If you are pregnant or planning a pregnancy in the UK and have just felt the familiar tingle of a cold sore starting, you may be wondering: is aciclovir safe in pregnancy? Based on the available evidence, aciclovir is generally considered suitable for use during pregnancy when clinically appropriate. A large Danish study found no increase in the rate of major birth defects after first trimester exposure to aciclovir. However, the right treatment for you should still be discussed with your midwife, GP or maternity team.
Many pages say that a medicine is "safe when the benefits outweigh the risks". While this is common wording, it does not explain much about the actual evidence. Here, we look at what the pregnancy safety data shows, including the numbers, how advice can differ by trimester, why cold sores and genital herpes are not the same issue during pregnancy, and how topical aciclovir compares with oral treatment. We also cover UK pregnancy medicine categories, breastfeeding, protecting a newborn if you have herpes symptoms, and who should help you make treatment decisions.
Key Things to Know
- Aciclovir has a substantial amount of human pregnancy data, rather than only animal studies
- The largest cohort study found no increase in the rate of major birth defects after first trimester exposure
- Lettered pregnancy categories are American, were retired in 2015, and have never been used in the UK
- When a cold sore is the only problem, topical treatment involves very little absorption into the body
- Your midwife, GP or maternity team can help decide whether treatment is appropriate based on your individual circumstances
First, the Correction: There Is No UK Pregnancy Category
You may see aciclovir described as a "category B" medicine in pregnancy, including on some pharmacy websites. This is not a UK pregnancy category, so it is worth clearing this up before looking at the evidence.
The lettered system is American, not British. It was created by the United States Food and Drug Administration and was retired under the Pregnancy and Lactation Labeling Rule in 2015. The system was changed because assigning a single letter to a medicine could oversimplify a complicated question and could be misunderstood. The UK has never used lettered pregnancy categories.
Neither the NHS nor the MHRA issues pregnancy categories. UK information is provided through sources such as the British National Formulary, the UK Teratology Information Service and the summary of product characteristics. These sources explain what is known about a medicine, along with any limitations or uncertainties, rather than reducing the information to a single letter.
If you have seen a category grade somewhere and found it reassuring, the evidence behind that information may still be useful. However, the letter itself is not a UK classification and should not be treated as one.
What the Evidence Actually Shows
This is the part that deserves numbers rather than general statements.
One of the largest studies of its kind was a Danish population based cohort covering 837,795 live births between 1996 and 2008. The researchers linked national prescription records with birth defect diagnoses.
| Group | Major birth defect rate | Adjusted comparison |
|---|---|---|
| Any antiviral exposure in the first trimester (1,804 pregnancies) | 40 infants, 2.2% | Prevalence odds ratio 0.89 (95% CI 0.65 to 1.22) |
| Aciclovir specifically (1,561 infants) | 32 infants, 2.0% | Prevalence odds ratio 0.82 (95% CI 0.57 to 1.17) |
| Valaciclovir (229 infants) | 7 infants, 3.1% | Prevalence odds ratio 1.21 (95% CI 0.56 to 2.62) |
| No exposure | 19,920 infants, 2.4% | Reference |
The rate of major birth defects after aciclovir exposure was not higher than the background rate in this study. The adjusted analysis also did not show an increased risk. The confidence interval included 1, so the study did not identify a statistically significant increase.
The researchers also looked at thirteen separate categories of birth defects and did not find an association with aciclovir exposure. However, the number of cases in each individual category was small, so these results need to be interpreted with some caution.
It is also useful to know that around 2.4 per cent of all births in the cohort involved a major birth defect, regardless of medicine exposure. This provides important context when looking at pregnancy medicine studies.
Available data have not identified an increased risk of miscarriage associated with aciclovir use. In registry data, spontaneous abortions occurred in 11% of exposed pregnancies. This was an observed figure and does not, on its own, show that aciclovir caused or increased the risk of miscarriage.
