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Pharmacology · Anti-herpesvirus agents

Valaciclovir !

An oral prodrug that the body turns into aciclovir. It is much better absorbed, so it is taken less often for herpes and shingles.

Updated 2026-11-01

  • Class: Anti-herpesvirus agents
  • Enzyme: Viral DNA polymerase (through its active form, aciclovir)
  • ATC code: J05AB11
  • WHO Model List of Essential Medicines

Also called: valacyclovir

The figure shows aciclovir triphosphate, the active form made from valaciclovir, in the herpes simplex virus 1 DNA polymerase bound to DNA (cryo-EM).

Target in 3D: Viral DNA polymerase (through its active form, aciclovir). PDB structure 8V1T (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Valaciclovir is the L-valyl ester of aciclovir. After absorption it is converted rapidly and almost completely into aciclovir and valine. From then on it acts like aciclovir: the viral thymidine kinase starts its activation, and aciclovir triphosphate inhibits the viral DNA polymerase and ends the DNA chain.

  1. Absorbed in the gut and converted to aciclovir
  2. Viral thymidine kinase phosphorylates aciclovir in infected cells
  3. Aciclovir triphosphate inhibits the viral DNA polymerase
  4. Viral DNA synthesis stops

Pharmacokinetics

Absorption
Well absorbed by mouth; aciclovir bioavailability about 55%, several times that of oral aciclovir
Metabolism
Converted to aciclovir and valine in the gut wall and liver
Elimination
As aciclovir, mainly in the urine
Half-life
Aciclovir about 2.5 to 3.3 hours with normal kidney function

Indications

Adverse effects

Common

  • Headache
  • Nausea
  • Abdominal pain

Contraindications

  • Hypersensitivity to valaciclovir, aciclovir or any component, including severe skin reactions

Key interactions

Other drugs
The label lists no clinically significant drug or food interactions; kidney function and hydration matter more

What is monitored

  • Kidney function in older people and in kidney disease
  • Mental status in older people

Administration and safety

  • Oral
  • Started as early as possible in an outbreak (ideally at the first symptoms)
  • Look-alike/sound-alike risk with valganciclovir, a different drug with serious toxicity

Nursing care

  • Ask about kidney disease and age before starting
  • Encourage fluids
  • Teach to report confusion, hallucinations or reduced urine
  • Explain that suppressive therapy lowers but does not remove the risk of passing on genital herpes; condoms and avoiding contact during outbreaks still matter

Pregnancy and breastfeeding

Decades of data have not shown a drug-related increase in major birth defects; untreated herpes in pregnancy carries risks to the baby.

Test yourself

Think of the answer, then tap the question to see it.

What is valaciclovir converted into after absorption?

Aciclovir (and the amino acid valine).

What is the advantage of valaciclovir over oral aciclovir?

Much better absorption, so higher aciclovir levels with fewer doses a day.

Which patients are at higher risk of confusion or hallucinations with valaciclovir?

Older people and people with kidney disease.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - VALACYCLOVIR HYDROCHLORIDE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026); Herpes simplex virus (World Health Organization, 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

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Representative model, for educational purposes only.