Carbimazole and methimazole

Date: February 2019, Version 2

Quick read

Women taking carbimazole or methimazole might be switched to a different medicine to control an overactive thyroid in pregnancy.

What is it?

Carbimazole (Neo-Mercazole®) and the related medicine methimazole are used to treat an overactive thyroid gland (hyperthyroidism). Hyperthyroidism is sometimes caused by an autoimmune illness called Graves’ disease.


What are the benefits of taking carbimazole or methimazole in pregnancy?

An untreated overactive thyroid in pregnancy can be dangerous for the woman and her baby. Carbimazole and methimazole help the thyroid gland to function normally and can prevent complications.


Are there any risks of taking carbimazole or methimazole in pregnancy?

Taking carbimazole or methimazole in early pregnancy slightly increases the chance of certain birth defects in the baby. However, most women taking carbimazole or methimazole will have a healthy baby.


Are there any alternatives to taking carbimazole or methimazole in pregnancy?

Yes. Because carbimazole and methimazole are linked to a slightly higher chance of birth defects, a different medicine might be recommended in pregnancy. However, if the different medicine is not suitable (for example due to liver problems), continued use of carbimazole or methimazole might be advised. If you are trying to conceive, please speak to your doctor about whether your medication should be altered.

No treatment

What if I prefer not to take medicines during pregnancy?

An overactive thyroid needs to be treated in pregnancy as it can cause life-threatening illness in the woman and complications, including stillbirth, low birth weight, and preterm delivery. Your doctor will only prescribe medicines during pregnancy when necessary.

Will I or my baby need extra monitoring?

Women in the UK are offered a detailed scan to look for birth defects, usually at around 20 weeks of pregnancy. Women who have been treated with carbimazole or methimazole during the first trimester may be offered this scan slightly earlier. It is important to understand that scans are unlikely to detect some of the birth defects that have been linked to carbimazole and methimazole and cannot rule out that a baby will have problems with learning. 

Extra scans in later pregnancy will also be offered to check the baby’s growth and thyroid function. Pregnant women with thyroid disease also have extra blood tests to monitor their thyroid hormone levels.

Are there any risks to my baby if the father has taken carbimazole or methimazole?

We would not expect any increased risk to your baby if the father took carbimazole or methimazole before or around the time you became pregnant.

Who can I talk to if I have questions?

If you have any questions regarding the information in this leaflet, please discuss them with your healthcare provider. They can access more detailed medical and scientific information from

How can I help to improve drug safety information for pregnant women in the future?

Our online reporting system (MyBump Portal) allows women who are currently pregnant to create a secure record of their pregnancy, collected through a series of questionnaires. You will be asked to enter information about your health, whether or not you take any medicines, your pregnancy outcome and your child's development. You can update your details at any time during pregnancy or afterwards. This information will help us better understand how medicines affect the health of pregnant women and their babies. Please visit the MyBump Portal to register.

General information
Sadly, miscarriage and birth defects can occur in any pregnancy.

Miscarriage occurs in about 1 in every 5 pregnancies, and 1 in every 40 babies are born with a birth defect. This is called the ‘background risk’ and happens whether medication is taken or not.

Most medicines cross the placenta and reach the baby. For many medications this is not a problem. However, some medicines can affect a baby’s growth and development.

If you take regular medication and are planning to conceive, you should discuss whether your medicine is safe to continue with your doctor/health care team before becoming pregnant. If you have an unplanned pregnancy while taking a medicine, you should tell your doctor as soon as possible.

If a new medicine is suggested for you during pregnancy, please make sure that the person prescribing it knows that you are pregnant. If you have any concerns about a medicine, you can check with your doctor, midwife or pharmacist.

Our Bumps information leaflets provide information about the effects of medicines in pregnancy so that you can decide, together with your healthcare provider, what is best for you and your baby.