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Date of issue: October 2023, Version: 3.1

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A corresponding patient information leaflet on USE OF ACICLOVIR/VALACICLOVIR IN PREGNANCY is available.

Aciclovir is an antiviral agent used systemically for the treatment of herpes simplex and varicella-zoster infections, or topically for herpes simplex infections of the skin, mucous membranes and eye. Valaciclovir, a prodrug of aciclovir, is used systemically in the treatment or prophylaxis of herpes simplex, varicella-zoster and cytomegalovirus infections.

Guidelines from the Royal College of Obstetricians and Gynaecologists (RCOG) recommend that aciclovir be considered for all pregnant women who present with a first episode of genital herpes. The UK Health Security Agency (UKHSA) recommends use of aciclovir or valaciclovir at any stage of pregnancy for treatment of chickenpox, or post-exposure prophylaxis.

Overall, the available data do not suggest an increased risk of congenital malformation or preterm delivery following exposure to aciclovir during pregnancy. First episode genital herpes infection in pregnancy is a recognised risk factor for preterm labour, and there is some evidence that treatment with aciclovir can reduce the risk of preterm delivery. Limited data suggest no increased risk of low infant birth weight or intrauterine death relating to aciclovir exposure. Other pregnancy/infant outcomes have not been adequately studied to facilitate an evidence-based assessment of risk, although the known risks posed by the conditions that aciclovir is used to treat should be considered when discussing treatment options with the pregnant woman.

Human pregnancy data for valaciclovir are limited to one small study which provided no evidence of an increased malformation risk. However, ongoing research is ideally required to confirm this. Other fetal outcomes have not been assessed in women taking valaciclovir.

Exposure to aciclovir or valaciclovir at any stage in pregnancy would not usually be regarded as medical grounds for termination of pregnancy or any additional fetal monitoring. 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 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 NHS health care professionals who are logged in.

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 to ensure you are using the most up-to-date version.