If you have searched for aciclovir tablets and breastfeeding, the short answer is that aciclovir does pass into breast milk, but only in very small amounts. Studies suggest that the amount is around 1% of a usual infant dose. Aciclovir is generally considered compatible with breastfeeding, and there is usually no need to pump and discard your milk or time feeds around your tablets.
If you have searched for the aciclovir pregnancy category, you may be looking for something that does not exist in the UK. This matters because the old category system can give you an outdated view of the evidence. If you are pregnant or breastfeeding and want to know whether aciclovir tablets are suitable for you, current evidence and UK guidance are more useful than an old category letter.
A category letter was designed to summarise the evidence available at a particular point in time. The letter associated with aciclovir was assigned decades ago in the United States, under a system that has since been replaced. Since then, more human evidence has become available. Looking at the current evidence gives a fuller picture than relying on the old letter alone.
This page looks at what human studies show about aciclovir in pregnancy, how tablet strengths and uses differ, whether monitoring is usually needed, when topical treatment may differ from oral treatment, how valaciclovir compares, what the evidence means across different stages of pregnancy, what breastfeeding means in practice, how treatment can help reduce the risk of herpes transmission to a newborn, and when to speak to a pharmacist, midwife, GP or prescriber.
Key Things to Know
- Lettered pregnancy categories were used in the United States, were retired in 2015, and have never been used in the UK
- Neither the NHS nor the MHRA uses these pregnancy categories. UK information is provided through sources such as the British National Formulary, UK Teratology Information Service and medicine product information
- Human safety data on aciclovir use in early pregnancy is fairly substantial
- 400mg is not the usual strength for oral cold sore treatment, although aciclovir is used at different strengths for other conditions
- Routine blood tests or kidney function tests are not usually required simply because someone is taking a standard course of aciclovir during pregnancy
Is There an Aciclovir Pregnancy Category?
Not in the UK, and the old US system is no longer used there either.
What the Letters Were
The lettered system, running from A through to X, was created by the United States Food and Drug Administration. It assigned medicines a letter intended to summarise what was known about their potential pregnancy risks. Some historical US sources described aciclovir (Zovirax) using the FDA pregnancy categories.
Aciclovir was historically classified as FDA Pregnancy Category B. This meant that animal studies had not shown harm to the developing baby, but adequate and well controlled studies in pregnant women were lacking. That reflected the evidence available at the time.
The system was retired under the Pregnancy and Lactation Labeling Rule in 2015 because a single letter could not adequately describe the available evidence. Medicine labels now use more detailed written information about pregnancy and breastfeeding instead.
The UK has never used these lettered categories. The NHS and MHRA do not issue them. UK information comes from sources such as the British National Formulary, the UK Teratology Information Service and the Summary of Product Characteristics.
Why the Label Is Not Just Foreign, It Is Out of Date
This is worth understanding because an old category letter does not change when new evidence becomes available.
The live version of this page states that aciclovir is Category B "whilst human studies are limited". That description reflected the evidence available when the category was assigned, but it does not reflect the larger amount of human evidence that has since become available.
For example, a Danish national cohort published in 2010 looked at more than 1,500 first trimester aciclovir exposures.
A category letter cannot update itself. It reflects the evidence available when it was assigned. For that reason, it is more useful to look at current human studies and UK guidance when considering aciclovir during pregnancy.
What the Evidence Actually Shows for Pregnant Women
One of the largest relevant studies was a Danish population based cohort covering 837,795 live births between 1996 and 2008. Researchers linked national prescription records with birth defect diagnoses.
| Group | Major birth defect rate | Adjusted comparison |
|---|---|---|
| Aciclovir in the first trimester (1,561 infants) | 32 infants, 2.0% | Prevalence odds ratio 0.82 (95% CI 0.57 to 1.17) |
| Any antiviral in the first trimester (1,804 pregnancies) | 40 infants, 2.2% | Prevalence odds ratio 0.89 (95% CI 0.65 to 1.22) |
| Valaciclovir in the first trimester (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 |
Registry data have also been generally reassuring, with no evidence of an increased risk of major birth defects following aciclovir exposure in pregnancy.
The rate following aciclovir exposure was not higher than the background rate in the study. No association was found across thirteen separate categories of birth defects, although the number of cases within individual groups was small.
