Iron tablets side effects and the risks of too much iron

What are the common side effects of iron tablets?

The NHS lists these as the most common side effects of iron tablets:

• constipation or diarrhoea
• tummy pain
• heartburn
• feeling sick (nausea)
• darker than usual poo

They are mostly linked to the amount of iron: the NHS notes that high doses – above about 20 mg a day – are more likely to cause constipation, sickness and stomach pain. Prescription tablets for a diagnosed deficiency contain several times that, which is why side effects are more common with them.

Darker poo is a normal effect of iron and nothing to worry about on their own. But black, sticky, tar-like poo – especially with stomach pain or feeling faint – can be a sign of bleeding and need urgent medical attention.

How do you stop stomach pain from iron pills?

• Take them with or soon after food. Iron is absorbed best on an empty stomach, but the NHS says taking iron tablets with or soon after food may reduce side effects.
• Drink plenty of water and eat enough fibre if you’re constipated.
• Don’t double up if you miss a dose – take the next one as normal.

If you’ve been prescribed iron, don’t stop taking it without talking to your GP or pharmacist: the NHS advises carrying on even if you get side effects, and your GP can adjust your treatment if they’re hard to live with. If you’re taking iron you bought yourself and it upsets your stomach, it’s worth asking whether you need it at all.

Why shouldn’t you take iron if you don’t need it?

Unlike many nutrients, the body has no active way of getting rid of excess iron. It controls iron by limiting how much it absorbs, and anything extra is stored – mainly in the liver. Over years, a build-up can damage the liver, heart and pancreas.

That’s why iron is one of the supplements you shouldn’t take “just in case”. Men and women after the menopause need relatively little iron and rarely benefit from a supplement unless a doctor has found a reason. If you suspect you’re low, get a blood test first – see Iron deficiency symptoms.

How do you tell if you’re taking too much iron?

Start by adding up the iron in everything you take – iron tablets, multivitamins and fortified drinks or powders all count. The NHS says 17 mg a day or less from supplements is unlikely to cause harm, and that you should only take more on your GP’s advice. The European Food Safety Authority’s safe level for adults is 40 mg a day from food and supplements combined.

In the short term, taking too much shows up as stomach upset – nausea, vomiting, stomach pain and constipation. Long-term iron build-up often causes no symptoms at first, so it can only be picked up with a blood test.

What are the early signs of iron overload?

An important cause of iron overload is haemochromatosis, an inherited condition in which iron levels in the body slowly build up over many years. It most often affects people of white northern European background, and is especially common where many people have a Celtic background, such as Ireland, Scotland and Wales.

Symptoms usually begin between the ages of 30 and 60 and include:

• feeling very tired all the time
• weight loss
• weakness
• joint pain, especially in the fingers
• erectile dysfunction
• irregular or missed periods
• brain fog, mood swings, depression and anxiety

These symptoms overlap with many other conditions – including, confusingly, iron deficiency – which is another reason not to treat tiredness with iron before you know the cause. See your GP if you have symptoms, or if a parent, brother or sister has haemochromatosis. People having treatment for haemochromatosis are usually advised to avoid iron or vitamin C supplements and breakfast cereals with added iron.

Why are iron tablets dangerous for children?

The NHS warns that very high doses of iron can be fatal, particularly if taken by children. Small children can mistake tablets for sweets, so always keep iron supplements out of their reach.

Does iron interact with medicines?

Yes. Iron can reduce the absorption of some medicines – including certain antibiotics, thyroid medicine (levothyroxine) and some medicines for osteoporosis and Parkinson’s – and antacids can reduce the absorption of iron. If you take any regular medicine, ask your pharmacist before starting iron and how far apart to take them. More on timing in When to take iron tablets.