USE OF TAMOXIFEN IN PREGNANCY

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(Date of issue: August 2015, Version: 2)

This is a UKTIS monograph for use by health care professionals. For case-specific advice please contact UKTIS on 0344 892 0909. To report an exposure please download and complete a pregnancy reporting form. Please encourage all women to complete an online reporting form.

A corresponding patient information leaflet on tamoxifen use in pregnancy is available at www.medicinesinpregnancy.org.

Summary

Tamoxifen is a selective oestrogen receptor modulator that acts as an oestrogen antagonist in some tissues, including breast, and as an agonist in others, including endometrium. Tamoxifen is used for prevention and treatment of breast cancer and has also been used off-licence in the treatment of infertility in females (ovulation induction), as well as for a number of other conditions such as mastalgia.

There are very limited data on the use of tamoxifen in human pregnancy. No large studies have been carried out to assess its pregnancy safety profile and human pregnancy data consist of only a small number of retrospective case reports, mainly to the manufacturer of tamoxifen.

It is therefore not currently possible to undertake an evidence-based risk assessment of whether exposure to tamoxifen in utero increases the risk of congenital malformations in the infant. Animal studies have suggested that tamoxifen exposure in utero and in the early neonatal period is associated with structural and histological changes of the female genital tract. Genital tract malformation as a consequence of altered oestrogenic signalling during embryogenesis is plausible, and in humans a small number of case reports have described abnormalities of female external genitalia following exposure in-utero. There are also a number of case reports of infants with craniofacial malformations following tamoxifen exposure (although there is no discernible malformation pattern), however there are also reports documenting numerous non-malformed infants. The retrospective nature of these data, as well as lack of details in the reports, co-exposure to other drugs in most cases, and a likely publication bias towards adverse outcomes, means that the available information cannot be used to assign causality or assess the overall risk of malformation following gestational exposure to tamoxifen.

There are no data on which to assess risk of other outcomes such as fetal loss, low or high birth weight, preterm delivery, neonatal complications or neurodevelopmental impairment.

A theoretical risk of transplacental carcinogenesis (increased risk of cancer in the offspring, specifically in this case of cancers of the reproductive tract) following in utero tamoxifen exposure has been proposed but remains to be proven.

Due to the lack of human pregnancy safety data the manufacturer states that pregnancy should be avoided for two months after stopping treatment with tamoxifen. However, where pregnancy has occurred in a woman with an underlying potentially life-threatening illness, maternal treatment with tamoxifen during pregnancy may be clinically indicated. In all cases patients should be involved in weighing up the risks and benefits of treatment options and be informed about the limited human pregnancy data for tamoxifen. 

Exposure to tamoxifen in pregnancy may warrant additional fetal monitoring, the need for which should be assessed on a case-by-case basis. However, other risk factors may be present in individual cases which may independently increase the risk of adverse pregnancy outcome. Clinicians are reminded of the importance of consideration of such factors when performing case-specific risk assessments.

This document is regularly reviewed and updated. Only use UKTIS monographs downloaded directly from TOXBASE.org or UKTIS.org to be sure you are using the most up-to-date version.

This is a summary of the full UKTIS monograph for health care professionals and should not be used in isolation. The full UKTIS monograph and access to any hyperlinked related documents is available to health care professionals at www.toxbase.org.

If you have a patient with exposure to a drug or chemical and require assistance in making a patient-specific risk assessment, please telephone UKTIS on 0344 892 0909 to discuss the case with a teratology specialist.

If you would like to report a pregnancy to UKTIS please click here to download our pregnancy reporting form. Please encourage all women to complete an online reporting form.

Disclaimer: Every effort has been made to ensure that this monograph was accurate and up-to-date at the time of writing, however it cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes of the measures recommended. The final decision regarding which treatment is used for an individual patient remains the clinical responsibility of the prescriber. This material may be freely reproduced for education and not for profit purposes within the UK National Health Service, however no linking to this website or reproduction by or for commercial organisations is permitted without the express written permission of this service. This document is regularly reviewed and updated. Only use UKTIS monographs downloaded directly from TOXBASE.org or UKTIS.org to ensure you are using the most up-to-date version.