Authorship note: This article is a summary of the clinical guide published by Vicky Vives Moya (Licenced in Veterinary Medicine, Higher Studies Certificate in Ophthalmology, AVEPA, GEMFE and iCatCare member) and Esther Garrido Cervera (Licenced in Veterinary Medicine) in the journal Centro Veterinario, 2025. Maricel Veterinaris, El Masnou, Barcelona. Last reviewed: December 2025.
For general information about GS-441524, see our treatments page. This article goes deeper into the clinical details of dosage, monitoring and the management of complex situations.
1. Oral or injectable: the initial decision
The first studies with GS-441524 used subcutaneous injections. Today, accumulated experience and Therapeutic Drug Monitoring (TDM) studies show that the oral route is equally effective for most stable patients.
| Oral route | Injectable route (SC) | |
|---|---|---|
| Convenience | Greater convenience for the owner | Requires injection technique |
| Pain | None | Pain at injection site |
| Skin risks | None | Abscesses, seromas, pruritus |
| Absorption | ~50% (variable) | Almost complete |
| Onset of action | More delayed | Faster |
| Recommended use | Stable cats | Critical cats or those unable to swallow |
When to ALWAYS start with injectable
iCatCare recommends starting with injectable remdesivir for the first 48 hours in these three situations, before switching to oral:
- Cats with neurological involvement preventing normal swallowing
- Cats with severe dehydration or very advanced disease
- Cats that cannot be medicated orally for any other reason
⚠️ Important warning: The ISFM protocol doses for injectable correspond to remdesivir, not injectable GS-441524. Remdesivir is not available in many countries. The doses of injectable GS-441524 are much lower than those of remdesivir. Using remdesivir doses with injectable GS-441524 is a potentially serious error.
2. Updated dosage protocols
Table 1 — ISFM Protocol (International Cat Care, updated 2024)
Administer oral GS on an empty stomach, waiting at least 30 minutes before offering food.
| Clinical presentation | Oral GS-441524 | IV/SC Remdesivir |
|---|---|---|
| Effusion without ocular/neuro signs | 15 mg/kg/24h (or in 2 doses) | 10 mg/kg/24h |
| No effusion, no ocular/neuro signs | 15 mg/kg/24h (or in 2 doses) | 12 mg/kg/24h |
| Ocular signs ± effusion | 20 mg/kg/24h (or in 2 doses) | 15 mg/kg/24h |
| Neurological signs ± effusion | 10 mg/kg every 12h | 20 mg/kg/24h |
Table 2 — Reference doses from clinical groups (FIP Warriors)
In severe cases, doses may be higher and administered every 12h.
| Clinical presentation | Oral GS | Injectable GS (SC) |
|---|---|---|
| Effusion without ocular/neuro signs | 12–18 mg/kg/24h | 6–8 mg/kg/24h |
| No effusion, no ocular/neuro signs | 12–18 mg/kg/24h | 6–8 mg/kg/24h |
| Ocular signs ± effusion | 16 mg/kg/24h | 8 mg/kg/24h |
| Neurological signs ± effusion | minimum 20 mg/kg/24h | minimum 10 mg/kg/24h |
The differences between oral and injectable reflect the ~50% oral absorption. The ISFM protocol uses higher doses because 15% of cats responded poorly to lower doses due to variability in absorption.
3. Treatment duration
The standard is at least 84 days, for two fundamental reasons:
- Experience with GC-376 in Pedersen’s 2018 study showed that short treatments produce relapses
- The half-life of macrophages in tissues is ~84 days — to eliminate the intracellular virus this barrier must be exceeded
Can 42 days be enough?
A study presented in June 2024 treated 20 cats with wet FIP for 42 days at 15 mg/kg/24h with 95% success. A subsequent study with 64 cats achieved 91% success with the same protocol.
These results suggest 42 days may be sufficient in selected cases (effusive FIP, adequate dose, good response). Duration always depends on normalisation of parameters and clinical progression.
Non-negotiable rule: dose adjustment with weight
One of the main causes of therapeutic failure is not increasing the dose when the cat gains weight.
- Weigh the cat weekly
- Adjust the dose with any weight increase
- Never miss a dose under any circumstances
- Administer always at the same time
4. Monitoring during treatment
Complete monthly blood work is recommended for all 3 months of treatment, plus reviews during the subsequent observation period (3 months). Severe cases require more frequent monitoring.
