April 2026
In this VETgirl online veterinary continuing education blog, Dr. Amy Kaplan, cVMA, DACVECC, MRCVS explores the evolving landscape of Feline Infectious Peritonitis (FIP) treatment, unveiling the latest antiviral strategies that are transforming outcomes for cats. Tailored for veterinary professionals, this Part 2 of a 2-part series, offers expert guidance on choosing the right nucleoside analogues, customizing doses for ocular and neurological cases, and mastering patient monitoring—empowering you to turn hope into healing.
Check out FIP Updates From 2025 Part 1: New Data, New Doses, New Hope HERE and the related podcast HERE!
FIP Updates From 2025 Part 2: New Data, New Doses, New Hope
By Dr. Amy Kaplan-Zattler, cVMA, DACVECC, MRCVS
VETgirl CE Program Manager
Last week we talked about Feline Infectious Peritonitis (FIP) and why FIP is so hard to diagnose; clinical signs are often nonspecific, confirmatory testing isn’t always feasible in real time, and the stakes are high when you’re deciding whether to move forward with treatment for FIP. If you didn’t check out Part 1, read that first HERE! In Part 2, we shift from how to recognize FIP to how to treat FIP, guided by the growing body of evidence from the past couple of years and the practical recommendations outlined by International Cat Care in An update on the treatment of feline infectious peritonitis. In today’s VETgirl blog, we focus on how to approach antiviral therapy for FIP thoughtfully and consistently in the real world — choosing a rational, patient-specific plan, setting expectations for response and monitoring, and knowing when to reassess if a cat isn’t responding to FIP treatment the way you expect.
As we move into 2026, nucleoside analogues remain the backbone of FIP therapy, and treatment protocols continue to evolve with clinical experience, emerging studies, and (in some regions) improved access to quality-assured drugs. In areas where legal access is limited, some veterinarians and cat owners have turned to unregulated products — often out of necessity. VETgirl echoes International Cat Care’s caution: black-market antivirals may have inconsistent active drug content, no quality assurance, and have been associated with adverse events, including reports of severe injection-site injuries linked to highly acidic formulations. And while none of us want to play “antiviral roulette,” if limited access guides your antiviral selection, just remember the goal is to treat thoughtfully, monitor patient’s closely, and give that cat a fighting chance.
With that context in mind, the current backbone of FIP treatment are the nucleoside analogue antivirals, which work by interfering with feline coronavirus RNA replication. This group includes GS-441524, remdesivir, molnupiravir, and EIDD-1931 (with legal access varying by country and, in some regions, requiring a compounding pharmacy). Molnupiravir and its parent drug EIDD-1931 carry additional safety considerations such as reported neutropenia, potential viral mutation concerns, and teratogenic risk, so they’re typically reserved as backup options for treatment failures or relapses, or for regions where access to GS-441524 and/or remdesivir is limited. Beyond nucleoside analogues, there are emerging successful reports using nirmatrelvir with ritonavir (Paxlovid™) in refractory or neurologic cases. Lastly, GC-376, a 3CL protease inhibitor, has gained attention from anecdotal reports, but its restricted access puts it in the “watch for now” category.
Next, we break down some of the major highlights and pearls we’ve learned so far in our treatment battle against FIP.
Update #1: Oral GS-441524 can be First-line Therapy From Day 1
We no longer need to start with injectable antivirals (i.e., remdesivir), unless the patient is severely dehydrated, cannot swallow, or otherwise cannot receive oral medications. But for critically ill or clinically dehydrated patients, hospitalization for ~48 hours for IV fluid therapy and injectable remdesivir is still advisable with the goal of transitioning to oral GS-441524 once able. While data on oral absorption (especially when complicated by various gastrointestinal conditions) is lacking, current recommendations are to give oral GS-441524 on an empty stomach and wait 30 minutes before a full meal – but hiding a crushed tablet in a small treat is still considered ok!
Update #2: Treatment Duration may be Effective in a Shorter (6 week) Course in SOME cats
Most FIP protocols still include a 12-week treatment plan, often with once daily GS-441524 administration for 84 days (or split q12h if neuro/ocular disease is in the mix). That said, newer data looking at cats with ‘wet’ FIP show that many did well with a 6-week course without incurring a higher relapse risk. The catch? This shorter course was studied only in cats that presented mostly with an effusive FIP form, had been diagnosed early in their disease, were treated early as a result, and were hospitalized for their first week of treatment. The study used oral GS-441524 at 15 mg/kg q24h and found most of these cats had a fast response to treatment with resolution of lab work abnormalities and elevated alpha-1 acid glycoprotein (AGP) levels, often within the first 28 days. But before shortening your patients’ treatment, be sure to prioritize a response-driven approach. Practical rule of thumb: treat at least 2 weeks past normalization of biochemistry abnormalities and AGP levels, as well as resolution of effusion/ocular/neuro signs. We don’t want our haste to be the reason a cat relapses!
Update #3: Follow a Streamlined Dosing Framework by FIP “Type” (with Special Attention to Eyes and Brains)
Ocular and neurologic presentations generally warrant dose adjustments to improve penetration across the blood-brain/blood-CSF and blood-ocular barriers. Dose increases may improve penetration, and dividing the total dose into twice-daily administration may maintain steadier blood concentrations throughout the day. Below are suggested antiviral doses by clinical signs, and extrapolated from icatcare.org.
Update #4: Expect Early Improvements; Otherwise, Consider Changing the Plan!
