Iron Tablets Side Effects: What Is Common and What Helps

Constipation, nausea, tummy pain, diarrhoea and darker poo are the side effects of iron tablets that UK health sources list as common, and the NHS says darker than usual poo is usually nothing to worry about³. Advice on easing them is not uniform: the NHS website, the British Society of Gastroenterology (BSG) and 10 NHS trust and health board documents differ on whether to take tablets with food, every day or every other day, and with or without vitamin C. Any change to how you take prescribed tablets is a question for your GP, pharmacist or the clinician who prescribed them.

Below is what those sources say, side by side. We opened each document in October 2026 and they are listed in the sources, covering England, Scotland and Wales. Four are pregnancy documents (NHS Fife, NHS Grampian, Hywel Dda and Cwm Taf Morgannwg), so their regimens are written for pregnancy, and one is written for people with inflammatory bowel disease. Several are past the review date printed on them: the three NHS medicine pages (February 2026), Southampton (December 2025) and NHS Greater Glasgow and Clyde (May 2025). Two leaflets, Milton Keynes and Barnsley, print no date at all. This is general information, not advice on your own treatment. If you want to read about the intravenous route alongside, we describe iron infusion at our Mayfair clinic on its own page.

Contains public sector information licensed under the Open Government Licence v3.0.

Which side effects of iron tablets are common

The NHS lists constipation or diarrhoea, tummy pain, heartburn, feeling sick and darker than usual poo as the side effects some people get with iron tablets². Hospital leaflets give much the same list, and the Scottish pregnancy leaflets add a metallic taste, headaches and vomiting^{10, 11}. The NHS also says many people have no side effects or only minor ones³. Where leaflets say how long the effects last, they say the effects usually improve as the body gets used to the iron^{6, 7}.

The BSG gives a sense of how often people stop: it reports rates of stopping oral iron because of side effects of 0% to 24% in clinical trials and up to 40% in observational and population studies¹.

Darker poo and when it needs attention

The NHS says darker than usual poo is common with iron and usually nothing to worry about, but black, tar-like or red poo, or blood in it, means talking to a doctor or calling 111 straight away³. It gives the same advice if you also feel unwell in any other way³. Wording varies between leaflets: NHS Grampian tells pregnant women not to be concerned if stools turn black¹¹, while NHS Fife lists dark, tarry, hard stools among possible side effects¹⁰. Unlike those two leaflets, the NHS page draws a line between "darker than usual" and "black, tar-like". If you are not sure which description fits your poo, ask a pharmacist or call 111.

What hospitals and health boards advise to ease side effects

The steps named most often are taking the tablets with or after food, moving to alternate days, and asking the prescriber about a lower dose or a different preparation, and the sources differ on which comes first. The NHS also says to swallow tablets whole with water, because sucking or chewing them can cause mouth ulcers or stain teeth⁴.

Taking tablets with or after food

Most sources say iron is absorbed best on an empty stomach, and the NHS says that if it upsets your stomach you can take it with or after food. Its wording is to take ferrous sulfate 30 minutes before eating or 2 hours after, "but if it upsets your stomach, you can take it with or after food"⁴. The BSG states the trade-off: taking iron with meals can cut how much is absorbed by as much as 75%¹. Milton Keynes and Barnsley advise an empty stomach, preferably an hour before a meal^{6, 8}, and Southampton adds that you might need a full stomach if tablets cause nausea⁷. NHS Highland and NHS Greater Glasgow and Clyde list taking it with food as an option when tablets cause side effects, noting it may reduce absorption^{12, 13}, and Guy's and St Thomas' tells people with inflammatory bowel disease to take the tablets after food⁹.

Alternate days instead of every day

Alternate-day dosing is the change the NHS, the BSG and most of the documents we read name for side effects from daily tablets, and the prescriber decides on it. The NHS says that if you get side effects your doctor may advise taking ferrous sulfate on alternate days⁴. The BSG recommends one tablet a day to start and, if that is not tolerated, says every-other-day dosing, a different oral preparation or intravenous iron should be considered¹. It notes that side effects may be lower with less than daily dosing, but says the effect of alternate-day therapy on compliance and on the eventual blood response is unclear¹. Southampton's leaflet says that taking iron every other day may increase its effectiveness, and that the clinician who prescribes the tablets will discuss the precise instructions with you⁷.

