Acyclovir for animals

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Functional classification: Antiviral

Acyclovir is a synthetic purine analogue (acyclic nucleoside analogue). It has antiviral activity against herpes virus. The action is related to the affinity for  the enzyme thymidine kinase (TK). However, resistance among some virus forms is possible because of changes in TK or in  the DNA polymerase. It is used for treatment of various forms of herpes virus infection in humans and also has been used for treatment of viral     infections in animals. However, feline herpes virus 1 (FHV1) is resistant to acyclovir and valciclovir, and studies are lacking  on the  susceptibility of other herpes  viruses.

Acyclovir for animals

The half-life is 9.6 hours for horses, 2.3 hours for dogs, and 2.6 hours for cats. By comparison, the half-life in humans is 2.5 hours. Unfortunately, it is not absorbed orally in horses (less than 3%), and there are little data to confirm oral absorption in other species. In humans, oral absorption is only 10%. Other forms (e.g., prodrugs and related compounds) are better absorbed in people, but these have been expensive to use in animals.

Indications and Clinical Uses

Acyclovir is an antiviral drug. The use in veterinary  medicine is limited because the activity against  some viruses (e.g., FHV1) is either poor or unknown. In cats, acyclovir had poor absorption and  produced toxicity. Acyclovir is able to inhibit replication of equine herpes virus (EHV1) in vitro. However, acyclovir  oral absorption in horses was poor and inconsistent and intravenous treatment is needed for treatment of EHV1 infection.


Adverse Reactions and Side Effects

The most serious adverse effect in humans is acute renal insufficiency. This may be prevented by slow intravenous infusion and proper hydration. Phlebitis can occur with intravenous administration. No adverse effects were identified in limited studies performed in horses. In cats, significant adverse effects have been observed, which included myelosuppression, hepatotoxicity,  and nephrotoxicity.

Contraindications and Precautions

Do not use in animals with compromised renal function.

Drug Interactions

Do not mix with biological solutions (e.g., blood products). Do not  mix with fluids that contain bacteriostatic preservatives. Do not use with other nephrotoxic drugs.

Instructions for Use

To prepare injectable formulation, dilute each vial with 10 or 20 mL of water to make 50 mg/mL. Do not use bacteriostatic water that contains benzyl alcohol or parabens. Further dilute solution to at least 100 mL to         a concentration of 7 mg/mL or less.

Patient Monitoring and Laboratory Tests

Monitor BUN  and creatinine during use. In horses,  doses should be administered to maintain plasma  concentrations  above 0.3 mcg/mL.


Acyclovir is available in 400-and 800-mg tablets, 200-mg capsules, 1-g and 500mg vials for injection (50 mg/mL), and 40-mg/mL oral  suspension.

Stability and Storage

After reconstitution of solution, at 50 mg/mL, it is stable for 12 hours at room temperature. More dilute solutions are stable for  24 hours. If refrigerated, a precipitate  will form, which should be re-dissolved at    room temperature before use.       

Store tablets and capsules in a tightly  sealed container, protected from light, and at room temperature.

Small Animal Dosage

Dogs and Cats

• Systemic doses have  not been determined.  Doses have been extrapolated from human use: 3 mg/kg  PO 5 times daily for 10 days,  up to 10 mg/kg PO 5 times daily for 10 days. Alternatively 10-20 mg/kg IV q8h  (slow infusion  for 1 hour).

Large Animal Dosage


• 10 mg/kg q12h IV infused over one  hour. Even after 20 mg/kg, oral acyclovir is not absorbed in horses well enough for systemic treatment.

Trade and other names: Zovirax and generic  brands


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