Signs iron tablets are working

How long do iron tablets take to work?

Most people start to feel better after 3 to 4 weeks, but the NHS says it can take 2 to 4 months for iron tablets to work fully. Your blood count moves first: the British Society of Gastroenterology (BSG) says haemoglobin usually starts to rise within 2 weeks. Its 2021 guideline recommends a check within the first 4 weeks, when a rise of 20 g/L, or a return to the normal range, counts as an adequate response.

Refilling your iron stores takes longer than fixing the blood count. The BSG recommends carrying on for around 3 months after haemoglobin is back to normal, and the NHS says 3 to 6 months after your condition improves. Your GP will set the length. If you bought iron from a pharmacy without a prescription, the NHS says not to take ferrous fumarate for longer than 6 months, and to see a GP if your symptoms get worse or don't improve.

These timings apply to treatment doses, usually one tablet giving 50 to 100 mg of iron a day. There's no equivalent timeline for a low-dose food supplement, which isn't designed to treat anaemia.

What are the signs iron pills are working?

The surest sign is a blood test, and the NHS says your doctor may recommend one within the first 4 weeks. The sign you'll notice yourself is the symptoms that sent you to the GP, such as tiredness and breathlessness, starting to ease, usually from about 3 to 4 weeks.

If you're taking iron to prevent anaemia rather than treat it, the NHS says you probably won't feel any different, and that doesn't mean it isn't working.

Lists online also promise thicker hair and stronger nails. We couldn't find good evidence on how quickly, or whether, those change, so don't judge your treatment by them, or by dark poo (see below).

What if iron tablets aren't working?

See your GP if you feel no better after a month or your blood test hasn't moved. Possible reasons include missed doses because of side effects, taking the tablets with food or tea, a condition that blocks absorption such as coeliac disease, and blood loss that is still going on. Options include taking iron every other day, a different oral preparation, or iron given by injection or drip, which the NHS says is usually done in hospital.

Do iron tablets make your poo black, and does that mean they're working?

They often make it darker, sometimes black, and the NHS says this is usually nothing to worry about. It isn't a sign they're working, because the colour comes from iron your body didn't absorb: the European Food Safety Authority describes black stools as reflecting "large amounts of unabsorbed iron in the gut". Most of each dose goes unabsorbed; in a Swiss study of iron-depleted women taking 60 mg doses, about 16 to 22% was absorbed.

Whether your poo turns dark depends partly on the dose and type of iron. In a Danish trial in pregnant women, 8% of those taking 25 mg of iron as bisglycinate reported black stools, against 31% of those taking 50 mg as ferrous sulfate, and black stools became more common as the dose went up. So poo that stays its usual colour doesn't mean the tablets have failed.

When is black poo a warning sign?

When it's tar-like, sticky or very smelly, has red blood in it, or comes with feeling unwell. Bleeding high in the gut, from a stomach ulcer for example, can also turn poo black. The NHS ferrous sulfate page lists darker poo as a common side effect, but says to talk to a doctor or call 111 straight away if your poo is black, tar-like or red, or has blood in or on it, or if you feel unwell in any other way. Its stomach ulcer page says to call 999 or go to A&E if your poo is black, sticky and very smelly, or if you're vomiting blood or something that looks like ground coffee.

If you can't tell whether your poo is just dark or black and tarry, treat it as the second and call 111. Be more alert if you also take anti-inflammatory painkillers such as ibuprofen or aspirin, or a blood thinner, since the NHS lists these as possible causes of bleeding in the gut.

What are the side effects of iron tablets?

The NHS lists feeling or being sick, stomach discomfort or heartburn, losing your appetite, constipation, diarrhoea and darker poo as common side effects of ferrous sulfate, plus black-stained teeth. A 2015 meta-analysis of 43 trials in 6,831 adults found ferrous sulfate more than doubled the odds of gut side effects compared with placebo (odds ratio 2.32), with no clear link to the dose. Up to 40% of people in observational studies stop oral iron, according to the BSG.

Side effects often settle as your body gets used to the tablets. The NHS suggests taking them with or just after a meal or snack if they make you feel sick or give you heartburn, and more fibre, water and activity for constipation, which is common in pregnancy. The NHS says a side effect with one type of iron may not happen with another, though the BSG found no good evidence that switching between the standard iron salts helps.

How do you stop diarrhoea when taking iron tablets?

Drink plenty of water or squash to avoid dehydration, and don't take any medicine for diarrhoea without asking a pharmacist or doctor first, the NHS says. If you take the combined or progestogen-only contraceptive pill and have severe diarrhoea for more than 24 hours, your pill may not protect you from pregnancy, so check the packet. Taking the tablet with food may help. If the diarrhoea carries on, ask your GP or pharmacist about a lower dose, taking it less often (such as every other day) or a different preparation. The NHS advises keeping going with prescribed iron in the meantime, even with side effects, unless you're told otherwise.