The printed regimens differ, as shown here:

NHS website: one 200 mg tablet usually once a day, sometimes 2 to 3 times a day; alternate days if you get side effects⁴
BSG: one tablet a day to start¹
Milton Keynes: rare to need tablets more than once a day; alternate days if taking them more often⁶
Southampton: instructions agreed with the prescriber; every other day may increase effectiveness⁷
NHS Greater Glasgow and Clyde: 50 to 100 mg of elemental iron once a day, taken fasting; one tablet on alternate days if side effects persist with food¹²
NHS Highland (GP guideline): once daily or alternate days for long-term prevention; a lower dose, such as alternate days, for people who do not tolerate oral iron¹³
Guy's and St Thomas' (for people with inflammatory bowel disease): once a day, alternate days if side effects occur⁹
NHS Fife (pregnancy): one tablet three times a week from 16 weeks, rising to daily if haemoglobin falls below 105 g/L¹⁰
NHS Grampian (pregnancy): one 200 mg tablet daily¹¹
Hywel Dda (pregnancy): ferrous sulfate 200 mg once daily, reduced to alternate days if not tolerated¹⁴
Cwm Taf Morgannwg (pregnancy): alternate-day dosing is recognised to aid compliance and reduce the risk of side effects¹⁵

The printed starting point therefore runs from three tablets a week to two or three a day, depending on the document, its age and whether it is written for pregnancy.

Vitamin C and orange juice

Sources disagree on vitamin C, and the question is about how much iron is absorbed rather than about side effects. The NHS says a doctor or pharmacist may recommend taking ferrous sulfate with orange juice or a vitamin C supplement, because vitamin C is believed to increase the iron absorbed⁴. Milton Keynes and Southampton recommend a vitamin C drink^{6, 7}, and NHS Grampian and Guy's and St Thomas' suggest orange juice^{11, 9}. The BSG does not recommend taking vitamin C with oral iron, citing a large randomised trial in which it neither improved the blood response nor reduced side effects¹.

Tea, coffee and milk

The NHS asks for a 2-hour gap between iron tablets and tea, coffee, milk, dairy or eggs, and leaflets range from one hour to two^{4, 5}. Milton Keynes and Southampton say to avoid food, tea and milk for one hour before and after^{6, 7}, Barnsley says 2 hours for milk and calcium-containing products⁸, and NHS Grampian says 2 hours for milk, tea and coffee¹¹.

A different iron salt

Whether a different iron salt causes fewer side effects is a point on which sources differ. The NHS says that if you have a side effect with one type of iron it may not happen with another, and that there is little difference in how well they work at the same dose of iron⁵. The BSG says switching to a different traditional iron salt is not supported by evidence¹, and NHS Highland notes that better tolerance may reflect a lower amount of elemental iron¹³. Milton Keynes suggests asking the prescriber about ferrous fumarate, citing anecdotal evidence that it is the best tolerated⁶.

Constipation

For constipation, the NHS suggests more fibre, plenty of water and regular exercise, and NHS Grampian adds that a GP or pharmacist can suggest medicines to ease it if that is not enough^{3, 11}.

What the BSG guideline says about tablets that are not tolerated

The BSG's recommendation is one tablet a day to start, and if that is not tolerated, every-other-day dosing, alternative oral preparations or parenteral (intravenous) iron should be considered¹. It calls the best option for people with significant intolerance, usually gut disturbance, "unclear" and says it depends on the individual¹.

When to go back to your GP or pharmacist

The NHS says to speak to a doctor or pharmacist if the coping advice does not help and a side effect is still bothering you or does not go away³. Southampton and Barnsley say the prescriber, GP or pharmacist can reduce the strength or frequency, or adjust the preparation^{7, 8}, and NHS Greater Glasgow and Clyde says people should return to discuss other treatment options if its measures fail¹². The NHS page on iron deficiency anaemia also says it is important to keep taking the tablets even if you get side effects².

Urgent advice is separate. The NHS says to talk to a doctor or call 111 straight away for black, tar-like or red poo, or blood in it³, and to call 999 if your lips, mouth, throat or tongue suddenly swell or you struggle to breathe³. It also asks that iron tablets are kept out of the reach of children, because an overdose in a young child can be fatal².

A GP may recheck your blood count: the NHS says a doctor may recommend a blood test within the first 4 weeks to see if the tablets are working⁵, and the BSG recommends monitoring the blood response in the first 4 weeks¹. What the NHS offers when tablets do not suit is covered in our guide to getting an iron infusion on the NHS or privately. Our iron infusion page lists "patients who can't tolerate oral iron" in its section on who needs an iron infusion.

Common questions

How long do the side effects of iron tablets last?

None of the leaflets we read gives a number of days, but those that address it say the effects usually improve as the body gets used to the iron^{6, 7}. NHS Greater Glasgow and Clyde says they usually settle down with time¹². If they do not go away, the NHS says to speak to a doctor or pharmacist³.

How long do iron tablets take to work?

The NHS says most people begin to feel better after 3 to 4 weeks, and it may take up to 2 to 4 months for the medicine to work fully⁵. Barnsley says 2 weeks to feel better and up to 4 weeks for full effect⁸. The BSG says that no rise of at least 10 g/L in haemoglobin after 2 weeks of daily tablets strongly predicts that the response will not be sustained, and a doctor decides what to do next¹.

Should I take iron tablets without a blood test?

The NHS says to talk to a doctor or pharmacist first if you feel tired but have not been diagnosed with iron deficiency anaemia, because there may be other reasons for low energy⁵. It says a simple blood test will confirm whether you are anaemic⁵, and to see a GP if you think you might have iron deficiency anaemia².