Famciclovir

What is famciclovir?

Famciclovir (Famvir®) is an antiviral medication used to control feline herpesvirus (FHV-1).

Its use in cats is off label (extra label). Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label.

How is famciclovir given?

• Famciclovir comes in tablet form; however, your veterinarian may compound it into a liquid form.
• Famciclovir should be given by mouth, with or without food. Try dosing on an empty stomach first; if your cat vomits or seems unwell after receiving the medication, give the next dose with a small amount of food.
• Measure liquid forms carefully.

This medication can take a few weeks before full effects are noted, but side effects can occur immediately. Gradual improvements are usually appreciable after a few days.

What if I miss giving my pet the medication?

If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. If it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects?

Famciclovir may cause the following side effects:

• Decreased appetite
• Vomiting, diarrhea
• Increased drinking (which may lead to increased urination)

Report any side effects to your veterinarian. Drug sensitivities can develop over time, with repeated exposure. Your pet may have no reaction after the first few doses but may develop a sensitivity later. It is important to watch for signs of an allergic reaction over the entire course of treatment.  

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately.  This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE famciclovir in pets that:

• Have a known hypersensitivity to it or to penciclovir (Denavir®)

USE WITH CAUTION in pets that:

• Have a history of kidney disease
• Are pregnant or nursing

Are there any drug interactions I should be aware of?

Certain medications may interact with famciclovir, so be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment.

Is any monitoring needed with this medication?

• If your cat needs this medicine long-term, your veterinarian may recommend periodic blood tests to ensure your cat’s body is tolerating the medicine well. Do not miss these important follow-up visits.
• Your veterinarian will let you know when and how they want to reassess your pet to ensure the medication is working and not causing side effects.
• At home, monitor for any side effects such as those noted above. If any occur, stop using the medication and contact your veterinarian.

How do I store famciclovir?

• Store famciclovir tablets in the original prescription bottle at room temperature (20°C-25°C or 68°F-77°F) and protect from light.
• If your veterinarian has prepared a liquid formulation, follow the storage recommendations and expiration date on the bottle.

What should I do in case of an emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet's weight and symptoms.