Last update Dec. 20, 2022

Acyclovir; Aciclovir

Very Low Risk

Safe. Compatible. Minimal risk for breastfeeding and infant.

Antiviral very active against Herpes simplex and Herpes zoster-varicella. Intravenous, oral and topical (dermatological and ophthalmological) administration. Its use is authorized in newborns and infants.

Despite its higher concentration in breast milk than in plasma (Mactal 2001,Bork 2000, Lau 1987), it is excreted in breast milk in a clinically insignificant amount (Drake 2012, Bork 1995, Taddio 1994, Don 1991, Meyer 1988, Lau 1987), much lower than the dose used in newborns and infants (Sheffield 2002), and no problems have been observed in infants whose mothers took it. (Agarwal 2019, Taddio 1994)

Systemic absorption of topical cutaneous acyclovir is negligible. (Mactal 2001)

List of WHO essential medicines: compatible with breastfeeding (WHO / UNICEF 2002). American Academy of Pediatrics: medication usually compatible with breastfeeding. (AAP 2001)

See below the information of these related products:


We do not have alternatives for Acyclovir; Aciclovir since it is relatively safe.

Suggestions made at e-lactancia are done by APILAM team of health professionals, and are based on updated scientific publications. It is not intended to replace the relationship you have with your doctor but to compound it. The pharmaceutical industry contraindicates breastfeeding, mistakenly and without scientific reasons, in most of the drug data sheets.

Jose Maria Paricio, Founder & President of APILAM/e-Lactancia

Your contribution is essential for this service to continue to exist. We need the generosity of people like you who believe in the benefits of breastfeeding.

Thank you for helping to protect and promote breastfeeding.

José María Paricio, founder of e-lactancia.

Other names

Acyclovir; Aciclovir in other languages or writings:


Variable Value Unit
Oral Bioavail. 15 - 30 %
Molecular weight 225 daltons
Protein Binding 10 - 30 %
VD 0.8 l/Kg
pKa 7.99 -
Tmax 1.5 - 2 hours
2 - 3 hours
M/P ratio 1 - 3 -
Theoretical Dose 0.9 - 1.5 mg/Kg/d
Relative Dose 1 - 2.5 %
Ped.Relat.Dose 1.8 - 3.6 %


  1. OTIS. Acyclovir (Zovirax®) / Valacyclovir (Valtrex®). 2022 Apr. Mother To Baby | Fact Sheets [Internet]. Brentwood (TN): Organization of Teratology Information Specialists (OTIS); 1994-. Consulted on April 21, 2022 Abstract
  2. Agarwal R, Maharana PK, Titiyal JS, Sharma N. Bilateral herpes simplex keratitis: lactation a trigger for recurrence! BMJ Case Rep. 2019 Mar 9;12(3). pii: e223713. Abstract Full text (link to original source)
  3. Drake AL, Roxby AC, Kiarie J, Richardson BA, Wald A, John-Stewart G, Farquhar C. Infant safety during and after maternal valacyclovir therapy in conjunction with antiretroviral HIV-1 prophylaxis in a randomized clinical trial. PLoS One. 2012;7(4):e34635. Abstract Full text (link to original source) Full text (in our servers)
  4. Sheffield JS, Fish DN, Hollier LM, Cadematori S, Nobles BJ, Wendel GD Jr. Acyclovir concentrations in human breast milk after valaciclovir administration. Am J Obstet Gynecol. 2002 Abstract
  5. WHO / UNICEF. BREASTFEEDING AND MATERNAL MEDICATION Recommendations for Drugs in the Eleventh WHO Model List of Essential Drugs. Department of Child and Adolescent Health and Development (WHO/UNICEF) 2002 Abstract Full text (link to original source) Full text (in our servers)
  6. Mactal-Haaf C, Hoffman M, Kuchta A. Use of anti-infective agents during lactation, Part 3: Antivirals, antifungals, and urinary antiseptics. J Hum Lact. 2001 Abstract
  7. AAP - American Academy of Pediatrics Committee on Drugs. Transfer of drugs and other chemicals into human milk. Pediatrics. 2001 Sep;108(3):776-89. Abstract Full text (link to original source) Full text (in our servers)
  8. Bork K, Kaiser T, Benes P. Transfer of aciclovir from plasma to human breast milk. Arzneimittelforschung. 2000 Abstract
  9. 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 Abstract
  10. Taddio A, Klein J, Koren G. Acyclovir excretion in human breast milk. Ann Pharmacother. 1994 Abstract
  11. Meyer LJ, de Miranda P, Sheth N, Spruance S. Acyclovir in human breast milk. Am J Obstet Gynecol. 1988 Abstract
  12. Lau RJ, Emery MG, Galinsky RE. Unexpected accumulation of acyclovir in breast milk with estimation of infant exposure. Obstet Gynecol. 1987 Abstract

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