What UK Sources Say
The British National Formulary states that aciclovir is not known to be harmful in pregnancy, while also noting the manufacturer's advice that it should be used when the potential benefit outweighs the possible risk. This type of wording is commonly used for medicines and does not mean that a specific pregnancy risk has been established for aciclovir.
Aciclovir can cross the placenta. This does not, by itself, mean that it is harmful. This is one reason why human pregnancy outcome data is important when considering treatment.
Aciclovir exposure alone would not usually mean that you need additional fetal monitoring beyond your usual antenatal care. However, your healthcare team may recommend extra monitoring if there are other reasons to do so.
The Honest Limits
No observational study can rule out a small risk completely. The evidence on systemic aciclovir exposure during pregnancy also has limitations because some less common birth defects occurred too infrequently for researchers to study them reliably.
Even so, aciclovir has a relatively large amount of pregnancy safety data compared with many other antiviral medicines. Valaciclovir has less pregnancy safety data available, although the evidence that is available is generally reassuring.
Cream or Tablets in Pregnancy?
If you have a straightforward cold sore on your lip, topical treatment may be considered when appropriate. The reason is fairly simple. Aciclovir cream applied to the skin results in very little absorption into the bloodstream, so much less medicine is expected to reach the rest of the body than after taking a tablet.
This does not mean that oral aciclovir is unsafe. The pregnancy data above is generally reassuring. It simply means that when a cold sore can be managed with a topical treatment, your healthcare professional may consider this option before oral treatment.
Oral aciclovir may be used when an outbreak is severe, when outbreaks are frequent, when the affected area is not suitable for cream, or when the infection is genital herpes rather than a cold sore. The appropriate treatment depends on the situation and should be decided by a healthcare professional.
As with any medicine, oral aciclovir can cause side effects. These can include nausea and headache. Kidney function may also be taken into account when prescribing oral aciclovir, particularly if you already have kidney problems.
There are also a couple of practical points to remember. Cold sore aciclovir cream is a skin preparation, and its patient information says not to apply it inside the nose or mouth. This advice applies during pregnancy as well.
Antiviral treatment works on actively replicating virus, so it is generally best started as soon as possible after symptoms begin, according to the product instructions or advice from your healthcare professional.
Trimester by Trimester
First Trimester
The first trimester is often the period people worry about most because this is when the baby's organs are developing. It is also the period specifically examined in the Danish cohort discussed above.
The general principle is still to use the most appropriate treatment for the condition. If a cold sore can be treated with a topical product, this may be considered. However, the available evidence does not suggest that necessary aciclovir treatment should automatically be avoided in early pregnancy.
If you are worried because you have taken aciclovir before realising you were pregnant, speak to your midwife, GP or maternity team. They can look at the timing, dose and reason for treatment and advise you based on your circumstances.
Second Trimester
The same general approach applies during the second trimester. If treatment is needed, your healthcare professional can weigh the potential benefits against any possible risks based on your individual situation.
If you are getting frequent cold sores during pregnancy, it may be worth mentioning this to your midwife or GP rather than dealing with each outbreak separately. They can help you understand your treatment options and whether anything else needs to be considered.
Third Trimester
The situation can be different later in pregnancy, particularly when the infection is genital herpes.
The main concern is not usually the aciclovir itself. With genital herpes, an active outbreak or viral shedding around the time of delivery can increase the chance of passing herpes to the baby during birth.
UK obstetric practice may therefore include antiviral treatment later in pregnancy for some women with genital herpes. This may be considered to reduce the chance of an outbreak around delivery and the possibility of needing a caesarean section because of an active infection. Aciclovir can also be used to treat a first episode of genital herpes during pregnancy when clinically appropriate.
The decision is made by your maternity team based on your medical history, including whether this is your first episode during the pregnancy and how close you are to delivery.