What is ferrous fumarate 210 mg, and how much iron does it contain?

Ferrous fumarate is an iron salt used to treat or prevent iron deficiency anaemia, and a 210 mg tablet gives about 65 to 70 mg of elemental iron, according to its product information (the table further down gives 69 mg). The NHS says you can get it on prescription or buy it from a pharmacy. The usual adult dose is one tablet, capsule or liquid dose once a day, at the same time each day, swallowed with water. You can take it on an empty stomach, or with food if that upsets you, and the NHS says alternate days are an option if side effects continue.

Ferrous sulfate 200 mg tablets, often spelt ferrous sulphate on packs, are the other standard choice. Each holds 200 mg of dried ferrous sulfate, equal to about 65 mg of iron. Both are treatment-sized doses meant for a diagnosed shortfall, so check with a GP or pharmacist before taking either as a general top-up.

How do you take ferrous sulfate and other iron tablets?

On an empty stomach if you can: the NHS suggests 30 minutes before eating or 2 hours after, though you can take it with or after food if it upsets your stomach. The BSG notes that taking iron with a meal can cut absorption by up to 75%. Swallow tablets whole with water, since sucking or chewing them can cause mouth ulcers or stain your teeth.

Leave a 2-hour gap before having tea, coffee, dairy foods or eggs, which all reduce absorption. Calcium, antacids, and zinc and magnesium supplements also interact, as do some medicines, including certain antibiotics, levothyroxine, bisphosphonates and some Parkinson's treatments. The NHS advises leaving a gap and asking your pharmacist how long.

Vitamin C is optional. The NHS says a doctor or pharmacist may suggest orange juice or a vitamin C supplement. But a 2020 trial in China, in 440 adults with iron deficiency anaemia, found that adding 200 mg of vitamin C to each iron dose made no difference to haemoglobin, ferritin or side effects, and the BSG doesn't recommend adding it. A glass of orange juice is fine if you like it.

Is it better to take iron every other day?

For absorption and side effects, trials point that way. A dose of 60 mg of iron raises hepcidin, a hormone that limits iron absorption, for about 24 hours, so the next day's dose is absorbed less well. In a 2017 study of 40 iron-depleted women given 60 mg doses, those dosing on alternate days absorbed 21.8% of the iron against 16.3% for daily dosing. A 2020 study in 19 women with iron deficiency anaemia found absorption 40 to 50% higher on alternate days.

A 2023 double-blind trial gave 150 iron-depleted Swiss women 100 mg of iron either daily or every other day, with the same total dose. Ferritin ended up the same, but gut side effects were about one and a half times as common with daily dosing. These trials were small and mostly in young women, and the BSG says the best regimen isn't clearly defined. It lists alternate-day dosing as an option if daily iron isn't tolerated. Ask your GP before switching a prescribed routine.

Can you take vitamin D and iron together?

Vitamin D isn't on the NHS list of things that interact with ferrous sulfate. Calcium is, so if your vitamin D tablet also contains calcium, leave a gap and ask your pharmacist how long. More pairings are in our guide to supplements you can take together.

How do you choose the best iron supplement?

There's no single best iron; it depends on whether a blood test has shown you're low (our guide to the signs of low iron explains the tests). If it has, your GP will usually prescribe one of the standard iron salts, which the BSG calls "inexpensive, effective, safe and readily available". The NHS says there's little difference in how well they work at the same dose of iron. Ferrous sulfate and fumarate are also sold in pharmacies. Their labels give the weight of the compound, which isn't the same as the iron it contains:

Iron content is from the BSG's 2021 comparison of UK preparations; the product information for the sulfate and fumarate tablets gives about 65 mg and 65 to 70 mg. If you're choosing an iron supplement in the UK without a diagnosis, check the milligrams of iron per daily dose, and add in any from a multivitamin or fortified cereals. The NHS says 17 mg a day or less from supplements is unlikely to cause harm, that doses over 20 mg can cause constipation, sickness and stomach pain, and that most people can get the iron they need from a varied diet. Take more only if a GP advises it.

What counts as a high strength iron tablet?

