Aciclovir in Pregnancy: Safety, Evidence and What UK Guidance Says

If you are pregnant and dealing with a cold sore, genital herpes or shingles, it is natural to worry about whether antiviral treatment is safe for your baby. Aciclovir (also spelled acyclovir) is the antiviral most often discussed in this situation.

Aciclovir has been used during pregnancy for many years, and available UK evidence is reassuring about several important pregnancy outcomes.

This guide explains what UK sources actually say about aciclovir in pregnancy, how the tablets and cream differ, what the evidence shows across each trimester, and how the confusing "pregnancy category" question really works.

Everything here reflects guidance from the NHS, the UK Teratology Information Service (UKTIS, which runs the patient-facing bumps service) and the licensed Summary of Product Characteristics (SmPC). It is educational and does not replace advice from your own midwife, GP or prescriber, who will weigh up your individual circumstances.

Is Aciclovir Safe in Pregnancy?

For most people, the short answer is yes, aciclovir can usually be used in pregnancy, but you should check with a pharmacist or doctor first. That is the exact position taken by the NHS, which states that "you can usually use aciclovir if you're pregnant, but check with a pharmacist or doctor first."

This advice is backed by decades of real-world use. A large post-marketing pregnancy registry, referenced in the aciclovir SmPC, found "no increase in the number of birth defects amongst aciclovir exposed subjects compared with the general population," and any defects that were seen "showed no uniqueness or consistent pattern to suggest a common cause." In plain terms, the available data have not shown an increased rate of birth defects among babies exposed to aciclovir during pregnancy.

It is important to remember that "usually suitable" is not the same as "appropriate without professional advice."

Aciclovir tablets are a prescription-only medicine, and the decision to use them in pregnancy is made with a healthcare professional who can weigh the potential benefits of treatment against any remaining uncertainties.

What the Evidence Says About Acyclovir and Pregnancy

The most useful UK summary comes from UKTIS/bumps, which reviews the published data on medicines in pregnancy. On aciclovir and valaciclovir, its conclusions are reassuring across the outcomes that matter most.

• Birth defects: "The available data do not suggest an increased risk of congenital malformation" after aciclovir exposure in pregnancy.
• Baby's birth weight: limited data suggest no increased risk of low infant birth weight.
• Stillbirth and intrauterine death: the limited data available do not point to an increased risk.
• Preterm birth: Aciclovir itself has not been shown to increase the risk of preterm delivery. A first episode of genital herpes is itself a recognised risk factor for preterm labour, and there is some evidence that treatment with aciclovir may reduce the risk of preterm delivery.

UKTIS also makes an important clinical point that may reassure anyone who has already taken aciclovir during pregnancy. This is considered reassuring within current UK teratology guidance.

The "Aciclovir Pregnancy Category" Question, Explained

Many people search for the aciclovir pregnancy category, acyclovir pregnancy class, or the grading of acyclovir in pregnancy, and end up confused. Here is what is actually going on.

The familiar A, B, C, D and X letters come from the former US FDA pregnancy category system. Under that old framework, aciclovir (and Zovirax, a brand of aciclovir) was assigned Category B, meaning animal studies had not shown a risk to the foetus and there was no clear evidence of harm in humans. This is the source of every "acyclovir pregnancy category B" and "zovirax pregnancy category" result you will find online.

However, two things are essential to understand:

1. The FDA retired the letter categories in 2015. They were replaced by the Pregnancy and Lactation Labeling Rule (PLLR), which uses a narrative summary of the evidence instead of a single letter, because a letter was considered too simplistic and often misleading.
2. The UK does not use the former US FDA A/B/C/D/X letter system as the basis for prescribing decisions. UK clinicians instead use sources such as the BNF, the medicine's SmPC and UKTIS.

So if a page claims "the MHRA classifies aciclovir as pregnancy category B," that is incorrect: it is mixing up the old US FDA label with the UK regulator. The accurate takeaway is that aciclovir was historically placed in the US FDA Category B system, which has since been retired. UK pregnancy decisions are based on current clinical evidence and UK sources such as the SmPC and UKTIS, rather than a letter category.

