Molnupiravir Treatment Guidelines

Treatment Recommendations with Molnupiravir:

Obtaining antiviral drugs (GS-441524, molnupiravir, or other) through prescription from licensed compounding pharmacies only is recommended and should always be used under the guidance of a veterinarian with a direct veterinary-client-patient relationship.

Caution is recommended when exceeding a dosage of 15 mg/kg q12hr. Side effects are dose dependent with Molnupiravir and thus extra vigilance is advised when using higher dosages (Evans & Jacque, 2024).

Molnupiravir (EIDD 2801) can be given with food. Adjusting the dose regularly as the cat gains weight is vital and owners should be encouraged to buy a pet or baby scale and weigh their cat weekly. One of the most common causes of an inadequate treatment response or treatment failure is not adjusting the dose in line with weight gain. 

Note: Cats with effusive FIP will lose weight initially as the fluid absorbs. Never decrease the dose with weight loss but instead maintain the initial starting dose until the cat has gained enough weight to warrant an increase (Taylor et al., 2025).

Treatment Duration Guidelines:

An 84-day (12-week) treatment course is currently the recommended treatment length. Further research is needed to determine if shorter treatment durations could be equally effective (Reagan et al., 2024; Coggins & Kennedy, 2024).

Response to Treatment:

Evans & Jacque (2024) have observed that Pyrexia and inappetence are expected to resolve within 2-7 days of beginning treatment. Pleural and abdominal fluid typically resolves within 1-2 weeks. Moreover, it is normal to see a mild decrease in the cat’s weight (in effusive cases), an increase in globulins and decreased HCT during this time and as long as the cat is improving clinically, there is no cause for concern. In some cases, a small amount of fluid may remain for weeks or months. It is usual to see improvement in neurological and ocular signs within 5 days of beginning treatment, and resolution is expected for most cats within 2 weeks.

When evaluating bloodwork, hyperbilirubinemia and leukogram abnormalities are expected to resolve within 2-3 weeks. The a/g ratio typically rises to above 0.6 within 6-10 weeks of treatment (Evans & Jacque, 2024) but is not a sensitive indicator of clinical resolution.

Monitoring Response to Treatment – Recommendations:

• Prioritize evaluating clinical response during the first 2-4 weeks of therapy
• Clinical exam and weight check every 2 weeks OR client can weigh cat weekly at home
• CBC and Biochemistry at 4 weeks. Repeat at 6-8 weeks and 10-12 weeks
• If a dosage increase is required, schedule a follow-up 1-2 weeks after dosage increase to evaluate response
• If a treatment extension is necessary, recheck every 2 weeks until remission is achieved

Supportive Care:

– Cats on Molnupiravir may need antiemetics. They also may not gain as much weight throughout the treatment period as cats on GS-441524 (Coggins & Kennedy, 2024).

– Consider prescribing antiemetics such as cerenia or ondansetron, as well as appetite stimulants, such as mirtazapine, for cats who appear nauseous or suffer from inappetence.

– Cats in critical condition may need additional supportive care, such as feeding tubes, anticonvulsants, IV or SubQ fluids, corticosteroids and at times a blood transfusion (Taylor et al., 2025). 

Navigating Use of Molnupiravir and When it is Indicated:

Molnupiravir has been shown to be effective treatment both as a frontline therapy and for relapse and refractory cases. When used within the recommended dosage range, it has been associated with minimal side effects. (Coggins & Černá, 2025).

Molnupiravir is recommended to be used for:

• Cats showing insufficient response to GS-441524 despite being treated at a therapeutic dosage
• Cats exhibiting signs of regression or possible resistance to GS
• Relapse cases that have been treated with GS-441524 at a therapeutic dosage (Taylor et al., 2025)
• Situations where access to GS-441524 is not feasible (Evans & Jacque, 2024)

Signs of regression or drug resistance may include:

• Recurring or persistent effusions (note: scant fluid at the end of treatment can be normal if all other clinical signs are normal) during GS-441524 treatment
• Pyrexia
• New or worsening ocular or neurological symptoms
• Declining appetite or noticeably worsening energy level

Consider the following steps:

• Reconfirm the FIP diagnosis and rule out other underlying conditions
• Increase the GS dosage by 5-10 mg/kg/day and consider splitting the dose q12hr if the dose previously was given q24hr 
• Switch to an alternative antiviral such as Molnupiravir. (Taylor et al., 2025) 

Relapses:

In some cases, relapsing cats may have initially presented with neurological or ocular FIP and if the starting dosage was too low this can contribute to a relapse. The timeframe of highest risk for relapse was found to be within 60 days of stopping GS treatment. Symptoms to monitor for include inappetence, lethargy, weight loss, pyrexia, ascites, jaundice, ocular symptoms, and neurological involvement. Lab work may or may not show the common FIP markers (Taylor et al., 2025).

