Valtrex (Valacyclovir)

  • Generic name: Valacyclovir hydrochloride
  • Brand name: Valtrex
  • Drug class: Antiviral, nucleoside analogue

What is Valtrex used for?

Does Valtrex help with canker sores? Valacyclovir has not been studied in the treatment of canker sores. Despite many studies trying to identify a causal microorganism, canker sores (aphthous stomatitis) does not appear to be infectious or contagious.


The only contraindication is a hypersensitivity to valacyclovir or any component in the formulation.

Drug interactions

Valacyclovir is not metabolized by CYP isoenzymes. No clinically significant drug interactions are known.

Pregnancy category

Pregnancy Category B. It is generally safe to take valacyclovir during pregnancy.

Valacyclovir is not thought to increase the risk for birth defects. The recent study found no increase in birth defects in over 200 infants exposed to valacyclovir during the first trimester of pregnancy3.

Valtrex during breastfeeding

Valacyclovir is excreted in breast milk. However, the amount of acyclovir (the active metabolite) in breast milk after taking valacyclovir is very low1. And the dose ingested by the child is less than 2% of therapeutic dosing for neonates. Also, the American Academy of Pediatrics considers acyclovir to be safe during breastfeeding2.


  • 1. 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 Jan;186(1):100-2.
  • 2. American Academy of Pediatrics. The transfer of drugs and other chemicals into human milk. Pediatrics 1994;93:13750.
  • 3. Pasternak B, Hviid A. Use of antiviral drugs in the first trimester of pregnancy and the risk of birth defects. JAMA. 2010 Aug 25;304(8):859-66. PubMed
  • 4. Pasternak B, Hviid A. Use of acyclovir, valacyclovir, and famciclovir in the first trimester of pregnancy and the risk of birth defects. JAMA. 2010 Aug 25;304(8):859-66 PubMed
  • 5. Gerald G. Briggs, Roger K. Freeman, Sumner J. Yaffe. Drugs in Pregnacy and Lactation, 6th Ed. LWW.

Last updated: February, 2015

Advanced Consumer Info

  • Pregnancy Category: B. Exposure to valacyclovir during the first trimester of pregnancy is not associated with an increased risk for birth defects4..
  • Breastfeeding: Valacyclovir is considered compatible with breast-feeding by the American Academy of Pediatrics5
  • Half-life:
    2.5 to 3.3 hours.
  • Mechanism of action: Valacyclovir disrupts the process by which the herpes virus reproduces itself. Valacyclovir is converted to acyclovir, which is converted to acyclovir triphosphate. Acyclovir triphosphate competitively inhibits viral DNA polymerase, incorporates into and terminates the growing viral DNA chain, and inactivates of the viral DNA polymerase.