Until 2017, a FIP diagnosis was a death sentence. The available treatments were exclusively palliative and cats died within weeks. Today, thanks to antivirals, more than 88–92% of treated cats make a full recovery. This is the story of how that change happened, and a detailed guide to all available antivirals — including those without their own page on this site.
Authorship note: This article was written by Vicky Vives Moya, Licenced in Veterinary Medicine specialising in Feline Medicine (Cert. Ophthalmology, AVEPA, GEMFE, iCatCare member), and Esther Garrido Cervera, Licenced in Veterinary Medicine. Last reviewed: November 2025. The authors have no conflict of interest with antiviral manufacturers or commercial suppliers.
For detailed information on the main antivirals, see our dedicated guides: GS-441524 · Molnupiravir · Dual therapy.
1. Before antivirals: the pre-2017 landscape
For decades, FIP treatment was limited to relieving symptoms with no real impact on survival. The drugs used fell into three strategies:
| Category | Drugs | Goal |
|---|---|---|
| Corticosteroids | Prednisolone, Dexamethasone | Anti-inflammatory and immunosuppressant |
| Alkylating agents | Cyclophosphamide | Immunosuppressant |
| Cytokine inhibitors | Pentoxifylline | Anti-vasculitis |
| Non-specific antivirals | Doxycycline, Cyclosporin A | Limited in-vitro antiviral activity |
| Immunostimulants | Feline recombinant omega interferon (rfIFN-ω), PPI | Immune stimulation |
| Experimental | Itraconazole, stem cells | No consistent efficacy |
The outcome was always the same: temporary relief of days or weeks, with no change to the fatal prognosis. Cyclosporin A showed some anti-coronavirus activity by blocking non-structural protein Nsp1, and recombinant omega interferon was used in monotherapy or combined with glucocorticoids, but neither altered disease progression.
Polyprenyl immunostimulant (PPI) produced some documented successes in non-effusive FIP, but without consistent reproducibility.
2. Dr. Niels Pedersen and the antiviral revolution (2017–2019)
Dr. Niels Pedersen, veterinarian and professor emeritus at the University of California, Davis, has been studying feline infectious diseases for decades. His work was decisive:
- 1970s: start of his research into FIP
- 2008: comprehensive review of all FIP scientific literature published to that date
- 2017: first study with GC-376 showing promising results in cats with FIP
- 2019: publication of GS-441524 data that forever changed the disease prognosis
His publications opened the door to what we know today: FIP is curable.
3. GS-441524 and Remdesivir: the gold standard
GS-441524 and remdesivir are nucleotide analogues that inhibit the RNA polymerase of feline coronavirus, stopping viral replication. GS-441524 is the active form of remdesivir.
Since 2019, thousands of treated cases worldwide confirm cure rates of 84–92% depending on the study and protocol. In 2023, the WSAVA included them on its list of essential medicines for dogs and cats.
→ Full guide on our dedicated page: GS-441524 for FIP
4. GC-376: the protease inhibitor
Mechanism of action
GC-376 works differently from GS-441524: it is a viral protease inhibitor of feline coronavirus (and SARS-CoV-2). It blocks the catalytic activity of the virus’s 3C-like protease, preventing it from processing the viral polyproteins needed for replication.
Developed by Anivive Lifesciences, it is under investigation for human and veterinary use.
Scientific evidence
Pedersen 2018 study (first clinical trial):
- 20 cats with effusive and non-effusive FIP (neurological cases excluded)
- Protocol: 15 mg/kg every 12 hours SC for 15 days
- 19/20 cats showed clinical improvement in the first two weeks
- Relapses in one third of cases between 1–7 weeks post-treatment
- Treatment extended to an additional 12 weeks in those that relapsed
- Final result: only 1/3 of cats survived long-term
2022 combination study (GS-441524 + GC-376):
- 46 cases treated with a combination of both antivirals
- Duration: 4–12 weeks according to clinical response
- Efficacy: 97.8% survival
- The distinct and synergistic mechanisms of both drugs explain the improved results
Current use
GC-376 in monotherapy has been superseded by GS-441524, which offers superior results and is more accessible. Its current use is reserved for:
- GS-441524 resistance: GS-441524 + GC-376 combination
- Refractory cases: when the standard protocol has failed
- Concurrent use: leveraging the complementary mechanisms
Dosage
| Parameter | Value |
|---|---|
| Dose | 15 mg/kg every 12 hours |
| Route | Subcutaneous (SC) |
| Duration | 15–84 days according to response |
Adverse effects
- Transient stinging at injection sites
- Subcutaneous fibrosis with prolonged use
- Hair loss at inoculation points
- Dental changes in kittens: delayed eruption of permanent teeth (second and third premolars most affected, retention of deciduous canines)
5. Molnupiravir: alternative for resistant cases
Molnupiravir (EIDD-2801, marketed as Lagevrio® by Merck) acts by inducing errors in viral RNA until the virus stops replicating. It has shown efficacy in FIP including neurological and ocular forms.
