extra ingredients, such as vitamin C, which people with haemochromatosis should avoid;
• child-resistant packaging, especially if children visit your home.

If you are not sure whether you need iron, what dose to take, or how to raise your iron through food, a registered dietitian can review your blood results with your GP's plan and build a diet around them. You can book a session with a dietitian.

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