Finding out that you need treatment for a cold sore, genital herpes or chickenpox during pregnancy can understandably feel worrying. Available evidence suggests that aciclovir can be used during pregnancy when a healthcare professional considers treatment appropriate. UK specialist guidance recommends aciclovir as a first line antiviral option for herpes infections during pregnancy when treatment is clinically needed, and current evidence does not suggest an increased risk of birth defects or preterm delivery.
For pregnant women in the UK who need treatment for cold sores, genital herpes or chickenpox, this guide explains what the evidence says about aciclovir 400mg during pregnancy, when it may be prescribed, how dosing can differ from use outside pregnancy, and what to know about suppressive treatment, chickenpox exposure, breastfeeding, fertility and common questions. It does not replace advice from your doctor, midwife or prescriber. They can consider your stage of pregnancy, symptoms, medical history and individual circumstances before recommending treatment.
Is Aciclovir Safe for Pregnant Women to Take During Pregnancy?
Aciclovir has been used during pregnancy for many years and is one of the better studied antiviral medicines in this setting. UKTIS, the UK Teratology Information Service, reports that available data do not suggest an increased risk of congenital malformations or preterm delivery following exposure to aciclovir during pregnancy.
Registry data involving more than 500 births, together with a further study involving more than 1,500 infants exposed to aciclovir during the first trimester, did not find an increase in birth defects above the expected background rate. Every pregnancy has a background risk of birth defects, estimated at around 3 to 5 per cent, whether or not a medicine is taken. Current evidence has not shown aciclovir to increase this risk.
Aciclovir crosses the placenta and can be detected in amniotic fluid. Available evidence does not suggest that it accumulates preferentially in the baby. Some studies have also not found an increased risk of low birth weight or intrauterine death following aciclovir exposure. However, some pregnancy outcomes have not been studied in enough detail to draw firm conclusions.
This is why treatment decisions during pregnancy should still be made with your healthcare professional. They can consider the available safety evidence alongside the benefits of treating the infection and your individual circumstances.
Why Might Aciclovir Be Prescribed in Pregnancy?
Aciclovir may be recommended in several situations during pregnancy. The reason for treatment and the approach used will depend on the infection, your symptoms and the stage of pregnancy.
A First Episode of Genital Herpes
A first episode of genital herpes can be more severe than a recurrence and may carry additional risks during pregnancy, including an association with preterm labour. RCOG and BASHH guidance recommends offering antiviral treatment to pregnant women who have a first episode of genital herpes in any trimester.
Your healthcare professional can advise whether aciclovir is appropriate for you and explain how long treatment should continue.
Cold Sores (Herpes Labialis, Herpes Simplex)
Most cold sores are mild, self limiting and clear without prescription treatment. Aciclovir cream may be recommended for a typical cold sore outbreak.
Oral aciclovir tablets may be considered for more severe, frequent or troublesome outbreaks, depending on your symptoms and your prescriber's assessment. If you are pregnant, speak to your healthcare professional before starting oral aciclovir.
Suppressive Therapy Later in Pregnancy
Women with recurrent genital herpes may sometimes be offered suppressive aciclovir from around 36 weeks of pregnancy through to delivery. The aim is to reduce the likelihood of an active outbreak around the time of birth, which may help reduce the need for some interventions and lower the risk of herpes transmission to the baby during delivery.
Your maternity team will consider your history and current circumstances when deciding whether suppressive treatment is appropriate.
Chickenpox (Varicella) Exposure or Infection
Chickenpox during pregnancy needs a different approach from herpes simplex, even though aciclovir may sometimes be used in its treatment.
If you have never had chickenpox and have been exposed to it during pregnancy, or if you develop chickenpox, contact your maternity or healthcare team promptly. Depending on your immunity, the stage of pregnancy and when the exposure occurred, management may include varicella zoster immunoglobulin, antiviral treatment or referral for specialist monitoring.
This is different from managing a cold sore or genital herpes infection, so do not assume that the same treatment plan applies to both.
How Is Aciclovir Dosed in Pregnancy?
The appropriate dose of aciclovir during pregnancy depends on the infection being treated and your individual circumstances. Pregnancy can also cause physiological changes that affect how medicines are handled by the body, which may be considered when treatment is prescribed.
The dose and duration can therefore vary depending on whether aciclovir is being used for a first episode, a recurrent infection, suppressive treatment or another condition. Your prescriber will confirm the appropriate dose, frequency and duration for you.
