Acyclovir (Zovirax®)
Information last updated February 2025
Most pregnancies result in healthy babies, but there are chances of complications and unexpected outcomes. These chances are called baseline risks. In Canada, the baseline risk of major birth defects is 3-5%. This means that 3-5 out of 100 babies born in the general population will be born with a major birth defect. There are also baseline risks for miscarriages (15-25 out of 100 pregnancies), premature birth and other outcomes. The information provided will summarize if taking this drug is likely to change these risks.
Summary:
Studies looked at the rates of birth defects in over 6,000 pregnancies where acyclovir was used intravenously (IV), as a tablet, cream or ointment, in the first trimester. Most studies found that taking acyclovir during pregnancy does not increase the risk of major birth defects above the baseline risk. The limited available data on other pregnancy outcomes are reassuring. To see more details, please click on the tabs below.
It is important to treat infections occurring during pregnancy. If infections are not treated, they may lead to complications for the pregnant individual and the developing baby. Health Canada recommends acyclovir for treatment of a first (primary) and of recurrent genital herpes infection in pregnancy. The Society of Obstetricians and Gynaecologists of Canada and Health Canada recommend that those with a history of genital herpes should be offered acyclovir or valacyclovir starting at 36 weeks of pregnancy until delivery to lower the risk of active lesions and shedding of the virus, when giving birth.
Please consult with your health care provider if you are considering stopping or making any changes to your regular medications.
This information about acyclovir is of a general nature and about medical use and does not replace the medical care and advice of your healthcare provider. For questions on dose, timing, side effects, interactions, etc. please consult your healthcare provider. Additionally, please read the patient insert provided with your medication.
Although participants in the studies referenced below may have used acyclovir in various combinations, the studies usually do not provide detailed information on drug combinations. This makes it challenging to comment on the safety of using this medication in combination with others during pregnancy or lactation
Acyclovir is an antiviral medication. It is used to treat herpes simplex virus, herpes zoster (shingles) and varicella (chickenpox). It can be given intravenously (IV), as a tablet to be taken by mouth (orally), and cream or ointment to be used topically. When it is used topically, only a minimal amount is absorbed into the skin and reaches the blood stream.
If the product you are using contains other active ingredients, please check our Exposures A to Z for available information on the ingredient(s).
Pronunciation
Please check back. We are in the process of reviewing if there is available information on the pre-pregnancy effects of acyclovir.
Studies looking at the rates of birth defects included over 3,000 pregnancies where acyclovir was used as a tablet or injection and over 3000 pregnancies where it was used as a cream or ointment, in the first trimester. Most studies found that taking acyclovir during pregnancy does not increase the risk of major birth defects above the baseline risk.
A study that looked at approximately 900 pregnancies with exposure to topical acyclovir reported no increased risks of miscarriages, prematurity (birth before 37 weeks of pregnancy) or low birth weight.
No increased risk of miscarriages or low birth weight were reported in studies that looked at a few hundred pregnancies with exposure to systemic (oral and IV injection) acyclovir. More data are needed to confirm that there are no increased risks of these outcomes.
Studies that looked at nearly 2000 pregnancies with exposure to oral acyclovir reported no increased risk of prematurity. One study found a lower rate of preterm birth in pregnancies where herpes infection was treated with acyclovir compared to leaving the infection untreated.
Health Canada recommends acyclovir for treatment of first (primary) and recurrent genital herpes infection in pregnancy.
To prevent a herpes simplex infection in the newborn, the Society of Obstetricians and Gynaecologists of Canada (SOGC) and Health Canada recommend offering a Caesarean section to those with active lesions or early signs or symptoms of genital herpes. To reduce the chance of needing a Caesarean section, SOGC and Health Canada recommend that those with a history of genital herpes should be offered acyclovir or valacyclovir (another antiviral) starting at 36 weeks to lower the risk of active lesions and shedding of the virus, when giving birth.
Please check back. We are in the process of reviewing if there is available information on the effects of paternal exposure to acyclovir.
We did not find published studies on the effects of acyclovir use in pregnancy on the newborn. We will update this section if studies become available.
If you are taking medications and you notice any new health concerns or symptoms in your nursing infant, please contact their health care provider. In case of emergency, please go to the emergency room or call 911.