The background figure is useful context. Around 2.4% of births in that cohort involved a major birth defect, regardless of whether aciclovir had been used. This helps put the 2.0% figure for aciclovir exposure into context rather than looking at the percentage on its own.
What UK Sources Say
The British National Formulary states that aciclovir is not known to be harmful in pregnancy. However, medicine information generally advises that treatment should be used when the potential benefits are considered to outweigh any potential risks.
Aciclovir does cross the placenta. This is one reason why evidence from human pregnancy studies is important when considering its use.
The Honest Limits
No observational study can rule out a small risk completely. Rare outcomes can also be difficult to study because there may not be enough cases to detect a small difference.
The available evidence on aciclovir is much larger than the evidence for valaciclovir. This is an important difference when considering uncommon outcomes, although the available data on both medicines have not shown an increased risk of major birth defects.
Aciclovir 400mg Specifically: What That Strength Is For
Since this page's title names the strength, it is worth clearing up a common misunderstanding.
400mg is not the usual strength for oral cold sore treatment. In UK practice, when oral aciclovir is prescribed for cold sores, 200mg is a commonly used strength.
Aciclovir tablets may also be prescribed for herpes simplex infections such as genital herpes. They can also be used for varicella zoster infections, including shingles and chickenpox, when clinically appropriate. The strength and treatment schedule can differ depending on the condition being treated.
This matters in pregnancy because someone looking for information about a cold sore on the lip may not be in the same situation as someone taking aciclovir for genital herpes or shingles.
If you have been prescribed 400mg during pregnancy, follow the instructions on your pharmacy label. If you are unsure why that strength has been prescribed, ask your prescriber or pharmacist rather than changing the strength yourself.
This page does not provide a dose or treatment schedule. The appropriate amount and duration depend on what the medicine is being used to treat and your individual circumstances.
Do You Need Monitoring While Taking It?
Routine monitoring is not usually needed simply because you are taking aciclovir during pregnancy.
A standard course of aciclovir does not normally mean that you need a scheduled blood test, kidney function test or additional pregnancy scan just because you are taking the medicine. Your usual antenatal care can continue as planned.
Kidney function is still relevant because aciclovir is cleared by the kidneys. If you already have kidney problems, your prescriber may need to take this into account when deciding whether aciclovir is suitable and what dose is appropriate.
Staying well hydrated is also sensible while taking aciclovir, unless you have been told to restrict your fluid intake for another medical reason. If you have a kidney condition, mention it to the healthcare professional prescribing your medicine.
Cream or Tablets in Pregnancy?
For a cold sore on the lip, topical treatment may be considered because aciclovir cream applied to the skin results in very little absorption into the bloodstream.
This does not mean that oral aciclovir is unsafe during pregnancy. The available human pregnancy data are generally reassuring. The choice between cream and tablets depends on the type and severity of the infection and your individual circumstances.
Oral treatment may be considered when an outbreak is severe, widespread or causing significant symptoms. Tablets may also be prescribed for certain infections such as genital herpes, shingles or chickenpox, depending on the clinical situation.
Treatment choices can also be different for a first episode of genital herpes during pregnancy or for people who have frequent outbreaks and may need preventive treatment.
If you are using aciclovir cream for a cold sore, follow the instructions for your specific product. Many products advise against applying it inside the nose or mouth or near the eyes.
Starting antiviral treatment early may be more helpful than waiting until a cold sore has fully developed. If you have been prescribed treatment, follow the instructions you have been given rather than changing how much or how often you use it yourself.
Valaciclovir in Pregnancy
Valaciclovir is converted into aciclovir in the body after it is absorbed, so the active medicine produced is aciclovir.
The main difference is the amount of pregnancy evidence available. In the cohort above, there were around 1,561 first trimester aciclovir exposures compared with around 229 valaciclovir exposures. Neither medicine was associated with an increased risk of major birth defects in that study.
Because there is more pregnancy data available for aciclovir, it may be preferred when either medicine would otherwise be suitable. Valaciclovir may still be prescribed in some situations. The choice should be made with your prescriber rather than by switching between the two medicines yourself.
What Changes Across the Trimesters
First trimester. This is the stage of pregnancy that often causes the most concern, and it was specifically examined in the large cohort study discussed above. The available evidence does not show an increased risk of major birth defects with aciclovir exposure, although no study can rule out every possible risk.
If treatment is clinically needed, pregnancy alone does not necessarily mean that aciclovir cannot be used. Your prescriber can weigh the potential benefits and risks for your situation.
