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IronWhat are the side effects of iron supplements?
What to know
- Iron supplements are one of the most commonly prescribed supplements in the UK, but side effects are very common, particularly digestive problems such as constipation, nausea and stomach cramps.
- Most side effects are harmless and settle within a few days or weeks. Simple changes such as taking iron with food, splitting doses or switching formulation can make a real difference.
- Skin reactions such as rash, itching or hives are uncommon but may signal an allergic reaction. Seek medical help immediately if your face, lips or throat swell, or if you have difficulty breathing.
- Iron is dangerous in overdose, particularly for young children. Always keep iron supplements out of the reach of children and store them in a child-resistant container.
Iron supplements are one of the most widely used supplements in the UK. GPs prescribe them routinely for iron deficiency anaemia, and many people take them over the counter to support their iron intake. They work, but they are not always easy to tolerate.
The NHS page on ferrous sulfate, the most commonly prescribed form of iron in the UK, lists digestive side effects as very common. The Cleveland Clinic notes that gastrointestinal problems are the main reason people stop taking iron supplements, even when they still need them.
This guide covers the side effects you are most likely to experience, which ones are serious, and what you can do about them.
Common side effects
Most side effects of iron supplements involve your digestive system. They tend to appear within the first few days of starting a supplement and often improve as your body adjusts. The NHS lists the following as common for both ferrous sulfate and ferrous fumarate, the two forms most widely used in the UK.
Constipation
This is one of the most frequently reported side effects. Iron slows the movement of food through your gut, which can make stools harder and less frequent. The GoodRx guide on iron side effects describes constipation as one of the top reasons people give up on iron supplements. Drinking plenty of water, eating high-fibre foods and staying physically active can help. If constipation persists, your GP may suggest a gentle laxative or a switch to a different formulation.
Nausea and vomiting
Feeling sick after taking iron is common, especially if you take it on an empty stomach. The NHS advises that taking iron with or just after food can reduce nausea, although this may slightly reduce absorption. If nausea is severe, speak to your GP or pharmacist about adjusting the timing or the dose.
Stomach cramps and discomfort
Stomach pain, cramping and a general feeling of discomfort in the upper abdomen are widely reported. The Cleveland Clinic lists abdominal pain, cramping and bloating among the most common gastrointestinal effects. These tend to ease after the first week or two.
Diarrhoea
Some people experience loose stools rather than constipation. The NHS lists diarrhoea as a common side effect of both ferrous sulfate and ferrous fumarate. If it persists beyond the first few days, contact your GP.
Heartburn
Iron can irritate the lining of the oesophagus and stomach, causing a burning sensation in the chest or upper stomach. The NHS lists heartburn as a common side effect of ferrous fumarate in particular. Taking your supplement with food, and avoiding lying down straight after, can help reduce it.
Dark or black stools
This side effect alarms many people, but it is completely harmless. The NHS reassures that dark or black stools are a normal effect of iron and not a cause for concern. The colour change is simply unabsorbed iron passing through your system. However, if you notice blood in your stools or your stools are tarry and sticky rather than simply dark, contact your GP, as this can occasionally indicate bleeding in the digestive tract.
Metallic taste
A metallic or unpleasant taste in your mouth is another common complaint, particularly with liquid iron preparations. It is harmless but can be off-putting. Rinsing your mouth after taking a liquid supplement, or drinking a small amount of juice, may help.
Tooth staining
Liquid iron supplements can stain your teeth grey or black. The NHS advises diluting liquid iron in water and drinking it through a straw to minimise contact with your teeth. If staining occurs, it is usually temporary and can be removed by brushing with bicarbonate of soda or by a dental hygienist.
Different forms of iron can cause different levels of side effects. Ferrous sulfate and ferrous fumarate, the most commonly prescribed in the UK, tend to cause more digestive symptoms than newer formulations such as ferrous bisglycinate or iron polymaltose. If side effects are making it difficult to continue, ask your GP or pharmacist about alternatives.
Less common but serious side effects
Serious side effects from iron supplements are uncommon, but they do happen. The NHS advises contacting your GP or calling NHS 111 if you experience any of the following.
Allergic reactions
Some people develop an allergic reaction to iron supplements or one of their inactive ingredients. Signs include skin rash, itching, hives, redness or swelling. Skin reactions to iron supplements are relatively uncommon but should not be ignored.
In rare cases, iron supplements can cause a severe allergic reaction (anaphylaxis). Call 999 or go to A&E immediately if you develop swelling of your face, lips, mouth or throat, difficulty breathing or swallowing, a tight feeling in your chest, or you feel dizzy or faint. This is a medical emergency.
Stomach ulceration
In rare cases, iron supplements can cause or worsen ulcers in the stomach or oesophagus, particularly if tablets are not taken with enough water or if they get stuck in the oesophagus. The Cleveland Clinic notes that swallowing iron tablets whole with a full glass of water and remaining upright for at least 30 minutes afterwards reduces this risk.
