Acyclovir
Acycloguanosine
Also known as: Zovirax · Aciclobene · Aciclotyrol · Acivir · Acyrax · Cicloviral · Geavir · Herpotern · Isavir · Nycovir · Supraviran · Viclovir · Virherpes · Viroxy · Xorox · aciclovirum · acycloguanosine · ACV · BW248U
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Herpesvirus-1 infections | 10-25 mg/kg PO twice daily | PO | q12h | — | 🌎 NA |
- Herpesvirus-1 infections: Never begin therapy until diagnostic evaluation is completed. May be toxic in cats; monitor CBC every 2-3 weeks.
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Treatment of Pacheco's Disease | 80 mg/kg PO q8h or 40 mg/kg q8h IM | PO/IM | q8h | Maximum 72 hours for IM | 🌎 NA |
| Treatment of Pacheco's Disease | 80 mg/kg in oral suspension once daily PO; mix suspension with peanut butter or add to drinking water 50 mg in 4 oz of water | PO | q24h | 7-14 days | 🌎 NA |
| Individual treatment | 80 mg/kg PO or IM twice daily | PO/IM | q12h | — | 🌎 NA |
| Prophylaxis (Exposed birds) | 25 mg/kg IM once, then acyclovir is added to drinking water at 1 mg/mL and to the food at 400 mg/quart of seed | IM, then PO | Once IM, then continuous PO | Minimum of 7 days | 🌎 NA |
| Prophylaxis (Quaker parrots) | 80 mg/kg q8h | PO (gavage) | q8h | 7 days | 🌎 NA |
- Treatment of Pacheco's Disease: Do not use parenterally for more than 72 hours as it can cause tissue necrosis at site of injection.
- Prophylaxis (Exposed birds): Give IM with caution as it is very irritating.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| EHV-1 outbreaks (prevention or treatment) | 10 mg/kg PO 5 times daily or 20 mg/kg PO q8h | PO | 5 times daily or q8h | — | 🌎 NA |
- EHV-1 outbreaks (prevention or treatment): Although efficacy is undetermined, anecdotal use may have had some efficacy. However, the drug's very low oral bioavailability suggests that oral dosing is unlikely to be of much benefit.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Acyclovir is a synthetic purine nucleoside analogue primarily utilized in veterinary medicine for its antiviral properties against specific herpesviruses.
Key clinical applications include:
- Avian Medicine: Treatment and prophylaxis of Pacheco's disease (a highly fatal herpesvirus infection in birds).
- Feline Medicine: Management of Feline Herpesvirus-1 (FHV-1) infections, particularly corneal or conjunctival presentations. Clinical Pearl: It has relatively mild activity against FHV-1 compared to newer antivirals like famciclovir, ganciclovir, or cidofovir, and carries a higher risk of systemic toxicity in cats.
- Equine Medicine: Investigational use for Equine Herpesvirus type-1 (EHV-1) myeloencephalopathy, though its extremely poor oral bioavailability (<4%) in horses makes therapeutic efficacy challenging.
While highly effective in humans, its use in veterinary species is often limited by variable pharmacokinetics, poor oral absorption in certain species (like horses), and potential toxicities (such as nephrotoxicity and bone marrow suppression in cats).
Mechanism of action
Acyclovir is a prodrug that requires activation by viral enzymes, making it highly specific to infected cells.
- Viral Entry: Acyclovir enters the virus-infected cell.
- Initial Phosphorylation: It is preferentially phosphorylated by viral thymidine kinase into acyclovir monophosphate.
- Subsequent Phosphorylation: Host cell kinases further convert it into the active form, acyclovir triphosphate.
- DNA Chain Termination: Acyclovir triphosphate acts as a competitive inhibitor of viral DNA polymerase. It incorporates into the growing viral DNA chain → lacks the 3'-hydroxyl group needed for further elongation → causes premature DNA chain termination and halts viral replication.
Safety & warnings
Contraindications
- Dehydrated states
- Pre-existing renal function impairment
- Hypersensitivity to acyclovir or related antivirals
- Pre-existing neurologic deficits
- Previous neurologic reactions to other cytotoxic drugs
Adverse effects
- Thrombophlebitis (with parenteral use)
- Acute renal failure
- Encephalopathologic changes (rare)
- Gastrointestinal disturbances (vomiting, diarrhea)
- Leukopenia (cats)
- Anemia (cats)
- Tissue necrosis (if given IM for >72 hours in birds)
Precautions
Renal Warning: Acyclovir is primarily excreted by the kidneys and can precipitate in renal tubules, especially if given rapidly IV or in dehydrated patients. Ensure adequate hydration during therapy.
