The treatment of feline infectious peritonitis (FIP) in the UK – an update FIP treatment protocols – what’s new?

Last updated: 07/2025

Sam Taylor BVetMed(Hons) CertSAM DipECVIM-CA MANZCVS FRCVS

Séverine Tasker BVSc BSc DSAM PhD DipECVIM-CA FHEA FRCVS

Danielle Gunn-Moore BSc(Hon), BVM&S, PhD, MANZCVS, FHEA, FRSB, FRCVS

Emi Barker BSc BVSc PhD PGCertTLHE DipECVIM-CA MRCVS

Stephanie Sorrell BVetMed(Hons) MANZCVS DipECVIM-CA MRCVS

Thank you to Richard Malik & Sally Coggins for their advice and assistance in producing this document.

The above specialists have come together to run the ‘FIP advice’ email address (fipadvice@gmail.com) answering queries on the new treatments on a voluntary basis and disseminating information to vets and vet nurses in the UK.

• FIP treatment protocols – what’s new?
• Use of oral GS-441524 for the whole treatment course, including at the start
• Injectable remdesivir is reserved for specific cases
• What dosages of GS-441524 and remdesivir should I use when treating FIP?
• What should I do in the event of relapse of FIP?

If you are a cat owner, please watch our latest webinar Feline Infectious Peritonitis in 2024: Updates for the Caregiver (2024)

FIP treatment protocols – what’s new?

(Updated July 2025)

Antivirals that veterinarians can legally prescribe vary by country, but are often acquired via compounding pharmacies. These include remdesivir, GS-441524, molnupiravir and EIDD-1931, which are all nucleoside analogues, acting by interfering with viral RNA replication. At the time of writing, countries with access to compounded nucleoside analogues include Australia, Canada, Cyprus, Czech Republic, Dubai, Finland, France, Germany, Hong Kong, India, Ireland, Japan, New Zealand, Norway, Portugal, Singapore, South Africa, Sweden, Switzerland, UK, and USA. Some countries, like the USA, UK and Australia, also have access to the EIDD-1931, which is the prodrug of molnupiravir (MPV, sometimes called EIDD-2801). Antivirals that inhibit the 3C-like protease inhibitor also exist. These include GC-376 and nirmatrelvir but they are far less commonly used than the nucleoside analogues and this stage, are not recommended as monotherapy, they are always combined with a nucleoside analogue.

Treatment of individual cases remains the responsibility of the attending veterinary surgeon. The dosages below are based on experience using reputable preparations of known antiviral content. Extrapolation is not applicable to other oral preparations where the active component and/or its content are not known or provided by the manufacturer.

Use of oral GS-441524 for the whole treatment course, including at the start

Oral GS-441524 (available as a suspension 50 mg/mL and tablets 50 mg) can be used from the start of, and for the entire, FIP treatment course (typically 6-12- weeks/42- 84-days; see later regarding the duration of treatment courses). It is important to support owners in medicating their cats, which can be challenging. Further study is needed to review the effect of food on GS-441524 absorption, but it is recommended to give GS-441524 on an empty stomach or in a small treat (tablets can be crushed for this), leaving a gap of 30 minutes or more before feeding a larger meal. Fasting cats overnight can increase their hunger to facilitate medicating in the morning, and similarly for an evening dose.

Injectable remdesivir is reserved for specific cases

Injectable veterinary reformulated remdesivir (10 mg/mL) or human-licensed remdesivir (100mg/vial) is effective in the treatment of FIP but is associated with side effects, particularly pain on subcutaneous injection. Previous FIP treatment protocols suggested that remdesivir should be used at the start of FIP treatment, before transitioning to oral GS-441254, but we now know that oral GS-441524 is effective when given from the start of FIP treatment. Injectable remdesivir, given intravenously to avoid subcutaneous injection pain, is now only indicated over oral GS-441524 in the following situations:

• Severe neurological signs with inability to swallow or tolerate oral medication; Extremely dehydrated/unwell cats;
• Cats that cannot be orally medicated for other reasons.

If a cat has a poor appetite and/or is sick, affecting the ability to medicate it, hospitalisation for supportive care and 48 hours of intravenous remdesivir can result in significant clinical improvement, facilitating transition to oral medication with GS-441524. The remainder of the treatment course can then be given as oral GS-441524. The transition from remdesivir to oral GS-441524 can be immediate, i.e., from one treatment to the next.

A compounded oral remdesivir preparation (capsule form) has been used successfully to treat cats with FIP when GS-441524 was not available, for example, in New Zealand (Renner et al., 2025).

What dosages of GS-441524 and remdesivir should I use when treating FIP?

