Can I take anastrozole while pregnant or breastfeeding?
A plain-words look at what the NHS, the US drug label and LactMed say about anastrozole.
In short
Pregnancy: Anastrozole is not recommended if you're pregnant.
Breastfeeding: Anastrozole is not recommended during breastfeeding.
Your doctor, midwife or pharmacist can help you decide.
What this means for you
Pregnancy
- This is because it affects hormone levels in both you and your baby, and there isn't enough information to say whether it's safe.
Breastfeeding
- How much anastrozole gets into breast milk is not known, but compared with most other medicines the amount is likely to be fairly high.
- Anastrozole affects hormones, and this may affect your baby's development.
When to get help
- If there's any chance you could be pregnant, speak to your doctor straight away.
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
Anastrozole in pregnancy
The NHS says
Anastrozole is not recommended if you're pregnant.
This is because it affects hormone levels in both you and your baby, and there isn't enough information to say whether it's safe.
If there's any chance you could be pregnant, speak to your doctor straight away.
In our words, from the NHS page Anastrozole (nhs.uk). Contains public sector information licensed under the Open Government Licence v3.0 (OGL v3.0). The NHS does not endorse this site.
What these words mean
- embryo-fetal
- about the embryo and fetus (a baby's early stages of growing in the womb)
Definitions adapted from the NCI Dictionary of Cancer Terms (National Cancer Institute).
The US drug label says
8.1 Pregnancy Risk Summary Based on findings from animal studies and its mechanism of action, anastrozole may cause fetal harm when administered to a pregnant woman [see Clinical Pharmacology (12.1) ]. There are no studies of anastrozole use in pregnant women. Anastrozole caused embryo-fetal toxicities in rats at maternal exposure that were 9 times the human clinical exposure, based on area under the curve (AUC). In rabbits, anastrozole caused pregnancy failure at doses equal to or greater than 16 times the recommended human dose on a mg/m 2 basis. Advise pregnant women and females of reproductive potential of the potential risk to a fetus. The background risk of major birth defects and miscarriage for the indicated population is unknown. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2% to 4% and 15% to 20%, respectively.
Prescribing label, section “8.1 Pregnancy” (opening excerpt): Anastrozole (oral), PD-Rx Pharmaceuticals, Inc. Read the full section on DailyMed. Quoted verbatim; US labels are written for US prescribers and patients.
Anastrozole while breastfeeding
The NHS says
Anastrozole is not recommended during breastfeeding.
How much anastrozole gets into breast milk is not known, but compared with most other medicines the amount is likely to be fairly high.
Anastrozole affects hormones, and this may affect your baby's development.
In our words, from the NHS page Anastrozole (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
No information is available on the use of anastrozole during breastfeeding. The manufacturer recommends that nursing mothers not breastfeed during anastrozole treatment and for at least 2 weeks after the last dose.
“Summary of Use during Lactation”, LactMed record “Anastrozole” (anastrozole), quoted verbatim. Everything LactMed reports about Anastrozole while breastfeeding.
What these words mean
- metabolite
- a substance made when the body breaks down a medicine
Definitions adapted from the NCI Dictionary of Cancer Terms (National Cancer Institute).
The US prescribing label says
8.2 Lactation Risk Summary There are no data on the presence of anastrozole or its metabolites in human milk, or its effects on the breast-fed child or on milk production. Because many drugs are excreted in human milk and because of the tumorigenicity shown for anastrozole in animal studies, or the potential for serious adverse reactions in the breast-fed child from anastrozole, advise lactating women not to breastfeed during treatment with anastrozole and for 2 weeks after the last dose.
Prescribing label, section “8.2 Lactation”: Anastrozole (oral), PD-Rx Pharmaceuticals, Inc. Label on DailyMed. Quoted verbatim; US labels are written for US prescribers and patients.
Other names
UK brand names (per the NHS): Arimidex.
Sources
- NHS, Anastrozole (nhs.uk), page last reviewed 6 March 2024. Paraphrased. Contains public sector information licensed under the Open Government Licence v3.0.
- US drug label: Anastrozole (oral), PD-Rx Pharmaceuticals, Inc., human prescription drug, label effective 5 August 2026, set ID 54da5c12-cfaf-7a14-e063-6394a90a3635. DailyMed (National Library of Medicine). Public domain; quoted verbatim.
- Drugs and Lactation Database (LactMed), National Institute of Child Health and Human Development (NICHD), via NCBI Bookshelf, record “Anastrozole” (anastrozole), 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.