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Feline infectious peritonitis, or FIP, was considered almost universally fatal for decades. That's changed. Here's what FIP is, how cats get it, and what current treatment looks like so you can give your cat the best life possible.
Key Takeaways:
- FIP develops when feline coronavirus mutates inside a cat's body. Only about 5% of cats exposed to the virus go on to develop FIP.
- There are two forms of feline infectious peritonitis: wet (effusive), with fluid buildup, and dry (non-effusive), with organ inflammation.
- A 2026 study in Scientific Reports following 629 cats treated with the antiviral GS-441524 found a 94% survival rate.
- GS-441524 isn't FDA-approved, but the FDA does not take enforcement action against veterinarian-prescribed compounded use.
- Mixlab compounds and delivers GS-441524, prescribed by your veterinarian.
What Is Feline Infectious Peritonitis (FIP) in Cats?
FIP is a viral disease caused by a mutated form of feline coronavirus, a common and usually harmless virus found in cat feces. Most cats exposed to feline coronavirus never develop FIP. It's the mutation inside an individual cat's body, not the original virus itself, that causes disease.
Definition: Feline coronavirus is a common, typically mild intestinal virus spread between cats. FIP develops only when the virus mutates within an infected cat, which is why FIP itself isn't considered contagious between cats even though the original virus is.
Feline Infectious Peritonitis (FIP) at a Glance |
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Wet (Effusive) FIP: |
Dry (Non-Effusive) FIP: |
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Hallmark sign |
Fluid buildup in abdomen or chest |
Organ inflammation without fluid |
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Progression |
Faster |
Slower |
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Common symptoms |
Abdominal swelling, breathing difficulty |
Eye inflammation, neurological signs, jaundice |
How Do Cats Get FIP?
Cats get FIP when feline coronavirus already in their system mutates, which happens in only about 5% of exposed cats, sometimes weeks to years after initial exposure. Certain cats face higher risk, including those under 2 years old, pedigree cats and cats in multi-cat households where coronavirus circulates more easily.
Our Take: The 5% figure surprises a lot of pet parents who assume any coronavirus exposure means FIP risk. In our experience filling FIP prescriptions, the more useful framing for owners is that FIP is a mutation event in one cat, not something that spreads cat to cat the way the underlying virus does.
What Are the Symptoms of FIP in Cats?
Early FIP often looks like general illness: persistent fever that doesn't respond to antibiotics, lethargy, reduced appetite and weight loss. Which symptoms show up next depends on whether the wet or dry form develops.
- Wet FIP: abdominal swelling, difficulty breathing
- Dry FIP: neurological signs like seizures or incoordination, eye inflammation (uveitis), jaundice or organ-related masses
No single test confirms FIP. Veterinarians typically combine bloodwork, imaging and fluid or tissue analysis, similar to how other hard-to-pin-down feline illnesses are diagnosed.
Is There a Cure or Treatment for FIP?
Yes, there is treatment for FIP, and this is the part that's changed the most in recent years. GS-441524, an antiviral, is now the standard treatment for FIP, prescribed by a veterinarian and available through compounding pharmacies.
A 2026 study in Scientific Reports following 629 cats treated with GS-441524 found a 94% survival rate, with relapse in less than 1% of cases.
GS-441524 isn't FDA-approved as a commercial drug. But the agency will not take enforcement action against compounded GS-441524 prescribed by a veterinarian for a specific cat's FIP treatment, since no FDA-approved alternative exists. Compounding follows FDA Guidance for Industry (GFI) #256.
For a deeper look at prognosis and what recovery looks like day-to-day, see Can a Cat Survive FIP? →
Has your veterinarian prescribed GS-441524? Mixlab compounds it as a QUADmix™ tablet or oral suspension.
How Is GS-441524 Given, and How Long Does Treatment Last?
Standard treatment of GS-441524 runs a full 12 weeks (84 days), and stopping early raises the risk of relapse even if a cat looks better sooner. Many cats show some improvement within the first several days of starting treatment, though every cat's course is different.
Cats need regular weigh-ins, at least every other week, since dosing is weight-based and often needs adjusting as treatment progresses. Your veterinarian should re-examine your cat before treatment ends to confirm it's the right time to stop.
Side Effects and Contraindications of GS-441524 for FIP Treatment |
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Consideration: |
What to Know |
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Common side effects |
Mild GI upset (vomiting or diarrhea), elevated white blood cell count or liver enzymes |
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Serious side effects |
None commonly reported in current literature |
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Corticosteroids |
Generally avoided during treatment, with limited exceptions your veterinarian may consider (topical steroids for uveitis, short-term use for severe neurological signs) |
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Who prescribes it |
A licensed veterinarian only, following an FIP diagnosis |
How Can I Order GS-441524 Through Mixlab?
Once your veterinarian prescribes GS-441524 for your cat, Mixlab compounds it as a QUADmix™ tablet or oral suspension with fast delivery. Your veterinarian submits the prescription directly, and Mixlab handles compounding to your cat's specific dose.
More Frequently Asked Questions About FIP
Is FIP contagious to other cats?
FIP itself isn't contagious from cat to cat, but the underlying feline coronavirus is, especially in multi-cat households. A cat with FIP developed it from a mutation of the virus already in its own body.
Can FIP spread to humans or dogs?
No. Feline coronavirus and FIP are specific to cats and don't infect humans or dogs.
Is FIP always fatal without treatment?
Historically, yes, FIP is fatal without treatment in the vast majority of cases. Before antivirals like GS-441524 became available, FIP had no effective treatment and was considered almost universally fatal.
Can FIP be prevented?
There's no guaranteed way to prevent FIP, but reducing stress and keeping litter boxes clean in multi-cat households can lower coronavirus transmission, which is the first step in the chain that can lead to FIP.
Do kittens get FIP more often than adult cats?
Young cats, especially under 2 years old, are considered higher risk for FIP, along with pedigree cats and cats in multi-cat households.
How much does FIP treatment cost?
Cost for FIP treatment varies by cat weight, dosing and treatment duration. Your veterinarian and Mixlab can provide a specific estimate based on your cat's prescription.
Is feline coronavirus the same thing as FIP?
No. Feline coronavirus is common and usually harmless. FIP only develops when the virus mutates inside a specific cat, which happens in a small percentage of exposed cats.
Mixlab Is Here to Keep Your Cat Happy and Healthy
FIP was once considered a death sentence, and for cats who don't receive treatment, it's still extremely serious. What's changed is that GS-441524 has turned FIP into a condition many cats now survive when it's caught and treated. If your veterinarian diagnoses your cat with FIP, Mixlab compounds and delivers GS-441524 quickly.
Ready to fill a GS-441524 prescription? Order through Mixlab's FIP medication page with fast delivery →
