Finding a cold sore when you are pregnant can be unsettling, and one of the first questions you may have is whether aciclovir cream is safe to use. Current UK guidance says aciclovir can usually be used during pregnancy, but you should check with a doctor or pharmacist first. Only a small amount of aciclovir is absorbed through the skin when the cream is used as directed.
The longer answer matters too, because "aciclovir in pregnancy" covers two different situations that are often mixed together online. If you are pregnant and dealing with a cold sore or genital herpes, the right approach can depend on what type of infection you have and when it occurs during pregnancy. A cold sore on the lip is different from genital herpes. Genital herpes can affect pregnancy management, particularly if it is a first episode later in pregnancy.
This guide explains what the available safety evidence says about aciclovir in pregnancy, how cream and tablets differ, when cold sores and genital herpes are managed differently, what to know about breastfeeding, how to protect a newborn, and why treatment decisions should be discussed with an appropriate healthcare professional.
Is Aciclovir Safe to Use While Pregnant?
UK medicines information does not describe aciclovir as completely risk free during pregnancy. Instead, it says that aciclovir can be used when a prescriber considers that the potential benefits outweigh any possible risks. This is because medicines are rarely tested in pregnant women through randomised clinical trials. Much of the reassurance around aciclovir comes from pregnancy data collected over many years.
The available data are generally reassuring. UKTIS reports that the available evidence does not suggest an increased risk of congenital malformations or preterm delivery following exposure to aciclovir during pregnancy. However, some pregnancy and infant outcomes have not been studied enough to give a complete picture of every possible risk.
What the Largest Study Found
One large population based cohort study in Denmark looked at 837,795 live born infants between 1996 and 2008. Researchers compared prescription records with birth defect diagnoses and found no increased risk of major birth defects following first trimester exposure to aciclovir or valaciclovir.
The study also looked at thirteen different groups of birth defects. No significant associations were found, although the researchers noted that the numbers in some of these groups were small.
This is one of the large studies that contributes to the reassuring pregnancy safety data for aciclovir.
What Is Less Well Established
It is important to be clear about the limits of the evidence. UKTIS states that limited data do not suggest an increased risk of low birth weight or intrauterine death following aciclovir exposure. However, other pregnancy and infant outcomes have not been studied well enough to make a full assessment of every possible risk.
The infection being treated also matters. In some situations, leaving a significant herpes infection untreated may carry its own risks. This is why treatment decisions during pregnancy need to consider both the infection and the medicine.
Why There Is No UK Pregnancy Category
You may see aciclovir described online as a Category B medicine in pregnancy. This refers to an American classification system, not a UK system. The United States has also moved away from the former letter based pregnancy categories.
The UK does not use lettered pregnancy risk categories. Instead, UK prescribing information explains the available evidence and the circumstances in which a medicine may be used during pregnancy.
If you see aciclovir described as "Category B in the UK", that information is not using a UK pregnancy classification system.
Cream or Tablets: How They Differ in Pregnancy
The main practical difference is how the medicine is absorbed and how much reaches the bloodstream.
Aciclovir cream is absorbed through the skin in very small amounts. This means exposure to the body is much lower than with tablets. For a straightforward cold sore, topical treatment may be considered where appropriate. Local side effects can include burning, stinging or skin irritation. NHS guidance says aciclovir can usually be used during pregnancy, but recommends checking with a pharmacist or doctor first.
Aciclovir tablets work through the bloodstream, so the medicine is absorbed throughout the body. That does not mean they cannot be used during pregnancy. Oral aciclovir is used in pregnancy in certain situations, including genital herpes, but the decision should be made by an appropriate prescriber based on your circumstances. UKTIS notes that the available pregnancy data do not suggest an increased risk of congenital malformations or preterm delivery following aciclovir exposure.
| Aciclovir 5% cream | Aciclovir tablets | |
|---|---|---|
| Exposure to the baby | Very low because only a small amount is absorbed through the skin | Systemic because the medicine enters the bloodstream |
| Typically used for | A localised cold sore on the lip | Genital herpes, more significant infections, or situations where a prescriber considers oral treatment appropriate |
| UK availability | Small packs are available from pharmacies, but check with a pharmacist if you are pregnant | Prescription only |
| Who should decide | Ask a pharmacist or doctor for advice | Prescriber, based on your individual circumstances |
One important practical point is that a small pack of aciclovir cream can be bought from a pharmacy without a prescription. However, the NHS advises checking with a pharmacist or doctor if you are pregnant. Being able to buy a medicine without a prescription does not mean that it is right for everyone.
A Cold Sore Is Not the Same Question as Genital Herpes
This distinction is important because cold sores and genital herpes can require different advice during pregnancy.
A Cold Sore on the Lip
A cold sore on the lip is not expected to infect a baby before birth because the virus does not cross the placenta in this situation. If you get recurrent cold sores while pregnant, the outbreak itself is generally not considered a risk to the pregnancy.
The main concern comes after birth. A newborn can catch herpes through close contact with someone who has an active cold sore, so extra care is important during the first few weeks after birth.
Recurrent Genital Herpes in Pregnancy
Genital herpes during pregnancy needs individual medical advice. The UK has joint guidance from the British Association for Sexual Health and HIV and the Royal College of Obstetricians and Gynaecologists covering herpes simplex infection during pregnancy and in newborns. The latest guidance was updated in 2024.