Table 3 — Normalisation of analytical parameters
| Parameter | Estimated normalisation time |
|---|---|
| Leucocytosis | 2–4 weeks |
| Lymphocytes (if lymphopenia) | 1 week |
| Bilirubin | 2–4 weeks |
| Globulins | Increase in wet FIP at 3–4 weeks, then normalise |
| Haematocrit | 6–8 weeks |
| Albumin | 8 weeks |
| A/G ratio | ≥0.6 at end of treatment |
| AGP (alpha-1 glycoprotein) | Progressive normalisation during treatment |
Table 4 — Normalisation of clinical symptoms
| Symptom / Sign | Estimated resolution time |
|---|---|
| Energy and appetite | Improvement in 1–2 days; normal by 2 weeks |
| Fever | 12–48 hours |
| Pleural effusion | Gone by day 7 |
| Abdominal effusion | Improvement at 10–14 days; gone by 2 weeks |
| Ocular symptoms | Not apparent by 7–14 days |
| Weight gain | 1–1.5 kg during treatment |
| Neurological symptoms | Variable |
Note: A 2022 study of 18 cases in complete remission for one year confirmed that abdominal lymphadenopathy may persist without clinical relevance. It should not cause alarm once treatment is finished.
5. Signs of lack of response
Table 5 — Warning thresholds
| Parameter / Sign | Warning threshold |
|---|---|
| Persistent fever | More than 7 days |
| Persistent effusions | More than 3–4 weeks |
| Hyperglobulinaemia | More than 6–8 weeks |
| A/G ratio <0.6 | More than 8–10 weeks |
| Hyperbilirubinaemia | More than 3 weeks |
| Abnormal white blood cell count | More than 3 weeks |
| Appearance of new FIP signs | At any time |
Possible causes of failure: incorrect diagnosis, concurrent disease, inadequate dose (not adjusted with weight), incorrect administration by the owner, poor-quality drug, or emergence of resistance.
Action on lack of response: increase the dose by 2.5–5 mg/kg for injectable and 5–10 mg/kg for oral, and continue until the next review.
6. Side effects
The safety profile of GS-441524 is broadly favourable. Most effects are mild and transient.
General (oral and injectable):
- Transient elevation of ALT (may be an effect of the disease or the drug)
- Mild peripheral eosinophilia
- GS uroliths: documented in 2 cats under treatment (very rare)
Injectable-specific:
- Pain, abscesses, seromas and wounds at the injection site
- Pruritus with self-injury
- Vagal reactions after parenteral administration
- Anaphylactic reaction (very rare)
Injectable remdesivir-specific:
- Development or worsening of pleural effusion in the first 48 hours
- Depression or nausea a few hours after IV administration
7. Drug interactions
Drugs with CNS penetration
Fluoroquinolones and some antiparasitic agents can cause neurological effects that are confused with progression towards neurological FIP. The clinician must weigh the pros and cons in each case.
Corticosteroids
They can be used in very specific cases and for limited periods:
- Immune-mediated haemolytic anaemia
- Neurological presentation
- Inflammatory bowel disease
- Dermatitis
Depot or long-acting corticosteroid formulations are completely contraindicated. Numerous cases have been reported of cats that deteriorated 5–6 weeks after administration of depot forms during antiviral treatment.
L-Lysine
It is recommended to discontinue L-Lysine use during treatment. It is hypothesised that it may cause arginine depletion and growth delays in kittens.
8. Special precautions
Persistent effusions
If effusion does not resolve with treatment, there are three possible causes:
- Persistent infection: inadequate treatment, poor-quality drug or antiviral resistance
- Circulatory damage: increased capillary pressure from low-grade infection or residual fibrosis
- Another pathology: heart disease, hepatopathy, hypoproteinaemia or neoplasia
If effusion persists: complete fluid study, increase the dose if active FIP is suggested, and consider exploratory laparotomy with biopsies if diagnosis is in doubt. Coronavirus PCR in fluid during treatment can give false negatives and must not be used as the sole criterion.
Pregnancy
The recommendation is to treat pregnant females. Cases have been documented with favourable outcomes for both the mother and the kittens. Pregnant queens are usually in subclinical stages when pregnancy occurs; immunosuppression inherent to gestation generally favours progression in the last trimester.
- Placental transmission is described but uncommon
- Kittens from untreated mothers usually die in the foetal or early postnatal phase
- Treated queens can give birth to healthy kittens and care for them properly
9. Comorbidities
Antiviral treatment does not interfere with treatment of other concurrent pathologies.