Aciclovir Cream in Pregnancy

When people ask about the acyclovir cream pregnancy category or the medication grading of acyclovir cream during pregnancy, the practical answer is simpler than the tablet version. Aciclovir 5% cream is applied to the skin, usually for cold sores around the lips, and only a very small amount is absorbed into the bloodstream.

Because systemic exposure is very low, topical aciclovir results in much less systemic exposure than oral treatment.

The NHS does not single out cream as needing different handling from tablets; both carry the same sensible advice to check with a pharmacist or doctor first. For many pregnant women with an ordinary cold sore, cream may be suitable for localised cold sores, subject to advice from a pharmacist or healthcare professional, and your pharmacist can confirm whether it suits you.

Aciclovir Tablets in Pregnancy

Aciclovir tablets, including aciclovir 400mg tablets, work throughout the body and are used when an infection is more than a minor cold sore, for example severe or frequent outbreaks, genital herpes, or shingles.

Tablets should only be used during pregnancy following assessment by an appropriate healthcare professional, but as covered above, the large body of registry and observational data has not shown this to translate into a higher rate of birth defects.

Pregnancy dosing is individual and set by your prescriber. In pregnancy your clinician decides whether, when and at what dose to prescribe, taking account of the severity of the infection, your stage of pregnancy and your kidney function. Because of this, we do not publish a "pregnancy dose" here. The right course is the one your prescriber gives you.

Aciclovir Cream vs Tablets in Pregnancy at a Glance

Neither form is universally "better." The suitable choice depends on the type and severity of the infection and on an individual clinical assessment. What matters is matching the treatment to the problem rather than reaching for the strongest option by default.

Aciclovir in the First Trimester

Concern is understandable if you took, or need, aciclovir in the first trimester, when the baby's organs are forming.

UKTIS reports that the available data do not suggest an increased risk of congenital malformation following aciclovir exposure at any stage of pregnancy and specifically notes that exposure would not usually be considered grounds for extra foetal monitoring or for ending a pregnancy.

If you have already taken aciclovir before realising you were pregnant, these findings may provide reassurance, and you can raise it with your midwife or GP for peace of mind.

As with any medicine in early pregnancy, aciclovir should only be used when a healthcare professional considers it appropriate, at the dose considered appropriate by your prescriber, and to keep them informed.

Genital Herpes, Shingles, and Chickenpox in Pregnancy: Risks and Treatment

Aciclovir may be used in pregnancy for conditions beyond cold sores when considered clinically appropriate by a healthcare professional, particularly in managing infections caused by herpes simplex and varicella-zoster viruses. Recurrent genital herpes is a significant concern in pregnant women, especially when a primary infection or reactivation occurs late in pregnancy. Active genital herpes near the time of delivery can increase the risk of neonatal herpes in the newborn, potentially causing severe complications such as neonatal encephalitis or disseminated infection.

In some circumstances, specialist maternity teams may recommend antiviral suppression later in pregnancy to reduce the risk of recurrent genital herpes around delivery to help suppress recurrences and reduce viral shedding around the time of delivery. The aim of this approach is to reduce the risk of herpes being present at the time of delivery, which can be life-threatening to the newborn. The decision to use aciclovir for suppression or treatment in pregnancy is carefully made by maternity specialists, considering the known risks and benefits.

Shingles and chickenpox, both caused by the varicella-zoster virus, can also present rare but serious complications during pregnancy. Primary varicella infection may lead to disseminated infection in the mother, increasing the risk of pneumonia and adverse pregnancy outcomes such as preterm birth. Aciclovir may be used to treat chickenpox or shingles during pregnancy when clinically indicated, with the aim of helping manage the infection and potentially reducing the risk of complications. If a pregnant woman is exposed to chickenpox or develops symptoms, prompt medical assessment is crucial to initiate antiviral treatment and provide preventative treatment if necessary.

Overall, aciclovir is an antiviral medicine that may be used when clinically appropriate during pregnancy in the prevention and treatment of infectious diseases caused by herpes simplex and varicella-zoster viruses during pregnancy.

The decision to use aciclovir during pregnancy depends on the infection, its severity and the stage of pregnancy, with the potential benefits of treating the infection weighed against any remaining uncertainties about treatment.

