Carbimazole is a medicine used to treat an overactive thyroid (hyperthyroidism). This is when your thyroid gland makes too many thyroid hormones.

Your thyroid controls things like your heart rate and body temperature. When it makes too many hormones, you can have symptoms such as weight loss, mood swings and feeling irritable.

Carbimazole helps to ease these symptoms by reducing the amount of hormones your thyroid produces.

This medicine is only available on prescription.

It usually comes as tablets. If you are unable to swallow tablets, your doctor may be able to prescribe carbimazole as a liquid.

Sometimes your doctor will prescribe carbimazole together with a beta blocker. This can help with your symptoms if you have a fast heartbeat, or feel jittery or anxious.

• You’ll have blood tests before and during treatment with carbimazole.
• When you first start taking this medicine, it can take up to 3 weeks to see an improvement in your symptoms.
• Side effects usually only happen in the first 8 weeks and tend not to last long.
• One rare side effect can be a sudden drop in your white blood cell count. This makes it harder to fight infections.
• If you are pregnant, your dose of carbimazole needs to be monitored carefully.

Most adults, children and babies can take carbimazole.

However, carbimazole is not suitable for everyone. To make sure it's safe for you, tell your doctor if you:

• have ever had an allergic reaction to carbimazole or any other medicines
• have had severe pancreas problems (acute pancreatitis) after taking carbimazole
• are due to have radioactive iodine treatment for your overactive thyroid
• have liver problems or a problem with your blood
• have a swollen thyroid (goitre)
• cannot have regular blood tests
• are pregnant or trying for a baby, or there's a chance you could get pregnant

Always follow the advice of a doctor, and the instructions that come with your medicine.

Swallow the tablets whole, with a drink of water. You can take carbimazole with or without food.

When you first start taking carbimazole, you will take it 2 or 3 times a day. Try to space the doses evenly throughout the day. If you take it 3 times a day, you could take it first thing in the morning, in the middle of the afternoon and at bedtime.

Dosage and strength

Carbimazole comes as 5mg, 10mg, 15mg and 20mg tablets. You may need to take more than 1 tablet to make up your dose.

Doses vary depending on your symptoms, your thyroid hormone levels and whether you have any other health conditions. Your doctor will tell you how many tablets to take.

The usual starting dose for adults is between 20mg and 60mg a day, split into 2 or 3 smaller doses.

Children usually start on 15mg a day, taken once a day, or split into 2 or 3 smaller doses.

Having blood tests during treatment

You need to have blood tests before you start taking carbimazole and during treatment. These tests are important to check the levels of thyroid hormones in your body.

Depending on the results, your doctor may need to increase or reduce your dose to get these hormones to the right levels. When your levels are stable, you can usually go onto a lower dose, taken once a day.

When you first start treatment, you'll have blood tests every 6 weeks or so. Once your hormone levels are stable, you'll have a blood test every 3 months, for as long as you continue to take carbimazole.

However, you may need blood tests more often if you:

• are pregnant
• have symptoms or side effects that could mean your dose is not quite right

What if I forget to take it?

If you forget to take a dose, take it as soon as you remember. If it is time for your next dose, take both doses together.

If you forget doses often, it may help to set an alarm to remind you. You can also ask your pharmacist for advice on other ways to help you remember to take your medicine.

What if I take too much?

Like all medicines, carbimazole can cause side effects, although not everyone gets them. If you do have side effects, these usually happen in the first 8 weeks of taking carbimazole.

Common side effects

Common side effects usually improve as your body gets used to carbimazole.

Talk to a doctor or pharmacist if these side effects bother you or do not go away:

• feeling sick (nausea)
• being sick (vomiting) or diarrhoea
• feeling dizzy
• headache
• painful joints
• itchy skin or rash
• thinning hair

Serious side effects

Serious side effects are uncommon.

Stop taking carbimazole and tell a doctor straight away if:

• you have a high temperature, sore throat, mouth sores, toothache or flu-like symptoms – these can be signs of a low white blood cell count
• you have sudden severe stomach (abdominal) pain – this can be a sign of an inflamed pancreas (acute pancreatitis)
• the whites of your eyes or your skin turns yellow (this may be less obvious on brown or black skin), dark pee – this can be a sign of liver problems
• you have swollen glands – this can be a sign of infection
• you feel faint, tired and sweaty – these can be signs of low blood sugar

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction to carbimazole (anaphylaxis).