Later pregnancy treatment may also be considered as prevention for women with recurrent genital herpes. This is different from treating an ordinary cold sore on the lip, and your maternity team should advise you about whether treatment is appropriate.
If you develop what you think is your first genital herpes outbreak late in pregnancy, tell your maternity team promptly rather than waiting for your next routine appointment. A first episode close to delivery may need different management from a recurrence.
Cold Sores and Recurrent Genital Herpes Are Not the Same Question
Most people reading this page will be thinking about a cold sore on the lip. It is important to separate this from genital herpes because the pregnancy advice is not the same.
A cold sore on the lip is not generally considered a risk to a baby during pregnancy or birth. It is contagious to other people, but it is not considered a route of transmission to a baby in the womb and would not usually affect your delivery plan.
Treatment can help manage the infection and symptoms, but it does not remove herpes simplex virus from the body.
Genital herpes is a different clinical situation, particularly if it is a first episode during pregnancy. Your maternity team can advise you about treatment and whether the infection could affect your delivery plans.
If you are not sure whether a sore is a cold sore or something else, speak to your GP, midwife or maternity team rather than trying to work it out from a website alone.
Protecting a Newborn After Birth
The main concern with a cold sore and a newborn is after birth rather than during pregnancy. This is important because herpes can spread through close contact.
Newborn babies have immature immune systems, and neonatal herpes is uncommon but can be serious. Transmission can occur through close contact with someone who has an active cold sore, including kissing.
Practical steps during the first few weeks include:
- If you have an active cold sore, do not kiss the baby. Avoid kissing the baby's face until the sore has healed
- Wash your hands thoroughly before handling the baby, especially after touching your face or applying cream
- Avoid touching the cold sore and keep it covered where practical
- Do not share flannels, towels or muslins with the baby
- If you are breastfeeding and have a herpes blister on the breast, do not feed from the affected breast and seek advice from a healthcare professional
- Ask visitors with an active cold sore not to kiss or have close contact with the baby
- Seek urgent medical advice if a newborn becomes unusually sleepy, floppy, feeds poorly, develops a temperature or develops any blistering rash
Having a cold sore does not mean that you cannot care for your baby. It means taking extra care to reduce the chance of passing the virus to them.
Breastfeeding
Aciclovir is generally considered compatible with breastfeeding when clinically appropriate. It passes into breast milk in small amounts, and babies can also receive aciclovir directly at doses much higher than the amount expected through breast milk.
If you are prescribed aciclovir while breastfeeding, tell your prescriber that you are breastfeeding so this can be taken into account when choosing your treatment.
Who Should Actually Make This Decision
When you are pregnant, your midwife, GP, maternity team or another healthcare professional involved in your care is best placed to advise you about medicines. They know your medical history, stage of pregnancy, other medicines and any other factors that may affect the decision.
An online pharmacy consultation is not a substitute for maternity care. If you are pregnant and unsure about treating a cold sore, your midwife or GP is usually a good place to start. A pharmacist can also advise you about suitable pharmacy medicines and when you may need to speak to another healthcare professional.
When you ask for advice, it can help to mention three things: how often you get cold sores, how far along you are in your pregnancy, and whether you are sure it is a cold sore or think it could be something else.
Medical Disclaimer
This article is provided for general information only and does not constitute medical advice. It is not a substitute for a consultation with a qualified healthcare professional, and it does not tell you what medicine to take or what dose to use.
If you are pregnant, planning a pregnancy or breastfeeding, speak to your midwife, GP, obstetric team or pharmacist before starting, stopping or changing any medicine, including medicines available without a prescription. Always follow the instructions on your pharmacy label and in the patient information leaflet.
Contact your maternity team promptly if you develop a first episode of genital herpes during pregnancy, if you have an outbreak near your due date, if you feel generally unwell with an outbreak, or if you have any concern about a newborn baby. Seek urgent medical advice if chickenpox or shingles is suspected during pregnancy, as these infections may require prompt assessment and treatment.