For those with active herpes lesion(s) please see Herpes Simplex Virus and Breastfeeding. In case of active lesion(s) on the breast(s) nursing from the affected breast(s) should be stopped. See the above link for additional details.
If you are taking a medication or substance while providing your breastmilk to an infant you need to know how much of the medication or substance is passing into your milk. One of the commonly used measurements to estimate this is the Relative Infant Dose (RID). The RID is estimated by comparing the dose of drug taken in by the infant through breastmilk to the dose that the nursing parent takes. Most medications with an RID of less than 10% are usually compatible with nursing a healthy infant. The RID does not need to be calculated for each person because most of the time it is expected to be similar to what has been found in research studies. We will provide the RID in the information below, when available.
Based on acyclovir breastmilk concentrations of three women receiving 200-800 mg oral acyclovir five times a day the RID is estimated to be 1-2%. In one case of a woman receiving 300 mg acyclovir IV three times a day the RID was estimated to be 7.3%. Acyclovir is also used as antiviral treatment for infants. The amount of acyclovir that passes into breastmilk is less than 1% of what is given to an infant to treat an infection. Acyclovir exposure through breastmilk is not expected to have effects on the nursing infant. Topical acyclovir is not expected to be a concern for the nursing infant.
Please reach out to a healthcare provider if you notice any changes in your baby.
Costs of some medications are covered for eligible people under provincial or national Indigenous drug benefit plans. Please visit the Ontario Drug Benefit (ODB) program Check medication coverage or the Non-Insured Health Benefits (NIHB) program Drug Benefit List to check if nitrofurantoin is covered for you.
Medications, if not taken as prescribed, if taken beyond the prescribed amount, or if taken in combination with certain other drugs, may cause harm to you and/or your pregnancy or your nursing child.
If you are using drugs or medications for non-medical reasons or beyond what was recommended by a healthcare practitioner and you are pregnant, providing your breastmilk to an infant, or parenting please click Harm Reduction for additional information. In case of emergency, please go to the emergency room or call 911.
Pregnancy:
Research examining the association between first trimester use of acyclovir and birth defects included over 3,000 pregnancies with systemic and over 3,000 with topical exposures. The majority of these studies found no increased risk of major birth defects.
A study that evaluated approximately 900 pregnancies with exposure to topical acyclovir reported no increased risks of miscarriages, prematurity or low birth weight.
Similarly, no increased risks of miscarriages or low birth weight were reported in studies that examined a few hundred pregnancies with systemic exposure to acyclovir. More research is needed to confirm these findings.
No increased risk of prematurity was reported in studies that included nearly 2,000 pregnancies exposed to systemic acyclovir. One study reported that treating a herpes infection during pregnancy with acyclovir was associated with lower rates of preterm birth, compared to cases where the infection was left untreated.
Health Canada recommends acyclovir for treatment of both newly acquired and recurrent genital herpes infection in pregnancy.
To prevent neonatal herpes simplex, the Society of Obstetricians and Gynaecologists of Canada (SOGC) and Health Canada recommend offering a Caesarean section to those with active lesions or early signs or symptoms of genital herpes. To reduce the chance of needing a Caesarean section, SOGC and Health Canada recommend that those with a history of genital herpes should be offered acyclovir or valacyclovir (another antiviral) starting at 36 weeks until delivery to lower the risk of active lesions and shedding of the virus, when giving birth.
Lactation:
For those with active herpes lesion(s) please see Herpes Simplex Virus and Breastfeeding. In case of active lesion(s) on the breast(s) nursing from the affected breast(s) should be stopped. See the above link for additional details.
One of the factors that helps to determine if a medication is compatible with nursing is the Relative Infant Dose (RID). The RID provides an estimate of infant’s exposure to a medication through breastmilk. It is the ratio between the infant’s and the nursing individual’s weight-adjusted doses. The infant weight adjusted dose is estimated based on the concentration of medication in breastmilk, and an assumption of infant daily milk consumption of 150 ml/kg/day. In general, for infants with normal growth and development, most medications with an RID of less than 10% are considered compatible with nursing. The RID does not account for infant’s drug metabolism, clearance, or infant blood levels. Although some variability may exist in the RID, in most cases the estimated RID is adequate for clinical purposes and does not need to be calculated for each individual. We will provide the RID in the information below, when available.