What to do about:

• feeling sick – stick to simple meals and try not eat rich or spicy food. It may help to take your carbimazole with food or just after eating.
• being sick (vomiting) or diarrhoea – drink lots of fluids, such as water or squash, to avoid dehydration. Take small, frequent sips if you are being sick. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee. Contact a doctor if this lasts for more than a few days. Tell your doctor if you get severe diarrhoea or vomiting from a stomach bug or illness that lasts for more than 2 days..
• feeling dizzy – lie down until the dizziness passes, then get up slowly. Move slowly and carefully. Avoid coffee, cigarettes, alcohol and recreational drugs. Do not drive, cycle or operate machinery until you feel ok again. If the dizziness does not get better, or this keeps happening, speak to a doctor.
• headaches – make sure you rest and drink plenty of fluids. Do not drink too much alcohol. Ask your pharmacist to recommend a painkiller. Headaches should usually go away. Talk to your doctor if they last longer than a week or are severe.
• painful joints – cooling treatments can help with joint pain. Try putting an ice pack wrapped in a cloth on the joint for about 15 minutes, and repeat several times a day. Avoid keeping the joint still and take gentle exercise to stop it becoming stiff. Ask a pharmacist to recommend a painkiller. Speak to your doctor if the pain is no better after a week.
• itchy skin or rash – use unperfumed moisturizer or skin lotion, wear loose cotton clothing, keep your nails clean, short and smooth, and pat or tap the skin instead of scratching it. Ask your pharmacist to recommend an antihistamine.
• thinning hair – no one knows if taking carbimazole causes this or if it's the changes in your thyroid hormone levels. Hair usually grows back but this may take a little while because the natural cycle of hair growth takes several months.

Carbimazole is not usually recommended in pregnancy. You'll usually be prescribed another thyroid medicine called propylthiouracil (PTU). This is because there's a small chance that carbimazole can cause problems for your baby's development in the first few months of pregnancy.

However, it's important to keep treating an overactive thyroid. If your thyroid hormone levels are too high, this can also cause problems for your baby.

If you become pregnant while taking carbimazole, keep taking your medicine and speak to your doctor.

If you have thyroid problems, then you will be asked to go to an antenatal clinic to see a pregnancy specialist (obstetrician) and a hormone specialist (endocrinologist). They can advise you on the best treatment options for you and your baby.

If you are taking carbimazole and thyroxine together (a "block and replace" regime), this will need to be changed, so it's important that you see a specialist early on in your pregnancy.

When taking carbimazole in pregnancy, you'll need regular blood tests to check you’re on the right dose. You may also have extra scans to check the health of your growing baby.

Carbimazole and breastfeeding

It's usually OK to take carbimazole while you are breastfeeding. Carbimazole passes into breast milk but only in small amounts. This is probably too little to affect your baby.

After a while, your doctor may take a blood test for the baby to check the levels of their thyroid hormones.

If you have any concerns about your baby's health, talk to your midwife, health visitor or doctor straight away.

For more information about how carbimazole can affect you and your baby during pregnancy, read this leaflet on the best use of medicines in pregnancy (BUMPS) website.

Taking carbimazole can affect the way certain medicines work, while some medicines are affected by your thyroid hormone levels. These levels will change as carbimazole starts to work.

For this reason, your doctor will want to review the medicines you take and possibly change the dosage. Tell your doctor before starting treatment with carbimazole if you take:

• digoxin, a heart medicine
• erythromycin, an antibiotic
• theophylline, used to treat breathing problems
• bladder medicines for urinary incontinence and other bladder problems
• blood pressure medicines called beta blockers – these are often prescribed together with carbimazole and your doctor will tell you how much to take
• anticoagulants such as warfarin
• steroids such as prednisolone

Mixing carbimazole with herbal remedies, supplements and other medicines

Some herbal remedies and supplements can affect the way carbimazole works. This includes black cohosh, sometimes taken to help with menopausal symptoms.

However, biotin supplements can affect the accuracy of thyroid function tests. Do not take biotin without talking to your doctor.