Second trimester. The basic approach does not change dramatically. If you are having frequent outbreaks, it may be useful to discuss this with your maternity team so that you can agree on an appropriate treatment plan.
Third trimester. With genital herpes, the main concern around delivery is the infection rather than aciclovir itself. A genital herpes outbreak or viral shedding around the time of delivery can increase the risk of passing herpes to the baby during birth.
UK obstetric practice may therefore include antiviral treatment later in pregnancy for some pregnant women with genital herpes. Suppressive treatment may reduce recurrent lesions and viral shedding around delivery and may reduce the chance of an outbreak at the time of birth.
This is a decision for your maternity team. Do not start, stop or change antiviral treatment based only on information from a website.
Cold Sores and Genital Herpes Are Different Questions
This distinction is important because cold sores and genital herpes can involve different risks during pregnancy.
A cold sore on your lip is not a route of transmission to a baby in the womb. It does not usually change your delivery plan simply because you have a cold sore.
A cold sore can still spread herpes to other people, particularly through direct contact with the sore. Good hygiene and avoiding direct contact with a newborn are therefore important after birth.
Genital herpes is a different clinical situation. It is usually caused by herpes simplex virus and can require specific advice during pregnancy, particularly if it is a first episode acquired during pregnancy.
If you think you may have a first episode of genital herpes while pregnant, contact your maternity team promptly. They can assess your symptoms and advise you about treatment and delivery planning.
Aciclovir and Breastfeeding
The breastfeeding evidence is generally reassuring.
Aciclovir passes into breast milk in small amounts. Studies suggest that even at higher maternal doses, the amount reaching a breastfed baby is around 1% of a dose that would be given directly to an infant. Aciclovir is therefore generally considered a suitable treatment for herpes infections during breastfeeding.
There is usually no need to pump and discard your milk or time feeds around your doses. If you have concerns about taking aciclovir while breastfeeding, speak to your GP, health visitor, pharmacist or maternity team.
Valaciclovir is also generally considered compatible with breastfeeding, but your healthcare professional can advise whether it is appropriate for your particular situation.
If you are using topical aciclovir on a small area away from the breast, very little medicine is expected to enter the bloodstream. Keep the baby away from treated skin and application sites.
If you have a herpes blister on your breast or nipple, get medical advice. You may be advised not to breastfeed from the affected breast while the lesion is present. Feeding from the unaffected breast may still be possible, depending on the circumstances.
Protecting a Newborn
The main concern with a cold sore and a newborn is direct contact after birth, rather than transmission during pregnancy or through breast milk.
- Do not kiss a newborn's face while you have an active cold sore. Avoid kissing the baby until the sore has healed.
- Wash your hands thoroughly before handling the baby, especially after touching your face or applying cold sore treatment.
- Try not to touch the cold sore. If you do touch it, wash your hands before touching the baby.
- Cover the sore where practical, and do not share flannels, towels or muslins.
- Ask visitors with an active cold sore to avoid close contact with the newborn until the sore has healed.
- Seek urgent medical advice if a newborn becomes unusually sleepy or floppy, feeds poorly, develops a temperature or develops any blistering rash.
Having a cold sore does not mean you cannot care for your baby. It does mean taking extra care to reduce the chance of passing the virus through direct contact.
Who Should Make This Decision?
Healthcare professionals such as your midwife, GP, pharmacist, maternity team or obstetrician can consider your stage of pregnancy, medical history and any other medicines you are taking when advising you about treatment.
A pharmacist can also advise you about suitable over the counter treatments where appropriate.
An online pharmacy consultation is not a substitute for maternity care. If you are pregnant and unsure about treating a cold sore or another herpes infection, speak to a healthcare professional who can assess your individual situation.
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 or dose is right for you.
If you are pregnant, planning a pregnancy or breastfeeding, speak to your doctor, midwife, GP, obstetric team or pharmacist before starting, stopping or changing any medicine, including medicines bought without a prescription.
Always follow the instructions on your pharmacy label and in the patient information leaflet. Do not change the strength, dose or frequency of a prescribed medicine based only on information on this page.
Contact your maternity team promptly if you think you may have a first genital herpes episode during pregnancy, have an outbreak close to your due date, develop symptoms affecting the eye, have spreading blisters with eczema, or have any concern about a newborn baby.