Based on acyclovir breastmilk concentrations of three women receiving 200-800 mg oral acyclovir five times a day, the RID is estimated to be 1-2%. In one case, in a woman receiving 300 mg of acyclovir IV three times a day the RID was estimated to be 7.3%. Acyclovir is also used for the treatment of viral infections in infants. The amount of acyclovir that passes into breastmilk is less than 1% of the recommended treatment dose for infants. Based on that, acyclovir exposure through breastmilk is not expected to have adverse effects on the nursing infant. Topical acyclovir is not expected to be a concern for the nursing infant. See above for information on active lesion(s) on the breast(s).
Harm Reduction:
If your patient may be using drugs or medications not as indicated during pregnancy, while providing breastmilk to an infant, or parenting please click Harm Reduction for additional information. In case of emergency, please advise them to go to the emergency room or call 911.
For additional resources see Health Canada Drug and Natural Health Product Monographs, Making Sense of Risk and Statistics.
Ahrens KA, Anderka MT, Feldkamp ML, Canfield MA, Mitchell AA, Werler MM. Antiherpetic medication use and the risk of gastroschisis: findings from the National Birth Defects Prevention Study, 1997-2007. Paediatric & Perinatal Epidemiology. 2013;27(4):340-5. [PMID: 104078003.}
Anonymous. Management of Genital Herpes in Pregnancy: ACOG Practice Bulletinacog Practice Bulletin, Number 220. Obstet Gynecol. 2020;135(5):e193-e202. [PMID: 32332414].
Bork K, Benes P. Concentration and kinetic studies of intravenous acyclovir in serum and breast milk of a patient with eczema herpeticum. J Am Acad Dermatol. 1995;32(6):1053-5. [PMID: 7751454].
Briggs GS, et al. Acyclovir in Drugs in pregnancy and lactation : a reference guide to fetal and neonatal risk 11th ed. ed. Philadelphia (PA): Wolters Kluwer; 2017. p. 21-2.
Drugs and Lactation Database (LactMed®) [Internet]. Acyclovir. Bethesda (MD): National Institute of Child Health and Human Development; 2006 [updated May 15, 2025 cited 2025 June 19]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK501195/.
Frenkel LM, Brown ZA, Bryson YJ, Corey L, Unadkat JD, Hensleigh PA, et al. Pharmacokinetics of acyclovir in the term human pregnancy and neonate. Am J Obstet Gynecol. 1991;164(2):569-76. [PMID: 1847004].
Greffe BS, Dooley SL, Deddish RB, Krasny HC. Transplacental passage of acyclovir. J Pediatr. 1986;108(6):1020-1. [PMID: 3012053].
Haddad J, Simeoni U, Messer J, Willard D. Transplacental passage of acyclovir. J Pediatr. 1987;110(1):164-5. [PMID: 3794883].
Health Canada. Genital herpes guide: Treatment and follow-up Ottawa (ON): Public Health Agency of Canada; 2022 [updated June 21, 2022; cited 2025 July 30]. Available from: https://www.canada.ca/en/public-health/services/infectious-diseases/sexual-health-sexually-transmitted-infections/canadian-guidelines/herpes-simplex-virus/treatment-follow-up.html.
Lau RJ, Emery MG, Galinsky RE. Unexpected accumulation of acyclovir in breast milk with estimation of infant exposure. Obstet Gynecol. 1987;69(3 Pt 2):468-71. [PMID: 3808527].
Li DK, Raebel MA, Cheetham TC, Hansen C, Avalos L, Chen H, et al. Genital herpes and its treatment in relation to preterm delivery. Am J Epidemiol. 2014;180(11):1109-17. [PMID: 25392064].
Li D-K, Ferber JR, Odouli R, Flanagan T, Avalos L, Garzaro P, et al. Does Treating Genital Herpes during Pregnancy Improve Birth Outcomes? Patient-Centered Outcomes Research Institute (PCORI) [NBK 599334] 2021 [updated April, 2021; cited 2025 July 31, 2025]. Available from: https://www.pcori.org/sites/default/files/Li347-Final-Research-Report.pdf. [PMID: 38232216].
