Famciclovir

What is this medication for?

• Feline herpesvirus-1 (FHV-1) — conjunctivitis, rhinosinusitis, keratitis, dermatitis, ulcerative mouth sores (adjunct)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Mild low red blood cell counts (Reported)
• Mild ↑ WBC (Reported)
• Well tolerated overall (Limited data but favorable so far)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Hypersensitivity to famciclovir/penciclovir
• has or has had: Reduce dose with renal impairment; safety in pregnant cats not studied

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Probenecid

Storage and handling

Room temperature, protect from light, tightly sealed container.

Good to know

• FHV-1 is resistant to acyclovir and valacyclovir — famciclovir/penciclovir is the preferred oral antiviral option for cats specifically because of this resistance pattern.
• Dosing recommendations have evolved significantly (early low-dose studies were ineffective) — use current higher-dose protocols (125 mg/cat q8–12h range), not older low-dose regimens.
• FHV-1 infection is controlled, not cured — recurrence is expected even with lifelong management; set owner expectations accordingly.