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Acyclovir (Zovirax®)

By: Dr. Nicholas Dodman

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Overview

  • Acyclovir [9-(2-hydroxyethoxymethyl)guanine], also known as ACV or acycloguanosine, is a selective antiviral agent indicated for the treatment of certain herpes virus infections.
  • Its inhibitory action against herpes viruses is highly selective due to its affinity for the enzyme thymidine kinase encoded by the virus. This viral enzyme converts acyclovir into its monophosphate, which is eventually converted into the triple phosphate. The triple phosphate stops replication of herpes viral DNA.
  • Peak plasma levels occur 2 hours after oral dosing.
  • Acyclovir is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.
  • Acyclovir is not approved for use in animals by the Food and Drug Administration (FDA) but may be legally prescribed by veterinarians as an extra-label drug.

    Brand Names and Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Zovirax® (GlaxoWellcome) and various generic preparations
  • Veterinary formulations: None

    Uses of Acyclovir

  • The use of Acyclovir includes the treatment of herpes virus infections, typically in avian species (treatment of Pacheco's disease), and in cats with corneal or conjunctival herpes infections.
  • Its use for treatment of other herpes virus infections of animals is not well documented

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, acyclovir can cause side effects in some animals.
  • Acyclovir should not be used in animals with known hypersensitivity or allergy to the drug.
  • Muscle necrosis can occur with repeated intramuscular injections. Therefore, it is often only used for a few days by intramuscular route.
  • Reversible leucopenia and mild non-regenerative anemia may occur in some cats. Therefore, it is important to monitor the complete blood count (CBC) during therapy.
  • Although acyclovir has low toxicity, renal dysfunction can occur and is not related to a rare sensitivity as acyclovir-treated dogs show subtle decrements in renal function. Acyclovir may also cause an obstructive nephropathy from accumulation of crystals in renal tissue. The dose of acyclovir should be reduced when renal function is compromised. Renal function should also be monitored because of potential nephrotoxicity.
  • Oral administration of acyclovir may cause gastrointestinal disturbances.
  • Acyclovir crosses the placenta and may be fetotoxic. For this reason, it should be used with caution or avoided in pregnancy.
  • Accidental overdose with acyclovir causes signs of vomiting, diarrhea, anorexia, and lethargy.

    Drug Interactions

  • Acyclovir may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with acyclovir.
  • Possible interactions may include:
  • Acyclovir bioavailability and terminal plasma half-life may be increased by probenecid while its renal clearance is decreased.
  • Plasma theophylline levels may be increased by concomitant treatment with acyclovir.
  • Plasma levels of hydantoins and valproic acid may be decreased by acyclovir.
  • Amphotericin B may potentiate the antiviral effects of acyclovir but increases chances for development of nephrotoxicity.

    How Acyclovir is Supplied

  • Acyclovir is available in 400 and 800 mg tablets and 200 mg capsules.
  • Acyclovir 300 mg per 5 ml suspension is available.
  • The injectable concentration is 50 mg/ml.
  • Powder for injection is available in 500 or or 1000 mg per vial concentrations.
  • Ointment 5% (50mg/gm) in polyethylene glycol base. 3 and 15gm tubes.
  • Cream - 5% acyclovir in an aqueous cream base. 2g tubes.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • Doses of acyclovir may vary widely depending on the reason for prescribing.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.
  • The usual dose in cats is one 200 mg capsule orally every 6 hours.



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