Famciclovir for Cats

PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet. 

What Is Famciclovir Used for in Cats?

Famciclovir is a human antiviral medication that is commonly used in the veterinary field to treat herpes infections in cats.  

Herpesvirus (FHV-1) infections can cause symptoms in cats including: 

• Eye infection 

• Eye ulcer 

• Sneezing 

• Runny nose 

• Fever 

• Lethargy 

• Loss of appetite 

While famciclovir is not a cure for FHV-1, it can help improve your cat’s symptoms, decrease viral shedding (the release of copies of itself in the body), help shorten the time your cat is sick, and prevent your cat from getting worse. 

Is Famciclovir FDA-Approved? 

While famciclovir is FDA-approved for human use, it is not currently FDA-approved as a veterinary medication.  

It is, however, readily utilized in the veterinary field, and veterinarians can legally prescribe certain human drugs for use in animals in certain circumstances. This is called extra-label or off-label use because this use isn’t described on the drug label. 

Compounded Famciclovir for Cats 

In certain circumstances, your veterinarian may recommend a compounded formulation of famciclovir.  

Compounded medications are prescribed, for example, if:  

• There’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if your pet has trouble taking pills in capsule form 

• The dosage strength is not commercially available 

• The pet is allergic to an ingredient in the FDA-approved medication 

Compounded medications are not FDA-approved. They are formulated by your veterinarian and dispensed either by a licensed pharmacist or veterinarian on an individual basis to best suit a patient’s particular needs. 

Your veterinarian will determine whether a compounded medication is right for your pet. 

How Famciclovir Works in Cats

Famciclovir is an antiviral medication that blocks the virus’s ability to make more DNA and replicate. It will not kill the viruses that are currently circulating in the body, but it can prevent the virus from replicating out of control.  

Famciclovir for Cats Directions

Follow the directions as provided by your veterinarian. Your veterinarian will determine the appropriate dosing regimen for your cat.  

• Treatment is sometimes complicated by viral latency or the ability of the virus to remain dormant within the host cell. Reactivation can then lead to clinical signs of infection in the animal even if it has not been re-exposed to the virus. 

• This medication is generally given two to three times daily.  

• Caution should be taken and the dose decreased in cats with renal insufficiency. 

• If your cat vomits after taking this medication, your veterinarian may recommend giving famciclovir with a small amount of food. 

Missed a Dose? 

Speak with your veterinarian about what to do If you forget to give a dose of famciclovir. Generally, they may advise you to give it when you remember.  

However, if it is almost time for your cat’s next dose, your veterinarian may instruct you to skip the missed dose and resume your normal dosing schedule. In most cases, they may instruct you to not give extra or double doses. 

Possible Side Effects of Famciclovir in Cats

Famciclovir is generally well-tolerated by cats, but side effects may include: 

• Diarrhea 

• Increased thirst 

• Increased urination 

• Weight loss 

• Vomiting 

If you believe your cat is experiencing any side effects, consult your veterinarian. 

Human Side Effects 

Famciclovir is also a prescription medication for humans, frequently with dosages different from those prescribed for your pet by a veterinarian. 

Due to possible side effects, humans should never use medicine dispensed for their pets and pets should not be given any medicine dispensed for a human’s use. 

If you accidentally ingest a pet medication, immediately seek medical attention, or call Poison Control at 800-222-1222. 

Monitoring Your Pet 

Specific monitoring or routine testing while your cat is on this medication may be recommended by your veterinarian depending on your cat’s individual needs, other medications they may be on, and/or the issue that initially caused your pet to be placed on this medication. 

Your veterinarian may recommend periodic blood tests to ensure your cat is tolerating the medicine well. 

When To Call Your Vet 

Call your veterinarian if any of the following apply: 

• Severe side effects are seen (see above) 

• Your pet’s condition worsens or does not improve with treatment 

• You see or suspect an overdose 

• You have additional questions or concerns about the use of famciclovir 

Famciclovir Overdose Concerns in Cats

Information and research are limited regarding overdoses of this medication in pets.  

Symptoms may include:  

• Vomiting 

• Diarrhea 

• Increased thirst 

If you suspect an overdose, immediately seek emergency veterinary care or contact an animal poison control center. Consultation fees often apply. 

• Pet Poison Helpline (855) 764-7661 

• ASPCA Animal Poison Control (888) 426-4435 

How To Store Famciclovir for Cats

Keep the following storage tips in mind:  

• Famciclovir should be stored at controlled temperatures between 68–77 F (20–25 C). Brief exposures to temperatures between 59–86 F (15–30 C) are acceptable. 

• Keep the container tightly closed to protect it from moisture and light.  

• Always confirm storage requirements by reviewing the label. 

• Compounded medications should be stored according to the compounding pharmacy’s label. 

• Keep out of reach of children and pets.