Acyclovir concentrations in human breast milk after valaciclovir administration

Am J Obstet Gynecol. 2002 Jan;186(1):100-2. doi: 10.1067/mob.2002.119183.


Objective: The purpose of this study was to determine the valaciclovir and acyclovir pharmacokinetic profiles in serum and breast milk after valaciclovir administration to women after delivery.

Study design: Valaciclovir (500 mg twice daily for 7 days) was given to 5 women after delivery who were breast-feeding healthy term infants. Matched serum and breast milk samples were obtained after the initial dose, on day 5 and 24 hours after the drug was discontinued. Infant urine was obtained on day 5.

Results: Valaciclovir was rapidly converted to acyclovir. The peak serum acyclovir concentration occurred 3 hours before the peak breast milk concentration (2.7 microg/mL at 1 hour vs 4.2 microg/mL at 4 hours). The serum acyclovir elimination half-life was 2.3 hours. The ratio of breast milk to serum acyclovir concentration was highest 4 hours after the initial dose at 3.4 and reached steady state ratio at 1.85. The median infant urine acyclovir concentration at steady state was 0.74 microg/mL.

Conclusion: Valaciclovir is rapidly converted to acyclovir and concentrates in breast milk. However, the amount of acyclovir in breast milk after valaciclovir administration is considerably less (2%) than that used in therapeutic dosing of neonates.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acyclovir / analogs & derivatives*
  • Acyclovir / analysis*
  • Acyclovir / blood
  • Acyclovir / metabolism
  • Acyclovir / pharmacology*
  • Acyclovir / urine
  • Female
  • Half-Life
  • Homeostasis
  • Humans
  • Infant, Newborn / urine
  • Milk, Human / chemistry*
  • Osmolar Concentration
  • Postpartum Period* / metabolism
  • Prodrugs / pharmacology*
  • Time Factors
  • Valacyclovir
  • Valine / analogs & derivatives*
  • Valine / metabolism
  • Valine / pharmacology*


  • Prodrugs
  • Valine
  • Valacyclovir
  • Acyclovir