(Date: August 2015. Version: 2)

This factsheet has been written for members of the public by the UK Teratology Information Service (UKTIS). UKTIS is a not-for-profit organisation funded by Public Health England on behalf of UK Health Departments. UKTIS has been providing scientific information to health care providers since 1983 on the effects that medicines, recreational drugs and chemicals may have on the developing baby during pregnancy.

What is it?

Tamoxifen (Soltamox®) is used to treat and/or prevent certain types of breast cancer. It works by blocking the activity of the female hormone oestrogen that is important for the growth of these cancer cells.

Is it safe to take tamoxifen in pregnancy?

There is no yes or no answer to this question. When deciding whether to take tamoxifen during pregnancy it is important to weigh up how necessary tamoxifen is to your health against any possible risks to you or your baby, some of which may depend on how many weeks pregnant you are.

The manufacturer of tamoxifen recommends stopping tamoxifen at least two months before trying to become pregnant because of the possibility that it may affect the development of a baby in the womb (see further details below). However, use of tamoxifen in pregnancy may be necessary for the health of both the mother and her unborn baby, for example to treat a cancer that might otherwise be terminal. Your doctor is the best person to help you decide what is right for you and your baby.

This leaflet summarises the scientific studies relating to the effects of tamoxifen on a baby in the womb. 

What if I have already taken tamoxifen during pregnancy?

If you have taken any medicines it is always a good idea to let your doctor know that you are pregnant so that you can decide together whether you still need the medicines that you are on and to make sure that you are taking the lowest dose that works.

Can taking tamoxifen in early pregnancy cause my baby to be born with birth defects?

A baby’s body and most internal organs are formed during the first 12 weeks of pregnancy. It is mainly during this time that some medicines are known to cause birth defects.

It is currently not known whether use of tamoxifen in early pregnancy might increase the risk of birth defects in the baby as no large scientific studies have investigated this. Because tamoxifen works by blocking the effects of the female hormone oestrogen, there is a theoretical possibility that exposure in pregnancy may affect the development of a female baby’s genitals or internal sex organs (e.g. womb and ovaries).

Early reports to the manufacturer of tamoxifen included details of babies born to 47 women who took tamoxifen in pregnancy. The majority of the babies were normally formed at birth. Twelve babies had a major (more serious) or minor (common, usually harmless) birth defect. These birth defects were mostly different, which suggests that they were not due to tamoxifen exposure. The reports did, however, include four girls with genital abnormalities.

Although reports to a medicine’s manufacturer are more likely to be of adverse outcomes (especially those that have already been identified as a potential risk, such as genital defects in this case), while normal outcomes are much less likely to be reported, these four cases may be a clue that tamoxifen can affect genital development before birth. Better studies are therefore required to answer the question of whether tamoxifen use in pregnancy increases the risk of birth defects in the baby, and specifically, of genital defects in girls.

Can taking tamoxifen in pregnancy cause miscarriage?

To date, no scientific studies have specifically assessed miscarriage rates in pregnant women taking tamoxifen. Although early reports to the manufacturer of 80 pregnant women who took tamoxifen do not suggest that they had an increased risk of miscarriage, large studies that compare miscarriage rates between groups of women taking and not taking tamoxifen are required to answer this question.

Can taking tamoxifen in pregnancy cause stillbirth?

No studies have specifically analysed stillbirth rates in pregnant women taking tamoxifen.

Can taking tamoxifen in pregnancy cause preterm birth, or my baby to be small at birth (low birth weight)?

No studies have specifically analysed rates of preterm birth or low birth weight babies in pregnant women taking tamoxifen. There are case reports of babies who were exposed to tamoxifen in the womb being born preterm (which itself increases the chance of a baby having a low birth weight), but many of these babies were born by induced early delivery due to the underlying illnesses in the mother. It is therefore unclear whether taking tamoxifen in pregnancy can directly cause premature birth or low birth weight in the baby and this should be the subject of future research.

Can taking tamoxifen in pregnancy cause learning and behavioural problems in the child?

A baby’s brain continues to develop right up until the end of pregnancy. It is therefore possible that taking certain medicines at any stage of pregnancy could have a lasting effect on a child’s learning or behaviour.

No studies have been carried out to specifically investigate whether exposure to tamoxifen while in the womb increases the chance of learning and behavioural problems in the child. A handful of case reports describe normal development up to five years of age in children who were exposed to tamoxifen in the womb, but larger scientific studies are needed to study this accurately.

Will I or my baby need extra monitoring during pregnancy?

Most women will be offered a scan at around 20 weeks of pregnancy to look for birth defects as part of their routine antenatal care.

Because the effects of taking tamoxifen in early pregnancy are largely unknown your doctor may suggest extra monitoring of your baby including more detailed scans for birth defects and monitoring of your baby’s growth in the womb. Women with the illnesses that tamoxifen is used to treat may generally be more closely monitored during pregnancy to ensure that they remain well throughout, and that their baby is growing and developing as expected. 

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 health care provider. They can access more detailed medical and scientific information from www.uktis.org.  

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General information 

Up to 1 out of every 5 pregnancies ends in a miscarriage, and 1 in 40 babies are born with a birth defect. These are referred to as the background population risks.  They describe the chance of these events happening for any pregnancy before taking factors such as the mother’s health during pregnancy, her lifestyle, medicines she takes and the genetic make up of her and the baby’s father into account.

Medicines use in pregnancy

Most medicines used by the mother will cross the placenta and reach the baby. Sometimes this may have beneficial effects for the baby.  There are, however, some medicines that can harm a baby’s normal development.  How a medicine affects a baby may depend on the stage of pregnancy when the medicine is taken. If you are on regular medication you should discuss these effects with your doctor/health care team before becoming pregnant.

If a new medicine is suggested for you during pregnancy, please ensure the doctor or health care professional treating you is aware of your pregnancy.

When deciding whether or not to use a medicine in pregnancy you need to weigh up how the medicine might improve your and/or your unborn baby’s health against any possible problems that the drug may cause. Our bumps leaflets are written to provide you with a summary of what is known about use of a specific medicine in pregnancy so that you can decide together with your health care provider what is best for you and your baby.   

Every pregnancy is unique. The decision to start, stop, continue or change a prescribed medicine before or during pregnancy should be made in consultation with your health care provider. It is very helpful if you can record all your medication taken in pregnancy in your hand held maternity records.



Disclaimer: This information is not intended to replace the individual care and advice of your health care provider. New information is continually becoming available. Whilst every effort will be made to ensure that this information is accurate and up to date at the time of publication, we cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes following decisions made on the basis of this information. We strongly advise that printouts should NOT be kept for any length of time, or for “future reference” as they can rapidly become out of date.

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