This sheet is about exposure to acyclovir or valacyclovir in pregnancy and while breastfeeding. This information is based on available published literature. It should not take the place of medical care and advice from your healthcare provider.

What is acyclovir/valacyclovir?

Acyclovir (Zovirax®) is an antiviral medication that has been used to treat symptoms from cold sores and genital herpes caused by the herpes virus. It is also prescribed to treat chickenpox and shingles. Acyclovir can help relieve the pain and help the healing of sores or blisters.

Valacyclovir (Valtrex®) is like acyclovir and has been used to treat the same types of infections. Valacyclovir changes to acyclovir once inside the body.

Acyclovir/valacyclovir have been prescribed during pregnancy when a person has a primary (first time) genital herpes infection. Primary infections can be life threatening or lead to complications in a pregnancy. A rare but serious infection called varicella pneumonia might also require treatment with these medications.

Sometimes when people find out they are pregnant, they think about changing how they take their medication, or stopping their medication altogether. However, it is important to talk with your healthcare providers before making any changes to how you take your medication. Your healthcare providers can talk with you about the benefits of treating your condition and the risks of untreated illness during pregnancy.

I take acyclovir/valacyclovir. Can it make it harder for me to get pregnant?

It is not known if acyclovir/valacyclovir can make it harder to get pregnant.

Does taking acyclovir/valacyclovir increase the chance of miscarriage?

Miscarriage is common and can occur in any pregnancy for many different reasons. Studies have not found an increased chance of miscarriage with acyclovir use.

Does taking acyclovir/valacyclovir increase the chance of birth defects?

Every pregnancy starts out with a 3-5% chance of having a birth defect. This is called the background risk. Use of acyclovir/valacyclovir is not expected to increase the chance of birth defects above the background risk. The manufacturer and the Centers for Disease Control and Prevention (CDC) looked at the effects of acyclovir on the developing fetus. No increase in birth defects was seen in over 500 births. Also, a separate study found no increase in birth defects in over 1,500 infants exposed to acyclovir and over 200 infants exposed to valacyclovir during the first trimester of pregnancy.

Does taking acyclovir/valacyclovir in pregnancy increase the chance of other pregnancy-related problems?

The use of acyclovir/valacyclovir later in pregnancy has not been well studied to see if it can increase the chance of other pregnancy-related problems, such as preterm delivery (birth before week 37) or low birth weight (weighing less than 5 pounds, 8 ounces [2500 grams] at birth). However, clinical experience has been reassuring. It is important to note that untreated infections could cause problems for the person who is pregnant, the pregnancy, and a newborn.

Does taking acyclovir/valacyclovir in pregnancy affect future behavior or learning for the child?

Studies have not been done to see if acyclovir/valacyclovir can cause behavior or learning issues for the child.

Breastfeeding while taking acyclovir/valacyclovir:

Acyclovir enters breast milk in small amounts. Acyclovir is commonly given to newborns and does not typically cause problems for babies. If you are applying acyclovir cream or ointment directly on your breast, clean the area well before nursing. Be sure to talk to your healthcare provider about all your breastfeeding questions.

If a male takes acyclovir/valacyclovir, could it affect fertility or increase the chance of birth defects?

Information on acyclovir/valacyclovir’s effect on male fertility (ability to get partner pregnant) is limited. One study in 20 males did not find lower sperm production when given high doses of acyclovir for six months. Studies have not been done to see if acyclovir/valacyclovir could increase the chance of birth defects above the background risk. In general, exposures that fathers or sperm donors have are unlikely to increase risks to a pregnancy. For more information, please see the MotherToBaby fact sheet Paternal Exposures at https://mothertobaby.org/fact-sheets/paternal-exposures-pregnancy/.

Selected References:

• Ahrens KA, et al. 2013. Antiherpetic Medication Use and the Risk of Gastroschisis: Findings From the National Birth Defects Prevention Study, 1997-2007. Paediatric and Perinatal Epidemiology. 27(4):340-345. [PMC free article: PMC3690801] [PubMed: 23772935]
• Braig S, et al. 2001. Acyclovir prophylaxis in late pregnancy prevents recurrent genital herpes and viral shedding. Eur J Obstet Gynecol Reprod Biol 96:55-58. [PubMed: 11311761]
• Centers for Disease Control. 1993. Pregnancy outcomes following systemic prenatal acyclovir exposure – June 1, 1984-June 30, 1993. MMWR. 42:806-809. [PubMed: 8413168]
• Douglas JM Jr, Davis LG, Remington ML. 1988. A double-blind, placebo-controlled trial of the effect of chronically administered oral acyclovir on sperm production in men with frequently recurrent genital herpes. J Infect Dis 157:588-93. [PubMed: 2830348]
• Drake AL, et al. 2012. Infant Safety During and After Maternal Valacyclovir Vherapy in Conjunction with Antiretroviral HIV-1 Prophylaxis in a Randomized Clinical Trial. PloS One. 7(4):e34635. [PMC free article: PMC3324503] [PubMed: 22509337]
• Drake AL, et al. 2012. Valacyclovir Suppressive Therapy Reduces Plasma and Breast Milk HIV-1 RNA Levels During Pregnancy and Postpartum: A Randomized Trial. The Journal of Infectious Diseases. 205(3):366-375. [PMC free article: PMC3256951] [PubMed: 22147786]
• Kimberlin DF, et al. 1998. Pharmacokinetics of oral valacyclovir and acyclovir in late pregnancy. Am J Obstet Gynecol. 179(4):846-851. [PubMed: 9790357]
• Pasternak B, Hviid A. 2010. Use of acyclovir, valacyclovir, and famciclovir in the first trimester of pregnancy and the risk of birth defects. JAMA 304(8):859-866. [PubMed: 20736469]
• Prober CG. 2001. Management of the neonate whose mother received suppressive acyclovir therapy during late pregnancy. Pediatr Infect Dis J 20:90-91. [PubMed: 11176581]
• Ratanajamit C, et al. 2003. Adverse pregnancy outcome in women exposed to acyclovir during pregnancy: a population-based observational study. Scand J Infect Dis 35:255-259. [PubMed: 12839155]
• Sheffield JS, et al. 2002. Acyclovir concentrations in human breast milk after valaciclovir administration. Am J Obstet Gynecol 186(1):100-102. [PubMed: 11810093]