The information in this guide is intended as general background only. Always follow the instructions on your prescription label and the patient information leaflet supplied with your medicine. Do not change the dose or stop treatment during pregnancy without speaking to your prescriber.
| Situation | General approach described in UK guidance |
|---|---|
| First episode of genital herpes or a significant cold sore outbreak | A treatment course lasting several days, with the exact regimen set by your prescriber based on trimester and severity |
| Recurrent outbreaks earlier in pregnancy | Managed episode by episode; treatment is not routinely required for short, mild, self-limiting recurrences |
| Suppressive therapy from around 36 weeks | Considered for women with a history of recurrent genital herpes; a higher-frequency regimen than typical non-pregnant suppressive dosing is generally used because of altered drug handling late in pregnancy |
| Chickenpox exposure or infection | Requires prompt specialist assessment; may involve referral to a fetal medicine specialist rather than antiviral treatment alone |
As with any prescription medicine, do not double up on a missed dose, and do not start, stop or change your regimen without speaking to your prescriber first. Full dosing detail is set out in the patient information leaflet supplied with your medicine, which is also available via the electronic medicines compendium (emc).
Breastfeeding and Aciclovir
Aciclovir passes into breast milk in small amounts, and the amount absorbed by a breastfed baby is generally expected to be low. Current UK specialist guidance generally considers aciclovir and valaciclovir compatible with breastfeeding. If you have been prescribed aciclovir while breastfeeding and have any concerns about continuing to breastfeed, speak to your healthcare professional.
There is one important situation to be aware of. If you have an active herpes lesion directly on your breast or nipple, you may need to avoid feeding from the affected breast until the lesion has healed. You should also follow professional advice about expressed milk and maintain good hand hygiene. This is because of the risk of direct transmission of the virus to your baby through contact, rather than because of the aciclovir in your breast milk.
If you are unsure whether a lesion could come into contact with your baby during feeding, speak to your midwife, GP, pharmacist or another healthcare professional for individual advice.
Aciclovir, Fertility and Trying to Conceive
If you or your partner takes aciclovir before or around the time of conception, current evidence does not suggest that aciclovir increases the risk of birth defects.
Information about the effect of aciclovir on male fertility is more limited. A small study involving 20 men who received high dose treatment for six months did not identify a reduction in sperm production. However, this is limited evidence and does not provide a complete picture of fertility.
If you are trying to conceive and have concerns about aciclovir, your infection or the timing of treatment, speak to your prescriber. They can consider your individual circumstances rather than relying on general information.
Aciclovir Cream in Pregnancy
Topical aciclovir cream, which may be used for cold sores, is only minimally absorbed into the bloodstream when applied to the skin. Because systemic exposure is low, it is generally considered suitable for use during pregnancy when clinically appropriate.
For a typical mild cold sore, topical treatment may be considered instead of oral treatment. However, the most appropriate option depends on the severity and location of the infection.
When using aciclovir cream, follow the instructions supplied with your product. Avoid getting it into your eyes or inside your mouth or nose, and wash your hands before and after applying it.
Questions Worth Asking Your Prescriber
If you are pregnant, breastfeeding or trying to conceive, it is completely reasonable to have questions before starting treatment. You could ask your prescriber:
- Does my stage of pregnancy affect whether aciclovir is appropriate for me?
- Is this a first episode or a recurrence, and does that change my treatment plan?
- Would cream, tablets or another treatment be more appropriate for my symptoms?
- Could I need suppressive treatment later in pregnancy, and if so, when would this usually be considered?
- What symptoms should prompt me to contact you or my maternity team?
- Is there anything else in my medical history or current medicines that you need to know before prescribing aciclovir?
Available Treatments
EverydayMeds offers a range of cold sore treatments following a short online assessment. Prices below were correct at the time of writing, but pricing can change, so check the live product page for the current price before ordering. If you are pregnant or breastfeeding, make sure you include this information during the assessment so the prescriber can consider whether the treatment is appropriate for you.
| Treatment | Price |
|---|---|
| Aciclovir Tablets | From £12.99 |
| Aciclovir 5% Cream | From £8.99 |
| Valaciclovir | From £18.99 |
| Valtrex | From £34.99 |
If you are pregnant or breastfeeding, mention this during your online assessment so that the prescribing team can assess treatment suitability appropriately .
Medical Disclaimer
This article is for general information only and is not a substitute for personalised medical advice. Medicine safety during pregnancy depends on individual factors, including your stage of pregnancy, medical history and the condition being treated. Always tell your prescriber, doctor, midwife or pharmacist that you are pregnant or breastfeeding before starting, stopping or changing any medicine.
If you have concerns about a medicine you are already taking, do not stop it without medical advice. Speak to your GP, midwife or pharmacist, or contact NHS 111 if you are unwell or need urgent advice. This page was prepared with reference to UKTIS (BUMPS), RCOG, BASHH and NHS sources and is reviewed by our clinical team. However, it cannot account for your individual circumstances.