Aciclovir and Breastfeeding

Many parents want to plan ahead for after the birth. Aciclovir passes into breast milk in small amounts after oral treatment. NHS Specialist Pharmacy Service considers aciclovir suitable for use during breastfeeding when needed, although monitoring the infant for unusual drowsiness, irritability, rash, vomiting or diarrhoea is advised as a precaution. Tell your doctor or pharmacist that you are breastfeeding before starting treatment.

For the cream, the NHS advises it is generally fine while breastfeeding, with one simple precaution: make sure your baby does not come into contact with the areas where you have applied it. As always, if you are breastfeeding, let your prescriber know before starting aciclovir tablets so the choice can be tailored to you and your baby.

Things to Know Before Using Aciclovir in Pregnancy

• Early assessment is advisable if symptoms develop. If treatment is required, it is generally considered most effective when started early in the course of an outbreak.
• Match the form to the problem. The most appropriate treatment depends on the type and severity of the infection.
• Tell your clinician you are pregnant or breastfeeding. This is the single most important step, and it allows them to weigh the potential benefits against any theoretical risks.
• Maintaining adequate fluid intake is generally advised while taking aciclovir tablets. Aciclovir is cleared by the kidneys, so drinking plenty of fluids matters, especially at higher doses.
• Do not self-diagnose genital herpes in late pregnancy. This needs a maternity review because of delivery planning.
• If you have already taken aciclovir during pregnancy, UKTIS guidance may provide reassurance. UKTIS states that aciclovir exposure alone would not usually be regarded as grounds for additional foetal monitoring or intervention.

How to Get Aciclovir Safely in the UK

Aciclovir 5% cream for cold sores is available over the counter, while aciclovir tablets are a prescription-only medicine, a status set under UK medicines regulation overseen by the MHRA.

Through a registered online pharmacy may provide access following an appropriate clinical assessment.

• You complete a short, confidential online health assessment covering your symptoms, medical history, pregnancy or breastfeeding status and current medicines.
• An independent prescriber reviews your answers to decide whether treatment is appropriate and which option fits your situation.
• If suitable, your medicine is dispensed by a GPhC-registered UK pharmacy and sent out with tracked delivery.
• If treatment is approved, medication is dispensed by a regulated UK pharmacy in accordance with UK medicines regulations.

If you are pregnant and have a herpes infection, particularly genital herpes or a significant infection, seek advice from your midwife, GP or maternity team so that treatment and pregnancy care can be assessed together.

You can complete an online assessment for cold sore treatment if appropriate, and returning customers can log in to reorder.

Aciclovir Pricing at EverydayMeds

The figures below are the standard prices displayed on the EverydayMeds site. Final pricing is confirmed at checkout after your assessment, and the exact product supplied depends on your prescriber's recommendation.

Valaciclovir is a related antiviral that the body converts into aciclovir. UKTIS reviews the two together, and its safety conclusions are broadly similar, though the human pregnancy dataset for valaciclovir is smaller. Whether aciclovir or valaciclovir suits you is a decision for your prescriber. Prices are updated regularly and may change.

Key Takeaways

• Aciclovir can usually be used in pregnancy, but check with a pharmacist or doctor first (NHS).
• Available UK evidence has not shown an increased risk of birth defects, low birth weight or stillbirth associated with aciclovir exposure during pregnancy, and UKTIS advises that exposure alone would not usually be grounds for additional foetal monitoring.
• The old "Category B" letter is from the retired US FDA system. UK pregnancy guidance does not use the former US FDA letter categories as the basis for prescribing decisions.
• Cream has very low systemic exposure and may be suitable for localised cold sores.
• Tell your clinician if you are pregnant or breastfeeding before taking tablets.

Medical Disclaimer

This guide provides general information about aciclovir use during pregnancy and breastfeeding and is not a substitute for personalised medical advice, diagnosis or treatment. The appropriate treatment depends on the type and severity of the infection, your stage of pregnancy, medical history and other medicines you may be taking. Do not start, stop or change aciclovir treatment without speaking to a doctor, midwife or pharmacist. If you develop genital herpes, shingles, chickenpox or another significant infection during pregnancy, seek prompt medical advice so that appropriate treatment and pregnancy care can be assessed. If you have already taken aciclovir before realising you were pregnant, speak to your midwife, GP or pharmacist for individual advice.