One of the key factors is whether the infection is a first episode or a recurrence, and when it happens during pregnancy.
A first episode of genital herpes later in pregnancy can carry a higher risk of transmission to the baby, particularly when it occurs close to delivery. In these situations, the maternity team may recommend specific treatment and a particular birth plan.
Recurrent genital herpes is generally managed differently, and the risk to the baby is much lower than with a first episode close to delivery.
If you think you have a first episode of genital herpes during pregnancy, speak to your maternity team or another appropriate healthcare professional rather than trying to manage it yourself.
Suppressive Treatment From 36 Weeks, and Its Limits
For people with known genital herpes, UK guidance includes considering daily suppressive antiviral treatment from around 36 weeks of pregnancy until birth. This can reduce lesions and viral shedding around the time of delivery and may reduce the chance of needing a caesarean birth because of herpes. The exact timing and treatment should be discussed with the maternity team.
It is important to understand the limits of this approach. Suppressive treatment has not been shown definitively to prevent every case of neonatal herpes. This is one reason why the maternity team will consider your individual history, symptoms and timing when planning your care.
There is also evidence that a first episode of genital herpes during pregnancy is associated with an increased risk of preterm labour. Some evidence suggests that antiviral treatment may reduce this risk. This is another reason why the risks of the infection and the potential benefits of treatment need to be considered together.
| Situation | What it generally means |
|---|---|
| Cold sore on the lip, any trimester | Not expected to pose a risk to the baby before birth; speak to a healthcare professional about suitable treatment |
| Known recurrent genital herpes | Generally lower risk to the baby; suppressive treatment from around 36 weeks may be considered |
| First episode of genital herpes, first or second trimester | Needs assessment and management by the maternity team; suppressive treatment may be considered later in pregnancy |
| First episode of genital herpes, third trimester | Needs prompt obstetric assessment because the risk to the baby can be higher, particularly close to delivery |
| Any suspected first episode in pregnancy | Speak to a healthcare professional so the infection can be assessed and managed appropriately |
Protecting a Newborn if You Get Cold Sores
Cold sores become particularly important after birth because a newborn can catch herpes through close contact with someone who has an active infection. Newborn babies are especially vulnerable to herpes infection during the first few weeks of life.
- Do not kiss a newborn while you have an active cold sore, and ask visitors to follow the same advice.
- Wash your hands thoroughly before handling the baby, especially after touching the cold sore or applying treatment.
- Avoid touching the cold sore and then touching the baby, a bottle, dummy or bedding.
- If you have a herpes lesion on your breast or nipple, speak to a healthcare professional about feeding from that side.
- Tell your midwife or health visitor if you develop a cold sore or another herpes outbreak during the early weeks after birth.
Seek urgent medical advice if a newborn becomes unusually sleepy or floppy, feeds poorly, has a temperature, becomes very irritable, or develops a rash or blisters. Neonatal herpes is uncommon but can be serious, so it is important to seek medical advice quickly if you are concerned.
Aciclovir and Breastfeeding
Aciclovir passes into breast milk in small amounts and is generally considered compatible with breastfeeding. NHS guidance says aciclovir cream or ointment can usually be used while breastfeeding, although the baby should not come into contact with areas where the medicine has been applied. If you are taking aciclovir tablets, tell your doctor that you are breastfeeding.
As with pregnancy, it is sensible to mention your medicine to your midwife, health visitor, pharmacist or doctor, particularly if your baby was premature or has health concerns.
Who Should Be Making This Decision
For anything related to herpes during pregnancy, your healthcare professionals are the right people to guide you. This may include your midwife, GP, pharmacist or obstetric team, depending on the situation.
They can consider how many weeks pregnant you are, your medical history, the type of infection you have and any other factors that may affect your care.
An online pharmacy assessment is not a replacement for maternity care. If you have a straightforward cold sore and want to ask about a topical product, a pharmacist can be a useful first point of contact. If you have genital symptoms, a suspected first episode, symptoms that are getting worse, or any concerns about your pregnancy, speak to your GP or maternity team.
Treatment suitability depends on your individual circumstances. Prescription medicines should only be used under the supervision of an appropriately qualified prescriber.
Things That Help That Are Not Medication
Whether or not you use an antiviral, a few simple measures can make a cold sore easier to manage.
- Keep the area clean and avoid picking or squeezing the sore. This can irritate the skin and may spread the virus to your fingers.
- Petroleum jelly or a plain barrier balm may help prevent the lip from cracking as the sore dries.
- A cool compress may help ease soreness.
- Drink enough fluids and choose softer foods if eating is uncomfortable. You may also find it helpful to avoid salty or acidic foods if they make the sore sting.
- If strong sunlight is one of your triggers, using a lip product with sun protection may help reduce exposure.
- Do not share towels, cups, cutlery or lip products while you have a cold sore.
Most cold sores heal within seven to ten days without treatment. If a cold sore has not healed after around nine days, keeps coming back, or is unusually severe, speak to a healthcare professional.
If Treatment Has Been Advised
If your midwife, GP or obstetric team has advised antiviral treatment and you want it reviewed and supplied online, you can complete an online cold sore assessment with a UK registered prescriber. Tell them that you are pregnant and how many weeks pregnant you are, as this may affect the treatment options available to you.
The prescriber will assess whether treatment is suitable for you and may advise you to speak with your maternity team if further assessment is needed.