With experience and as yet unpublished data on therapeutic drug monitoring (TDM), dosage recommendations have increased for antivirals from initial FIP treatment protocols. However, one must remember that published evidence shows that over 85% of cats respond to the previously recommended drug dosages, which is still a great response. We believe that individual cats vary in their absorption of oral GS-441524, with those absorbing poorly possibly requiring higher dosages to achieve clinical and biochemical remission. It is important that the dosage of oral GS-441524 is adjusted according to clinical response if needed.

Based on our collective experience, recommendations are:

• The daily dosage of oral GS-441524 can be given once daily (q24h) or divided (split) twice daily (q12h);
• Some cats may benefit from q12h treatment to optimise serum levels of GS-441524, but this is not uniform;
• For cats that are challenging to medicate and responding well, q24h treatment is acceptable;
• Higher dosages of oral GS-441524 (20 mg/kg divided into 10 mg/kg q12h) may overcome issues with poor absorption in some cats and have a better chance of crossing the blood- brain barrier and the blood-eye barrier in cats with ocular or neurological signs
• Some have used a standard dosage of 15 mg/kg/day q24h in all cats, regardless of clinical signs (Zuzzi-Krebitz et al., 2024). In this study, all cats were hospitalised for intensive care in the first week of treatment, and this may contribute to the success of this dosage of GS-441524;
• Dosage should be adjusted according to the clinical response of the cat and there can be variation in dosage required despite dosing for type of FIP.

Table 1: ^PO (orally) – give fasted with a water bolus or a tablespoon of wet food/treat, with a full meal at least 30 minutes later.

† Divided dose may improve plasma concentrations when using oral GS-441524 and is preferred by some authors, but q24h dosing remains highly effective and appropriate, particularly where compliance may be an issue. Additionally, GS-441524 at a standard dosage of 15 mg/kg q24h PO has been successfully used by one research group for all types of FIP, regardless of signs; it may be that the very prompt treatment, hospitalisation and intensive care of the cats given this one standard dosage contribute to the reported success.

*Slow IV (intravenously) – Give as constant rate infusion (CRI) over 30 minutes to 2 hours, can be diluted in saline.

**These EIDD-1931 dosages are currently recommended for relapses; for first-line therapy, lower dosages may be considered as 20 mg/kg Molnupiravir = ~16 mg/kg EIDD-1931, hence consider reducing the dose by a 5th; but more studies are needed.

Cats should be re-examined after 1-2 weeks (sooner if not improving or deteriorating) and dosage adjusted depending on monitoring at this point.

NOTE ON WEIGHING CATS: It is very important to weigh cats weekly during treatment, using accurate scales, e.g., cat or baby scales. Weight gain and/or growth in kittens will occur with successful treatment, necessitating an increase in dose to ensure that the dosage of antiviral administered is still appropriate for the type of FIP being treated as in Table 1. Not increasing the dose as the kitten grows appears to be one of the most common causes for a poor response to treatment, and treatment failure. However, if the cat’s weight decreases as a result of resolution of effusions, the dose should not be decreased; maintain the same dose and adjust upwards if weight gain occurs.

What should I do in the event of a poor response during treatment or relapse?

Examples of this include recurrence or lack of resolution of effusion, pyrexia, development of new ocular or neurological signs, or persistent clinical pathology abnormalities (note persistent mild hyperglobulinaemia has been reported and not associated with relapse, provided other parameters normalised). Firstly, ensure that you are still confident that the cat has FIP; review the diagnosis, look for additional pathology, and consider repeat sampling (e.g., external laboratory analysis and culture of any fluid; cytology or biopsy of lymph nodes ± feline coronavirus antigenor RNA detection by immunostaining or reverse-transcriptase PCR, respectively, but bear-in- mind that finding coronavirus is more difficult when the cat is on treatment). Reassess acute phase proteins; however, they can remain normal with relapse.

If relapse occurs during treatment; increase the dosage of GS-441524 (or remdesivir) by 5-10 mg/kg/day and consider splitting into q12h doses if treated orally q24h) and monitor as above, ensuring treatment is not stopped before the cat has been normal clinically and on clinical pathology results for at least 2 weeks. The increased dosage used will depend on the dosage the cat is on at the time of the relapse, the nature of the relapse and finances, but can be up to that recommended for neurological FIP (see Table 1) or even higher (please seek guidance when considering this). Similarly, if relapse occurs on treatment with molnupiravir or EIDD-1931, consider switching to GS-441524 if available, or increase the molnupiravir or EIDD-1931 dosage to the maximum recommended (see Table 1), or consider nirmatrelvir, as discussed earlier under ‘Treatment protocols’.

In the event of relapse after completion of treatment

Relapse is most likely to occur in the first month after completing treatment and may present differently to the original diagnosis (e.g., effusive case developing neurological signs). If relapse occurs after completion of treatment on GS-441524, restart GS-441524 (or remdesivir) course at a higher dosage (20 mg/kg/day; splitting oral GS-441524 into q12h doses); the optimum duration for repeat treatment is not known, but 12-week repeat treatment has been used successfully. The increased dosage used will depend on the dosage the cat was previously treated with and the nature of the relapse, but can be up to that recommended for neurological FIP (see Table 1).