There's no official cut-off we could find, but the NHS figures give a guide. It says 17 mg a day or less from supplements is unlikely to cause harm, and that doses over 20 mg can cause constipation, sickness and stomach pain. A ferrous sulfate 200 mg tablet holds about 65 mg of iron, nearly four times the 17 mg figure, and it is the sort of strength a GP prescribes for a diagnosed shortfall. So a high strength iron tablet is one to use on medical advice, once a blood test has shown you need it. If your GP advises a higher dose, the NHS says to keep taking it. Without that advice, stay near the 17 mg level and count the iron in multivitamins and fortified cereals too.

Is iron bisglycinate a gentle iron supplement?

It may be gentler on the stomach, but the best evidence is in pregnant women. Ferrous bisglycinate is iron bound to the amino acid glycine. A 2023 meta-analysis of 17 trials found that in pregnant women (9 trials) it raised haemoglobin a little more than other iron supplements and caused fewer gut side effects. It found no difference in children, and too few trials in other adults to say. In a Danish trial of 78 pregnant women, 25 mg of iron as bisglycinate caused fewer stomach complaints than 50 mg as ferrous sulphate, although the doses differed, so it wasn't a like-for-like test. The standard iron salts your GP prescribes in the UK are sulfate, fumarate or gluconate. If tablets upset you, the NHS suggests taking them with food or on alternate days, and a pharmacist can advise on a different type.

Is liposomal iron better than ordinary iron?

We don't know yet, and most of the research isn't in healthy adults. Liposomal iron is iron packaged in tiny fat-based capsules, developed to improve absorption and tolerability. A 2026 meta-analysis of four randomised trials in 454 children with iron deficiency anaemia found a larger rise in haemoglobin at 6 months than with conventional iron (0.96 g/dL) and fewer side effects. The results varied widely between trials, and the authors call the evidence preliminary. Other trials we found were in children or in people with chronic kidney disease, and we found no good trial in otherwise healthy adults. Whatever the format, the iron per serving is what matters, so read the label, and ask your GP or pharmacist before swapping a prescribed iron for a supplement.

Is liquid iron better than tablets?

Not necessarily, but it can suit people who find tablets hard to swallow. The NHS lists ferrous sulfate drops (125 mg of ferrous sulfate per 1 ml) and ferrous fumarate liquid as alternatives to tablets, and a sodium feredetate liquid gives 27.5 mg of iron in each 5 ml, so check the label of any liquid iron supplement for the iron per dose. Measure with the syringe or spoon in the pack, because the NHS says a kitchen spoon won't measure the right amount. The sodium feredetate product information says it doesn't discolour teeth, though it does colour poo black and contains sorbitol, which can cause stomach discomfort and a mild laxative effect at higher intakes. The same 2-hour gap from tea, milk and eggs applies, and liquids need to be kept out of children's reach.

Do iron gummies work?

We couldn't find a trial of iron gummies in adults, so we can't say how they compare with tablets. What matters is the same as for any iron: the milligrams of iron per serving, and whether you need iron at all. A gummy sold as a food supplement is a top-up and isn't a treatment for iron deficiency anaemia, which needs a blood test and a GP's advice. The NHS warns that an iron overdose can be fatal for a young child, and gummies look like sweets, so keep them well out of sight and reach.

Do women need iron tablets or supplements?

Many women get enough iron from food, but some need more. The NHS says women aged 19 to 49 need 14.8 mg of iron a day, against 8.7 mg for men, and that women with heavy periods are at higher risk of iron deficiency anaemia and may need iron supplements. In pregnancy, the NHS says your doctor or midwife may suggest iron if blood tests show your levels are low. If you think you're low, see a GP: a blood test shows whether iron is the cause, and heavy periods can be treated too. Without a diagnosis, the NHS says 17 mg a day or less from supplements is unlikely to cause harm. Women who still have periods after 50 may need the same amount of iron as women aged 19 to 49. Our guide to the signs of low iron covers the symptoms and tests.

Who should be careful with iron?

Check with a doctor before taking iron if you have haemochromatosis or haemosiderosis (conditions that raise iron levels), another type of anaemia, sickle cell disease or thalassaemia, a stomach ulcer or other bowel problem, or have had repeated blood transfusions. The NHS says people being treated for haemochromatosis are usually advised to avoid iron and vitamin C supplements.

Where Natura fits

Daily Iron, in the Natura cabinet (coming soon), is 14 mg of iron as bisglycinate in one capsule: close to the 14.8 mg a day the NHS says women aged 19 to 49 need, and about a fifth of the iron in one ferrous sulfate tablet. It's a food supplement for topping up intake and isn't a treatment for iron deficiency anaemia, and neither is the Blood Pack, which pairs it with Daily B12 (1000 µg). Iron contributes to the normal formation of red blood cells and haemoglobin, and vitamin B12 contributes to normal red blood cell formation. Find them with the daily bottles and the body-system packs.