Infectious:
- FeLV and FIV: FIP cure is possible. The owner must be informed that 90–95% of cats with progressive FeLV do not survive beyond 3 years
- Panleukopenia: if the cat survives panleukopenia, the FIP prognosis is favourable
- Haemoplasma: worsens anaemia and requires specific treatment (antibiotic ± corticosteroids)
Immune-mediated: pemphigus foliaceus, atopic dermatitis, lymphoplasmacytic gastroenteritis, immune-mediated haemolytic anaemia (may require corticosteroids or alkylating agents).
Neoplastic: lymphoma (simultaneous chemotherapy or after FIP), chronic lymphocytic leukaemia.
Other: dermatophytosis, heart disease (the FIP–feline myocarditis relationship has recently been described), polycystic kidney disease, gingivostomatitis complex, feline asthma.
10. Treatment of relapses
Definition and when they occur
Relapses can occur in two ways:
- During treatment: with worsening of symptoms
- During the observation period: in the 12 weeks following the end of treatment
The most plausible explanation is that the virus may have taken refuge in the CNS after a treatment without neurological or ocular involvement but with insufficient doses to cross the blood-brain barrier. Relapses may present differently from the initial clinical picture.
Re-treatment protocol
Re-treatment must be instituted for another full 12 weeks, with doses always higher than those at the end of the previous treatment:
| Route | Minimum re-treatment dose |
|---|---|
| Injectable | 10 mg/kg/day |
| Oral | 20 mg/kg divided into 2 doses |
There is currently no test that can predict which patients will relapse. This remains one of the main challenges of FIP treatment.
Frequently asked questions
Can I start directly with the oral form? Yes, provided the cat is stable: able to swallow, not dehydrated and not in a very advanced stage. If any of the three exclusion criteria apply, start with injectable remdesivir for the first 48 hours.
My cat has gained 0.5 kg. Do I need to change the dose? Yes, necessarily. The dose is calculated in mg/kg, so when weight increases without dose adjustment, the cat receives proportionally less drug. This is one of the main documented causes of therapeutic failure.
At 3 weeks, the effusion hasn’t completely disappeared. Is that normal? Abdominal effusion should be almost resolved between weeks 2 and 3. Pleural effusion normally disappears before day 7. If it persists at 3–4 weeks this is a warning sign that should be communicated to the vet.
Is it safe to give corticosteroids with GS-441524? In very specific cases and for limited periods, yes. Depot formulations are completely contraindicated due to the risk of deterioration at 5–6 weeks.
My cat has FeLV as well as FIP. Does it make sense to treat it? Yes. FIP cure is possible in cats co-infected with FeLV or FIV. You must be honest with the owner about the long-term prognosis of FeLV.
Does a relapse mean the cat can’t be cured? No. Relapses respond to re-treatment with higher doses for another 12 weeks.
Can a pregnant queen be treated? Yes. The current recommendation is to treat. Not treating carries a high risk of foetal loss or neonatal death.
Scientific references
- Krentz D, et al. Curing Cats with FIP with an Oral Multi-Component Drug Containing GS-441524. Viruses. 2021;13(11):2228.
- Zwicklbauer K, et al. Long-term follow-up of cats in complete remission after treatment of FIP with oral GS-441524. J Feline Med Surg. 2023;25(8).
- Pedersen NC, Jacque N. Use of oral GS-441524 for FIP treatment v10. UC Davis, 2021.
- Zuzzi-Krebitz AM, et al. Short Treatment of 42 Days with Oral GS-441524 Results in Equal Efficacy as 84-Day Treatment. Viruses. 2024;16(7):1144.
- Addie DD, et al. Alpha-1 Acid Glycoprotein Reduction in Cats Treated for FIP. Viruses. 2022;14(4):744.
- Taylor SS, et al. Retrospective study of 307 cats with FIP treated with remdesivir and GS-441524. J Feline Med Surg. 2023.
- Allinder M, et al. Uroliths composed of GS-441524 in 2 cats with FIP. J Vet Intern Med. 2024;38(1):370–374.
- Pedersen NC. Feline Infectious Peritonitis in Pregnant Cats. DVM PhD, 2021.
- Yang L, et al. Biotransformation and transplacental transfer of remdesivir and GS-441524 in pregnant rats. eBioMedicine. 2022;81:104095.
- Buchta K, et al. Myocarditis in Cats with FIP Cured with GS-441524. Animals. 2025;15(11):1660.
- AVEPA–GEMFE Protocol, 2024.
Medical disclaimer: This article is informational and educational, based on the clinical article by Vicky Vives Moya and Esther Garrido Cervera (Centro Veterinario, 2025). It does not replace professional veterinary consultation, diagnosis or prescription. FIP treatment must always be carried out under specialised veterinary supervision.