Meyer LJ, de Miranda P, Sheth N, Spruance S. Acyclovir in human breast milk. Am J Obstet Gynecol. 1988;158(3 Pt 1):586-8. [PMID: 3348321].
Mint Pharmaceuticals Inc. PrMINT-ACYCLOVIR Acyclovir Tablets USP 200 mg, 400 mg and 800 mg Mississauga (ON): Mint Pharmaceuticals Inc.; 2022 [updated January 21, 2022; cited 2025 June 19]. Available from: https://pdf.hres.ca/dpd_pm/00064477.PDF.
Money DM, Steben M. No. 208-Guidelines for the Management of Herpes Simplex Virus in Pregnancy. J Obstet Gynaecol Can. 2017;39(8):e199-e205. [PMID: 28729112].
Nakubulwa S, Kaye DK, Bwanga F, Tumwesigye NM, Nakku-Joloba E, Mirembe F. Effect of suppressive acyclovir administered to HSV-2 positive mothers from week 28 to 36 weeks of pregnancy on adverse obstetric outcomes: a double-blind randomised placebo-controlled trial. Reprod Health. 2017;14(1):31. [PMID: 28253893].
Pasternak B, Hviid A. Use of acyclovir, valacyclovir, and famciclovir in the first trimester of pregnancy and the risk of birth defects. JAMA. 2010;304(8):859-66. [PMID: 20736469].
Public Health Agency of Canada. Congenital Anomalies in Canada 2013: A Perinatal Health Surveillance Report. [Internet]. Ottawa: Public Health Agency of Canada; 2013 [updated 2013 September; cited 2023 September 8]. Available from: https://publications.gc.ca/collections/collection_2014/aspc-phac/HP35-40-2013-eng.pdf
Public Health Agency of Canada. Family-centred maternity and newborn care: National guidelines Chapter 7: Loss and grief. [Internet]. Ottawa: Public Health Agency of Canada; 2022 [updated 2022 Aug 10; cited 2023 September 9]. Available from: https://www.canada.ca/en/public-health/services/publications/healthy-living/maternity-newborn-care-guidelines-chapter-7.html
Ratanajamit C, Vinther Skriver M, Jepsen P, Chongsuvivatwong V, Olsen J, Sorensen HT. Adverse pregnancy outcome in women exposed to acyclovir during pregnancy: a population-based observational study. Scand J Infect Dis. 2003;35(4):255-9. [PMID: 12839155].
Sheffield JS, Hollier LM, Hill JB, Stuart GS, Wendel GD. Acyclovir prophylaxis to prevent herpes simplex virus recurrence at delivery: a systematic review. Obstet Gynecol. 2003;102(6):1396-403. [PMID: 14662233].
Stone KM, Reiff-Eldridge R, White AD, Cordero JF, Brown Z, Alexander ER, et al. Pregnancy outcomes following systemic prenatal acyclovir exposure: Conclusions from the international acyclovir pregnancy registry, 1984-1999. Birth Defects Res Part A Clin Mol Teratol. 2004;70(4):201-7. [PMID: 15108247].
Taddio A, Klein J, Koren G. Acyclovir excretion in human breast milk. Ann Pharmacother. 1994;28(5):585-7. [PMID: 8068994].
U.S. Centers for Disease Control. Herpes Simplex Virus and Breastfeeding – Breastfeeding special circumstances Atlanta, (GA): U.S. Centers for Disease Control; 2024 [updated February 6, 2024. Available from: https://www.cdc.gov/breastfeeding-special-circumstances/hcp/illnesses-conditions/herpes.html#print
UpToDate Lexi Drugs. Acyclovir – Topical [Internet] Waltham (MA): UpToDate; 2010 [updated July 15, 2025. Available from: http://online.lexi.com.
Verstegen RHJ, Anderson PO, Ito S. Infant drug exposure via breast milk. Br J Clin Pharmacol. 2022 Oct;88(10):4311-4327. Epub 2020 Sep 13. [PMID: 32860456].
Disclaimer
First Exposure does not offer health care treatment. If you have an urgent question about your pregnancy or your baby’s health, you should contact your health care provider directly. If you don’t have a health care provider and you live in Ontario, you have a variety of health care options. In the case of an emergency, visit a hospital emergency room or call 911.