If the cat is already receiving a high dosage of GS-441524 (and/or TDM serum levels are adequate, if measured or available), consider switching to an alternative antiviral, e.g., EIDD-1931, and seeking guidance (FIP advice email or your local feline specialist). If considering Paxlovid™ for a non-responding FIP case, a dose of nirmatrelvir 75 mg/cat plus ritonavir 25 mg/cat, q12h by mouth, can be considered alongside continuing nucleoside analogue treatment. Ritonavir may interfere with the metabolism of other drugs processed by cytochrome P450, so check for potential interactions before prescribing.

Treatment with Molnupiravir (EIDD-2801)

Molnupiravir (EIDD-2801) is another nucleoside analogue that inhibits viral replication and is metabolised into EIDD-1931 (NHC). In the USA and Australia, molnupiravir and EIDD-1931 are available from compounding pharmacies. Initial use was as a second-line antiviral for cats that failed to respond to remdesivir / GS-441524. However, recent studies suggest that it may be used as a primary treatment option. Molnupiravir is subject to restrictions on its use in Europe but is legally available in other regions. It has been used successfully to treat FIP, used as both a first-line agent and as a rescue treatment in cats that relapse following GS- 441524 treatment. It can be mixed with food for administration. Safety concerns include a narrower therapeutic window, potential for neutropenia and risk of generating FCoV mutations (see ‘Side effects of antivirals’). Current recommendations suggest caution when exceeding 15 mg/kg every 12 hours. When considering the use of molnupiravir, it is also important to note its mutagenic and teratogenic properties, and the observation of viral resistance observed in COVID-19 cases.

Use of molnupiravir should be reserved for:

• Cats failing to respond to treatment with GS-441524 (or remdesivir) despite adequate dosage;
• Cats relapsing after treatment with GS-441524 (or remdesivir) at adequate dosages;
• Regions where molnupiravir is the only antiviral legally available to veterinarians to prescribe.

Treatment with EIDD-1931

EIDD-1931 is the active form of molnupiravir and is legally available for veterinarians to prescribe in some countries in a 60 mg tablet form. Our knowledge of the usage of EIDD-1931 is far less than for GS-441524. Similar to molnupiravir, it appears to be associated with more severe side effects than GS-441524, and also has teratogenic concerns, thus, its use should be reserved for difficult cases and where it is the only antiviral legally available to veterinarians to prescribe in these circumstances.

Treatment with feline interferon (IFN), polyprenyl immunostimulant, or mefloquine

• Combinations of IFN omega, polyprenyl immunostimulant and/or mefloquine have been used in the period following the end of treatment with GS-441524 (or remdesivir) in some cats. However, currently, there is no evidence to suggest they are needed as high response rates of over 85% are seen without these adjunct treatments.
• Mefloquine has also been used to treat cats with FIP when cost constraints absolutely prohibit the use of a full course of, or increased dosage of, more effective antivirals such as GS-441524. Studies are needed to evaluate its effectiveness but it should only be used when absolutely no alternatives are available, as GS- 441524 is known to be very effective, and far more effective than mefloquine.

For information about what to monitor while treating FIP please read ‘What to look out for when treating FIP’. We also have some hints and tips based on common questions from vets which you can read FAQs about treating FIP with oral GS-441524 and/or injectable remdesivir or Feline Infectious Peritonitis (FIP) Updates | BOVA. If you prefer webinar content, please watch ‘Feline Infectious Peritonitis – A new era in diagnosis and treatment’ or find out recent webinars here ‘Feline Infectious Peritonitis (FIP) Updates | BOVA’.

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Further reading

Tasker, S.; Addie, D.; Egberink, H.; Hartmann, K.; Hofmann-Lehmann, R.; Hosie, M. J.; Truyen, U.; Belak, S.; Boucraut-Baralon, C.; Frymus, T.; Lloret, A.; Marsilio, F.; Pennisi, M. G.; Thiry, E.; Mostl, K., ABCD Guidelines Feline Infectious Peritonitis – factsheets & diagnostic tools. 2022, accessed 16^th January 2023. https://www.abcdcatsvets.org/portfolio-item/factsheets-tools-for-feline-infectious-peritonitis-fip/

Thayer, V.; Gogolski, S.; Felten, S.; Hartmann, K.; Kennedy, M.; Olah, G. A., 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines. Journal of Feline Medicine and Surgery 2022, 24, (9), 905-933. https://journals.sagepub.com/doi/full/10.1177/1098612X221118761

Figure 1: Oral GS-441524 tablets

Figure 2: Remdesivir for intravenous or subcutaneous injection

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