(Date: August 2018. 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?

Aciclovir (Zovirax®) is an antiviral medicine used to treat oral herpes (cold sores), herpes infection of the eyes or genitals, and chickenpox. It can be used as a cream, taken in tablet form, and is occasionally given intravenously (into a vein) in hospital.

Is it safe to use aciclovir in pregnancy?

There is no yes or no answer to this question. When deciding whether to take aciclovir during pregnancy it is important to weigh up how necessary aciclovir 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.

Use of aciclovir cream on a small area of skin (such as a cold sore) is unlikely to have an effect on the baby as only a very small amount will be absorbed into your bloodstream.

Use of aciclovir in tablet form during the four weeks prior to delivery may sometimes be advised to prevent the recurrence of genital herpes so that a mother does not pass the infection on to her baby during delivery.

Use of aciclovir tablets might also be advised by a doctor if a pregnant woman who has never had chickenpox or been vaccinated against the illness is exposed to a person with chickenpox or shingles, or develops chickenpox herself. This is important because chickenpox in pregnancy can be harmful to both mother and baby.

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 aciclovir on a baby in the womb. 

What if I have already used aciclovir during pregnancy?

If you have taken any medicine 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 using aciclovir in 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.

None of four studies of women taking aciclovir in the first trimester of pregnancy have shown a higher chance of their babies having a birth defect. Together, these studies have analysed information on around 5,600 pregnancies. These studies are designed to investigate whether a medicine can commonly cause birth defects. However, they may not show if a medicine increases the chance of a particular type of birth defect.

Two studies investigated whether a number of specific birth defects were more likely in babies exposed to aciclovir in the womb. These studies provided no evidence that aciclovir use in early pregnancy increases the chance of the baby having an orofacial cleft; or defects of the nervous system, eye, ear, face, neck, heart, respiratory system, digestive system, abdominal wall, urinary tract, genitals, limbs, or muscles and skeleton. A further study showed a possible link between use of aciclovir and related medicines in early pregnancy and gastroschisis in the baby (where some of the abdominal organs protrude through a hole next to the tummy button). However, the study also showed a possible link between untreated genital herpes and gastroschisis, suggesting a possible link with the mother’s illness rather than the medicines used to treat it.

Can taking aciclovir in pregnancy cause miscarriage?

A single study found no link between miscarriage and use of aciclovir cream or tablets in early pregnancy. More research is required to confirm this finding.

Can taking aciclovir in pregnancy cause stillbirth?

Two studies found no link between stillbirth and use of aciclovir in pregnancy. Stillbirth rates need to be studied in more pregnant women using aciclovir to confirm this finding.

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

Two studies have found no increased chance of preterm birth or low birth weight in the baby following use of aciclovir during pregnancy. One of these studies suggested that for women with genital herpes, use of aciclovir in late pregnancy may protect against preterm birth. While this is reassuring, rates of these pregnancy outcomes in more pregnant women using aciclovir need to be studied to confirm these findings.

Can taking aciclovir 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 scientific studies have investigated whether exposure to aciclovir in pregnancy in the womb can affect a child’s learning or behaviour.

Will 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. Taking aciclovir in pregnancy is not expected to cause problems that would require extra monitoring of your baby.

Are there any risks to my baby if the father has taken aciclovir?

No studies have specifically investigated whether aciclovir used by the father can harm the baby through effects on the sperm, however most experts agree that this is very unlikely.  More research on the effects of medicine use in men around the time of conception is needed.

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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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