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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Molnupiravir is an oral antiviral that veterinary researchers have tested in cats with feline infectious peritonitis (FIP). The published results are encouraging, but they come from small studies, most of them nonrandomized, and access to the drug varies by country. This article reviews that published evidence: what the studies found, which doses were used in research, which side effects have been reported, and what to ask your veterinarian. It is not a dosing guide or a treatment recommendation.
FIP is a serious disease that has historically been fatal when untreated. If your cat has fluid build-up in the abdomen or chest, fever, weight loss, or other concerning signs, see a veterinarian promptly for assessment rather than starting any antiviral on your own.
What molnupiravir is and why it is studied in cats
Molnupiravir is a nucleoside analogue, a class of antiviral drugs that is incorporated into the genetic material of a replicating virus and disrupts its copying. In cats it has been studied as a treatment for FIP, most often compared with GS-441524, the nucleoside analogue used as the comparator in the studies below. For that reason the two drugs should be read together: a molnupiravir result is usually a result against, or alongside, GS-441524.
Molnupiravir for cats: what the published studies show
Three sources carry most of the feline evidence. They differ considerably in design, so the table is the quickest way to see what each can and cannot tell you.
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| Study | Design | Cats | Reported outcome | Main limitation |
|---|---|---|---|---|
| Reagan et al., Journal of Veterinary Internal Medicine, 2024 | Open-label trial in naturally occurring effusive FIP; molnupiravir given as first-line treatment | 10 treated; 10 historical GS-441524 controls | 8 of 10 alive and in remission at 16 weeks; non-inferior to historical controls | Very small; controls were historical, not concurrent or randomized |
| Sase et al., Frontiers in Veterinary Science, 2024 | Single-center observational study in Japan | 118 (59 per drug) | Remission in 51 of 59 molnupiravir cats and 48 of 59 GS-441524 cats; no significant survival difference | Treatment was not randomly assigned |
| Clark et al., Australian Veterinary Journal, 2025 | Retrospective report | 54 cases (according to the title) | Not stated in the verifiable listing | Outcome details not reported here |
The open-label effusive FIP trial
Reagan and colleagues gave oral molnupiravir as first-line treatment to ten cats with naturally occurring effusive (wet) FIP and compared their outcomes with ten historical GS-441524 controls. The authors reported non-inferiority to the historical controls. Non-inferiority means the new treatment was not meaningfully worse than the comparison; it does not mean the two drugs were tested side by side. Because the controls came from earlier cases rather than being randomly assigned at the same time, the design cannot separate the drug’s effect from differences in patients, care, or timing. The authors concluded that “molnupiravir is an effective antiviral treatment for effusive FIP.” That claim rests on ten animals and should be read as promising rather than settled.
The Japanese observational cohort
Sase and colleagues compared 118 cats treated at a single Japanese center, 59 with each drug. Because treatment was not randomly assigned, the two groups may have differed in ways the authors could not control. Their conclusion was that “our data indicate that GS-441524 and molnupiravir show similar effects and safety in cats with FIP.” That is a finding about this cohort. It is not proof that the two drugs are interchangeable for every cat, every FIP presentation, or every clinic.
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The 2025 guideline and the Australian retrospective report
The 2025 feline FIP guideline from the European Advisory Board on Cat Diseases (ABCD) summarizes the full feline evidence base: small case series, comparative observational work, and the open-label trial. It notes that the studies used variable formulations and protocols, and that larger studies with longer follow-up would help clarify where molnupiravir fits in treatment.
A 2025 retrospective report by Clark and colleagues in the Australian Veterinary Journal describes treatment in 54 cases, according to its title. Only the title and a search listing of that paper could be verified for this article, so its outcome figures are not reported here. Its listing mentions neutropenia and transient liver-enzyme elevations, which are discussed under side effects. Treat its specific results as provisional until you or your veterinarian have read the full paper.
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Molnupiravir dosage for cats: what study protocols used
No single feline dose is established in published studies to date. The figures below describe what researchers gave in specific studies. They are not a recommendation for any individual cat.
| Source | Route and schedule | Dose reported | Treatment length | How the dose was assigned |
|---|---|---|---|---|
| Sase et al., 2024 (Japanese cohort) | Oral | 20 to 40 mg/kg/day | 84 days | By disease type: effusive compared with ocular or neurologic signs |
| ABCD guideline (2025), summarizing other reports | Oral, divided twice daily | Varies by study and disease presentation; no single figure | Often about 84 days | Set by each study’s protocol |
| Reagan et al., 2024 (open-label trial) | Oral | Not stated in the available summary | Not stated in the available summary | Not stated in the available summary |
Why study doses cannot be copied
- Each protocol belongs to a specific study population and clinical team, with its own disease presentations and monitoring.
- The guideline notes that formulations and protocols varied across studies, so a dose in one paper does not transfer cleanly to another product or cat.
- A veterinarian decides the dose, formulation, duration, and monitoring plan for the individual cat, based on its disease presentation, body weight, and laboratory results.
- Human tablets, online or unregulated products, and dose changes made by owners are not appropriate and should be avoided.
Molnupiravir side effects in cats
Side effects reported with molnupiravir in cats come from small or retrospective studies. They describe what has been observed, not how often it happens, and a cat may have none of them.
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- Transient increases in ALT and other lab abnormalities in the Japanese cohort. The authors described these as transient and resolving without specific intervention.
- In the open-label trial, no biochemical or hematologic adverse event required stopping treatment.
- Transient neutropenia (a low count of neutrophils, a type of white blood cell), elevated liver enzymes, and reduced appetite, as summarized in the ABCD guideline.
- Nausea after administration, reported in a feline pharmacokinetic study summarized in the same guideline.
- Neutropenia and transient liver-enzyme elevations, according to the summary listing of the 2025 Australian report.
Monitoring and when to call the veterinarian
- Ask which bloodwork is planned and when. No universal laboratory schedule has been established in the studies to date.
- Contact your veterinarian promptly if the cat stops eating, vomits, worsens, or develops new signs.
How molnupiravir compares with GS-441524
Comparisons are only meaningful when they use the same axes:
- Study design and strength of evidence
- Disease presentation: effusive, non-effusive, ocular, or neurologic
- Outcome definitions and length of follow-up
- Safety observations
- Formulation and local legal availability
The Japanese cohort found no significant survival difference between the drugs. The open-label trial reported non-inferiority only against historical controls. Neither design settles whether one drug is better, and neither supports calling either drug the best choice for all cats.
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Legal status and access
The 2025 ABCD guideline states that no licensed molnupiravir product is available for use in cats, and that use is not legally permitted in every country. It identifies the European Union as a jurisdiction where, according to a 2023 reference it cites, the drug was listed as restricted to human use. That is a dated, jurisdiction-specific statement. Rules change, so confirm the current position with your national veterinary or medicines regulator and with your veterinarian before making any decision about where a drug would come from.
Quick Recap
Questions to ask your veterinarian
- Which study protocol informs your plan, and how does it fit this cat’s disease presentation?
- Where would the drug come from, and is that source legal for cats in our country?
- How long is the planned course, and what result would lead you to change or stop it?
- If the cat is not improving at the planned review point, what happens next, and how does molnupiravir compare with GS-441524 for this case?
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