Can I take carbimazole while pregnant or breastfeeding?
A plain-words look at what the NHS, the US drug label and LactMed say about carbimazole.
In short
Pregnancy: Treating a thyroid problem properly in pregnancy is important.
Breastfeeding: If your baby is healthy, you can usually take carbimazole while breastfeeding, and treating your thyroid problem is important.
Your doctor, midwife or pharmacist can help you decide.
What this means for you
Pregnancy
- Having thyroid hormone levels that are too high or too low can lead to problems for your baby.
- Carbimazole is not often recommended in pregnancy, and you'll usually be moved to a different thyroid medicine called propylthiouracil (PTU).
- This is because, very occasionally, carbimazole can cause problems with your baby's development in early pregnancy.
- However, PTU is not suitable for everyone, and sometimes staying on carbimazole may be the safest choice.
- If you become pregnant while on carbimazole, keep taking it and talk to your doctor as soon as you can.
- You'll be invited to appointments with an obstetrician (pregnancy specialist) and an endocrinologist (hormone specialist), who can advise you on the best treatment for you and your baby.
- If you stay on carbimazole in pregnancy, you'll need regular blood tests to make sure your dose is right.
- You may also have extra scans to check on your growing baby's health.
Breastfeeding
- Small amounts of carbimazole reach breast milk.
- It's unlikely to give your baby side effects or to affect their thyroid hormones.
- Side effects in breastfed babies from medicines their mother takes are very rare, but it's important to keep an eye on your baby whenever you take a medicine while breastfeeding.
- Your doctor may do a blood test on your baby to check that all is well.
- Keep taking the medicine and carry on breastfeeding as normal until you've had advice about your baby's symptoms.
When to get help
- Contact a healthcare professional or call 111 (NHS 111 in the UK) as soon as you can if your baby is feeding less well than normal or not gaining weight as you'd expect, is unusually sleepy or irritable, has trouble pooing (constipation), has a rash, or keeps getting infections.
- For symptoms not mentioned here, or if you're worried about your baby, you can also speak to any healthcare professional or call 111.
Talk to your doctor or pharmacist
This page is educational, not advice for your own situation. Talk to your doctor, midwife or pharmacist before you start, stop or change any medicine in pregnancy or while breastfeeding.
The details: what each source says
Carbimazole in pregnancy
The NHS says
Treating a thyroid problem properly in pregnancy is important. Having thyroid hormone levels that are too high or too low can lead to problems for your baby.
Carbimazole is not often recommended in pregnancy, and you'll usually be moved to a different thyroid medicine called propylthiouracil (PTU).
This is because, very occasionally, carbimazole can cause problems with your baby's development in early pregnancy. However, PTU is not suitable for everyone, and sometimes staying on carbimazole may be the safest choice.
If you become pregnant while on carbimazole, keep taking it and talk to your doctor as soon as you can. You'll be invited to appointments with an obstetrician (pregnancy specialist) and an endocrinologist (hormone specialist), who can advise you on the best treatment for you and your baby.
If you stay on carbimazole in pregnancy, you'll need regular blood tests to make sure your dose is right. You may also have extra scans to check on your growing baby's health.
In our words, from the NHS page Carbimazole (nhs.uk). Contains public sector information licensed under the Open Government Licence v3.0 (OGL v3.0). The NHS does not endorse this site.
Carbimazole while breastfeeding
The NHS says
If your baby is healthy, you can usually take carbimazole while breastfeeding, and treating your thyroid problem is important.
Small amounts of carbimazole reach breast milk. It's unlikely to give your baby side effects or to affect their thyroid hormones.
Side effects in breastfed babies from medicines their mother takes are very rare, but it's important to keep an eye on your baby whenever you take a medicine while breastfeeding.
Contact a healthcare professional or call 111 (NHS 111 in the UK) as soon as you can if your baby is feeding less well than normal or not gaining weight as you'd expect, is unusually sleepy or irritable, has trouble pooing (constipation), has a rash, or keeps getting infections.
Your doctor may do a blood test on your baby to check that all is well.
For symptoms not mentioned here, or if you're worried about your baby, you can also speak to any healthcare professional or call 111.
Keep taking the medicine and carry on breastfeeding as normal until you've had advice about your baby's symptoms.
In our words, from the NHS page Carbimazole (nhs.uk). Contains public sector information licensed under the Open Government Licence v3.0 (OGL v3.0). The NHS does not endorse this site.
LactMed says
Carbimazole is not approved for marketing in the United States by the U.S. Food and Drug Administration, but is available in other countries. Doses of carbimazole of 15 mg daily have not adversely affected the few breastfed infants studied. Carbimazole is a prodrug for methimazole which has been studied extensively during breastfeeding; maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Some experts now recommend that methimazole should be considered the antithyroid drug of choice in nursing mothers.[1-4]
“Summary of Use during Lactation”, LactMed record “Carbimazole” (LM850), quoted verbatim. Everything LactMed reports about Carbimazole while breastfeeding.
Sources
- NHS, Carbimazole (nhs.uk), page last reviewed 1 May 2025. Paraphrased. Contains public sector information licensed under the Open Government Licence v3.0.
- Drugs and Lactation Database (LactMed), National Institute of Child Health and Human Development (NICHD), via NCBI Bookshelf, record “Carbimazole” (LM850), bulk release of 22 September 2026. LactMed on NCBI Bookshelf. Public domain; quoted verbatim. LactMed is a registered trademark of the U.S. Department of Health and Human Services.
Sources checked 26 September 2026. How we source and check every page.
Educational summary of named sources, not medical advice.