- Feline Toxicity: Cats are particularly sensitive to bone marrow suppression (leukopenia, anemia) from acyclovir. Use with extreme caution and monitor CBCs closely.
- Tissue Irritation: IM injections can be highly irritating and may cause tissue necrosis if used for more than 72 hours (noted in avian species).
- Pregnancy/Nursing: May be fetotoxic at high dosages (FDA Category C). It crosses the placenta and is excreted in milk at concentrations higher than maternal plasma. Use with caution in nursing patients.
- Renal Impairment: Dosage reduction is required in patients with renal insufficiency.
Drug interactions
Concomitant administration with IV acyclovir may increase the potential for nephrotoxicity. Amphotericin B may potentiate antiviral effects but significantly increases nephrotoxic risk.
Concomitant use may cause additional CNS depression.
Monitoring
- Renal function tests (BUN, Serum Creatinine) with prolonged or IV therapy
- Complete Blood Count (CBC) every 2-3 weeks in cats to monitor for leukopenia and anemia
- Hydration status
Pharmacokinetics
Half-life
Absorption
In dogs, bioavailability is dose-dependent (~80% at ≤20 mg/kg, declining to ~50% at 50 mg/kg). In horses, oral bioavailability is very low (<4%). In humans, oral absorption is poor (~20%) and not significantly affected by food.
Distribution
Widely distributed throughout body tissues and fluids including the brain, semen, and CSF. It has low protein binding and crosses the placenta.
Metabolism
Primarily hepatically metabolized (in humans).
Elimination
Primarily eliminated via the kidneys. Renal disease does not significantly alter half-life unless anuria is present.
Overdose
Acute oral overdosage is unlikely to cause significant toxicity.
- Clinical Signs: Gastrointestinal signs (vomiting, diarrhea, lethargy) predominate. Renal failure is possible with higher doses due to drug crystallization in the renal tubules. Crystalluria and elevated renal values occurred at 188.7 mg/kg in a dog.
- Treatment: Consider gastrointestinal decontamination at doses of 150 mg/kg or higher. Below 150 mg/kg, manage symptomatically as GI signs will likely predominate. Ensure adequate hydration to maintain urine flow and prevent crystalluria.
Available products
Formulations
- Oral Tablets
- Oral Capsules
- Oral Suspension
- Powder for Injection
- Sodium Injection (IV infusion)
- Topical Ointment
- Topical Cream
Veterinary
- None
Human-labeled
- Acyclovir Oral Tablets: 400 mg & 800 mg (Zovirax®, generic)
- Acyclovir Oral Capsules: 200 mg (Zovirax®, generic)
- Acyclovir Oral Suspension: 200 mg/5 mL (Zovirax®, generic)
- Acyclovir Sodium Injection: 50 mg/mL (generic)
- Acyclovir Powder for Injection: 500 mg/vial, 1000 mg/vial (generic)
- Acyclovir Ointment: 5% (Zovirax®)
- Acyclovir Cream: 5% (Zovirax®)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Capsules and tablets should be stored in tight, light-resistant containers at room temperature. Suspension and sodium sterile powder should be stored at room temperature. Reconstituted solutions (with standard electrolyte/dextrose) are stable at 25°C for 24 hours. Do not use bacteriostatic water with parabens or benzyl alcohol for reconstitution.
Client information
Acyclovir is an antiviral medication used to treat specific viral infections, such as herpesvirus.
- Administration: Give exactly as directed by your veterinarian. Do not skip doses, as maintaining consistent drug levels is crucial for fighting the virus.
- Hydration is Key: Ensure your pet always has access to plenty of fresh drinking water. This drug is processed by the kidneys, and good hydration helps prevent kidney damage.
- Side Effects: The most common side effects are stomach upset (vomiting, diarrhea) and lethargy.
- Special Note for Cats: Cats are more sensitive to this medication. It can temporarily lower their white and red blood cell counts. Watch closely for signs of extreme weakness, pale gums, or fever, and contact your vet immediately if these occur. Your vet will likely require regular blood tests to monitor your cat safely.
- Storage: Store at room temperature away from light and moisture.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.