Coach owners to be your at-home “FIP vital signs team”: track resting respiratory rate/effort, appetite, and overall behavior. Cats should start looking better quickly, so if they’re not trending upward, or if cat owners observe faster/labored breathing, inappetence, erythematous/painful eyes, or new/worsening neurologic signs (e.g., depression, ataxia, seizures), the cat should be evaluated ASAP for a possible dose adjustment. Before calling the failed response a “drug failure”, start with the basics: confirm the medication is actually getting into the cat and being given on schedule, because “I gave it” doesn’t always mean the cat got it! Also remember that in the first 48 hours (while drug levels are building) it’s possible to see worsening or new onset of clinical signs.
Update #5: Therapeutic Drug Monitoring (TDM) is Entering the Chat (in Some Regions)
TDM isn’t mainstream (yet), but it’s likely where we’re headed. More personalized dosing – especially for cats who aren’t improving on schedule – may eventually let us fine-tune therapy for the tricky cats, especially those with questionable GI absorption (hello, IBD cats!).
Update #6: For Adjunct Drugs, ‘Less’ is Often ‘More’
Corticosteroids: Try to avoid, but don’t shy away if they’re needed! Yes, FIP cats can come with comorbidities that may benefit from steroids (e.g., IMHA, IBD). For uveitis, a short course of topical corticosteroids is reasonable for comfort during the first 1–2 weeks of FIP treatment. If clinical signs still require corticosteroids beyond 2 weeks, that’s more suggestive of undertreated FIP, so the antiviral dose may need a bump. Immune-mediated hemolytic anemia has been rarely reported along with FIP, and the good news is we have weak evidence suggesting 0.5-2 mg/kg/day prednisolone does not negatively impact survival. So if you need steroids for a true immune-mediated problem, or to dampen inflammation, they’re not automatically off-limits!
Immunostimulants: As tempting as these may sound, the evidence isn’t quite there (yet). Current guidelines don’t recommend routine immunostimulant use with FIP, largely because we’re already seeing excellent response rates with antiviral therapy alone. So for most cats, this is one more add-on we can skip.
Update #7: When can FIP Cats get Back to Routine Veterinary Care?
- Spay/neuter: Ideally, wait >1 month after finishing antivirals. If surgery has to happen during treatment (life happens!), it’s likely OK – just continue antivirals 2–4 weeks post-op to keep treatment momentum on your side.
- Parasite prevention: Deworming and flea/tick meds can continue during treatment – no need to pause the basics.
- Vaccines: We still need prospective data, but current thinking is: if the cat is clinically well, killed/non-live vaccines are probably reasonable during antiviral therapy. Live vaccines, are still a question mark, so it’s recommended to save those for >1 month after treatment ends.
Do cats with FIP relapse after treatment? The first month after stopping antivirals is the prime relapse-risk window. Thankfully, relapse is uncommon (often quoted at <10%) and becomes rare once a cat cruises past the 4-week mark. That said, prep cat owners that relapses can happen, and they don’t always look like the cat’s original FIP presentation. Some cats throw new curveballs, including new neurologic, ocular, or even gastrointestinal signs despite originally presenting with effusion. That’s why a recheck exam around 4 weeks off meds is typically recommended. And if you have it, a normal post-treatment AGP is a nice little confidence boost that things are really staying on track.
Conclusion
When in doubt, for the newly diagnosed cat with FIP the gameplan should be: start treatment early, dose according to the cat’s clinical signs, and know when to tweak the plan/ Build in early check-ins so you can pivot fast if the cat isn’t responding quickly to treatment. As our knowledge (and collective clinical experience) with FIP therapy grows, expect these recommendations to evolve right along with it. And the best update of all? We’re no longer asking whether to treat FIP – we’re fine-tuning how to treat it. That’s a pretty fantastic problem to have…because it means more cats are surviving long enough for us to fine-tune the details.
Abbreviations:
A:G: albumin-to-globulin ratio
AGP: alpha1-acid glycoprotein
ALT: alanine aminotransferase
CSF: cerebrospinal fluid
FCoV: feline coronavirus
FIP: feline infectious peritonitis
IBD: irritable bowel disease
IMHA: immune-mediated hemolytic anemia
RNA: ribonucleic acid
SAA: serum amyloid-A
TDM: therapeutic drug monitoring
References:
1. Taylor S, Tasker S, Barker E, et al. An update on the treatment of feline infectious peritonitis (July 2025). International Cat Care; Accessed January 26, 2026. https://icatcare.org
2. Barua S, Kaltenboeck B, Juan YC, et al. Comparative Evaluation of GS-441524, Teriflunomide, Ruxolitinib, Molnupiravir, Ritonavir, and Nirmatrelvir for In Vitro Antiviral Activity against Feline Infectious Peritonitis Virus. Vet Sci. 2023;10(8):513. doi:10.3390/vetsci10080513.
3. Mulligan AJ, Browning ME. Quality assessment and characterization of unregulated antiviral drugs for feline infectious peritonitis: implications for treatment, safety, and efficacy. Am J Vet Res. 2024;1-9. doi: 10.2460/ajvr.23.10.0221.
4. Kamiyoshi T, Kamiyoshi N, Jintake C. High-dose induction therapy and treatment termination criteria for feline infectious peritonitis with remdesivir, GS-441524 and adjunctive mefloquine: 46 cases (2023). J Small Anim Pract. 2025;66(9):617-626. doi: 10.1111/jsap.13869.
5. Andrews ALMM, Izaguirre E, Green J, et al. Treatment With Remdesivir Alone or in Combination With GS-441524 in Cats With Ocular Involvement of Feline Infectious Peritonitis: An Observational Case Series. J Vet Intern Med. 2025;39(6):e70253.
6. Roman N. Feline infectious peritonitis. Merck Veterinary Manual. Updated November 2025. Accessed January 26, 2026.
7. Thayer V, Gogolski S, Felten S, et al. 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines. J Feline Med Surg. 2022;24(9):905-933. doi: 10.1177/1098612X221118761.






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