International Cat Care mentions it for cases of GS-441524 therapeutic failure.
→ Full guide: Molnupiravir for FIP
6. Mefloquine: the economical option
Mechanism of action
Mefloquine is a human antimalarial (Lariam®) used against chloroquine-resistant Plasmodium falciparum. Its activity against feline coronavirus was discovered following COVID-19 research.
In vitro, it reduces FIP viral load in feline renal cells (Crandell) without cytotoxic effect, inhibiting viral replication.
Scientific evidence
2013 study (chloroquine): Chloroquine, from the same pharmacological family, demonstrated an inhibitory effect against FIP virus replication and anti-inflammatory activity in vitro. Experimentally infected cats treated with chloroquine showed better clinical outcomes.
2020 study (mefloquine — in vivo pharmacokinetics): Study that determined the efficacy of current doses, described haematological and biochemical changes during treatment and established the safety profile in clinically normal cats.
Dosage
| Parameter | Value |
|---|---|
| Standard dose | 62.5 mg, 2–3 times per week |
| Duration | Up to completing 84 days |
| Route | Oral |
Use in combination
Mefloquine is used primarily as a supplement to GS-441524, potentiating antiviral effects, or as a more economical alternative in the International Cat Care protocol when access to GS-441524 is limited.
Adverse effects
- Vomiting (administer with food to minimise)
- Dermatopathies (skin reactions)
- Elevated SDMA (renal function marker)
7. Nirmatrelvir: current research
The journal Veterinary Sciences published an article in 2023 comparing the in-vitro efficacy of different antivirals against FIP, with promising results for nirmatrelvir.
Nirmatrelvir is marketed together with ritonavir under the name Paxlovid® (Pfizer), for exclusive human use against COVID-19. According to personal communication from Dr. Richard Malik, it has been used in cats refractory to other treatments.
Reported protocol: 75 mg nirmatrelvir + 25 mg ritonavir every 12 hours, oral route.
This dosage is based on individual cases under specialist supervision. It is not approved for veterinary use and requires a special prescription. Formal clinical studies are needed to establish definitive protocols.
8. Comparative table of clinical studies
| Study | Cases | Survival | Relapse rate |
|---|---|---|---|
| Pedersen 2019 (original GS-441524) | 31 | 77% | 30.8% |
| GTA-AVEPA 2023 — Spain | 89 | 77.5% | 7.2% |
| International Cat Care 2023 | 307 | 84.4% | 10.8% |
| Vives Moya 2025 | 500+ | 88.1% | 8% |
Clear trend: the most recent studies, with optimised protocols and higher doses, show increasing survival rates and declining relapse rates. The doses used in Pedersen’s original study (2 mg/kg) were far lower than current protocols (4–8 mg/kg depending on FIP form).
9. Antiviral selection algorithm
| Clinical situation | First line |
|---|---|
| Effusive FIP without complications | GS-441524 or remdesivir |
| Non-effusive FIP | GS-441524 or remdesivir |
| Neurological FIP | High-dose GS-441524 (6–8 mg/kg) |
| Ocular FIP | GS-441524 or remdesivir |
| GS-441524 resistance | GS-441524 + GC-376 |
| Post-treatment relapse | Restart GS-441524 (higher dose) |
| Economic limitation | Mefloquine + GS-441524 (reduced dose) |
| Failure with multiple antivirals | Nirmatrelvir (experimental use, specialist) |
10. Treatment monitoring
Dr. Pedersen established the monitoring criteria in 2019 that remain the standard:
- Clinical evaluation: weekly for the first 2–4 weeks, then fortnightly
- Complete blood count: every 2–4 weeks
- Serum biochemistry: total protein, albumin, globulins, bilirubin, ALT, creatinine
- Body weight: weekly
- Temperature: daily at home during the first weeks
- Ultrasound: if effusion present, to monitor its resolution
Indicators of good response:
- Resolution of fever within the first 24–48 hours
- Improvement of appetite and activity within 3–7 days
- Decrease in globulins and increase in albumin
- Resolution of effusion within 2–4 weeks
11. Organisations and reference sources
- UC Davis - Dr. Niels Pedersen — original publications on GS-441524 and GC-376
- International Cat Care — updated treatment guidelines
- GEMFE / AVEPA — feline medicine specialty group (Spain)
- ISFM — International Society of Feline Medicine
- ABCD — European Advisory Board on Cat Diseases
- FIP Warriors Spain — support group for owners
Medical disclaimer: This guide is informational and educational, based on peer-reviewed scientific literature. It does not replace professional veterinary consultation, diagnosis or treatment. FIP treatment with antivirals must always be carried out under veterinary supervision. The authors have received no financial compensation and declare no conflict of interest.