Iron supplements and your skin
If you have searched for information about iron supplements and skin side effects, you are not alone. The most common skin-related effects fall into two categories.
Allergic skin reactions, as described above, include rash, itching and hives. These are a reason to stop the supplement and speak to your GP or pharmacist. They may indicate a sensitivity to the iron compound itself or to a filler, coating or colouring in the tablet.
Itching without a visible rash is occasionally reported. This can sometimes relate to iron deficiency itself being corrected rather than to the supplement. Iron deficiency is a known cause of generalised itching (pruritus), and in some people the sensation changes as iron levels normalise. If itching develops or worsens after starting iron, mention it at your next GP appointment.
There is no good evidence that standard oral iron supplements cause acne, pigmentation changes or other skin conditions in people who are not allergic to them.
How to reduce side effects
Side effects are the main reason people stop taking iron before their course is finished. The following strategies, drawn from NHS guidance and clinical experience, can help you stay on track.
- Take iron with food if nausea or stomach cramps are a problem. This slightly reduces absorption but makes the supplement much easier to tolerate.
- Start with a lower dose and build up gradually. The Cleveland Clinic notes that starting at half the prescribed dose for a few days can allow your gut to adjust.
- Take it at a different time of day. Some people tolerate iron better in the evening than in the morning.
- Drink plenty of water and eat fibre-rich foods if constipation is a problem.
- Use a straw for liquid iron to reduce tooth staining.
- Avoid taking iron with tea, coffee or calcium-rich foods at the same time. These reduce absorption, which may mean you need a higher dose and experience more side effects.
- Ask about alternate-day dosing. Emerging research suggests that taking iron every other day may improve absorption and reduce side effects. Discuss this with your GP before changing your schedule.
If side effects are putting you off, speak to your GP or pharmacist before stopping. Switching from ferrous sulfate to a different form of iron, reducing the dose, or changing the timing can often make the difference between tolerating the supplement and giving up on it.
Iron overdose and children
Iron is dangerous in overdose. This is particularly important for young children, for whom even a small number of adult-strength iron tablets can be life-threatening.
Keep iron supplements out of the reach of childrenIron overdose is one of the most common causes of poisoning deaths in young children. The NHS advises storing all iron supplements in child-resistant containers and keeping them well out of sight and reach. If you suspect a child has swallowed iron tablets, call 999 or take them to A&E immediately, even if they seem well. Do not wait for symptoms to appear.
Symptoms of iron overdose in adults include severe stomach pain, vomiting (sometimes with blood), diarrhoea, rapid heartbeat and a drop in blood pressure. If you think you have taken too many iron tablets, contact NHS 111 or go to A&E. Do not try to make yourself sick.
When to speak to your GP
Most iron side effects are manageable and settle with time. However, you should contact your GP or pharmacist if:
- Side effects are severe enough that you are thinking of stopping the supplement.
- You develop a skin rash, itching, hives or swelling.
- You notice blood in your stools, or stools that are tarry and sticky rather than simply dark.
- You have persistent vomiting or severe stomach pain.
- You are pregnant or breastfeeding and experiencing side effects (your GP may adjust the dose or switch formulation).
If you experience swelling of your face, lips or throat, difficulty breathing, or a tight feeling in your chest, call 999 immediately. These are signs of a serious allergic reaction.
Common questions about iron supplement side effects
How long do iron supplement side effects last?
Most digestive side effects, such as constipation, nausea and stomach cramps, ease within a few days to two weeks as your body adjusts. If side effects are still troublesome after two to three weeks, speak to your GP. They may suggest a different formulation, a lower dose or alternate-day dosing.
Can iron supplements cause skin reactions?
Yes, although this is uncommon. Some people develop a rash, itching or hives, which may indicate an allergic reaction to the iron compound or to an inactive ingredient in the tablet. Stop the supplement and speak to your GP or pharmacist. If you develop swelling of the face, lips or throat, or difficulty breathing, call 999 immediately.
Should I take iron supplements on an empty stomach or with food?
Iron is absorbed best on an empty stomach, but this often causes nausea and stomach cramps. The NHS advises that taking iron with or just after food reduces side effects. You may absorb slightly less iron, but you are more likely to finish the course. Your GP can advise on what works best for you.
Is it normal for iron supplements to cause black stools?
Yes. Dark or black stools are a harmless and very common effect of iron supplements. The colour is simply unabsorbed iron. However, if your stools are tarry, sticky or contain visible blood, contact your GP, as this may indicate a different problem.
Can I switch to a different type of iron if I get side effects?
Yes. Different iron formulations cause different levels of side effects. If ferrous sulfate is causing problems, your GP or pharmacist may suggest ferrous fumarate, ferrous gluconate, or a newer formulation such as ferrous bisglycinate, which tends to be gentler on the stomach. Liquid iron is another option, though it can stain teeth. Do not switch without checking with your GP first, especially if your iron was prescribed.
Independent guides to natural remedies and supplements, checked against NHS and EFSA references.