As Evans & Jacque (2024) explain, for rescue therapy – it does not matter what dosage (even if significantly high) of GS-441524 the cat was on. The recommended starting dosages for Molnupiravir still apply regardless. This is a completely different antiviral, and the previous dosage of GS has no bearing on the dosage used for Molnupiravir when dealing with a relapse or resistance case. It is recommended to start fresh with Molnupiravir dosages and choose the appropriate dosage depending on the symptoms. 

Moreover, for relapse and refractory cases you may also choose to combine Molnupiravir, which is a nucleoside analog, with a protease inhibitor such as Paxlovid (Evans & Jacque, 2024).

Side Effects and Precautions:

A number of studies have documented several adverse effects with Molnupiravir treatment, including nausea, hyporexia, brittle whiskers, hair loss, ears folding, leukopenia, and elevated ALT. Typically, if you stay within the recommended dosage range with Molnupiravir you will not see the side effects mentioned (Evans & Jacque, 2024). All side effects generally resolve when the medication is stopped (Taylor et al., 2025; Reagan et al., 2024; Sase et al., 2024; Sase. 2023; Roy et al,. 2024).

Molnupiravir’s mechanism of action involves targeting RNA synthesis and is mutagenic for human RNA (Zhou et al., 2021).It also has teratogenic properties and should not be used in pregnant cats (Taylor et al., 2025).

It is recommended for caregivers to wear gloves when handling the medication. Pregnant women should avoid handling the medication. (Coggins & Černá, 2025).

Additionally, there is the risk of viral and host mutagenesis with Molnupiravir (Evans & Jacque, 2024). Some variants of concern of COVID-19 have been shown to be driven by molnupiravir use in humans; thus, there is concern for mutations in feline coronavirus to be driven by this drug.

Molnupiravir (EIDD 2801) vs EIDD 1931:

In terms of efficacy, administering the prodrug (Molnupiravir) is analogous to using the core nucleoside analog (EIDD 1931). They can likely be considered therapeutically equivalent (Coggins & Kennedy, 2024), though much less data exists on the use of EIDD 1931 in cats.

Side effects appear to be dose dependent for Molnupiravir and adverse effects such as leukopenia, folded ears and broken whiskers were generally only seen at dosages above 23 mg/kg BID (Roy et al., 2022). 

We only have one study so far using EIDD 1931 (Evans & Jacque, 2024), and a higher incidence of side effects was noted. Moreover, in one prospective study of 9 cats treated with EIDD 1931, side effects did not seem to be dose dependent, and cats on the lower end of the dosage range also experienced effects such as neutropenia, broken whiskers and hyporexia (Coggins & Kennedy, 2024).

References:

Coggins, S. & Černá, P. (2025). Pharmacologic approaches to feline infectious peritonitis. https://todaysveterinarypractice.com/pharmacology/pharmacologic-approaches-to-feline-infectious-peritonitis/

Coggins, S. & Kennedy, A. (2024, Dec. 12). The use of EIDD-1931 and Molnupiravir in the treatment of FIP (Webinar). Epicur Pharma. https://epicurpharma.com/webinars/on-demand-webinar-eidd-1931-and-molnupiravir-in-fip-treatment

Evans, S. & Jacque, N. (2024). The evolution of FIP treatment: Understanding compounded Molnupiravir and its role in treating Feline Infectious Peritonitis (Webinar). Wedgewood Pharmacy https://event.on24.com/wcc/r/4760171/870A0E49FA78A4DD8090C04FC18AA841

Reagan, K. L., Brostoff, T., Pires, J., Rose, A., Castillo, D., & Murphy, B. G. (2024). Open label clinical trial of orally administered molnupiravir as a first-line treatment for naturally occurring effusive feline infectious peritonitis. Journal of veterinary internal medicine, 38(6), 3087–3094. https://doi.org/10.1111/jvim.17187

Roy, M., Jacque, N., Novicoff, W., Li, E., Negash, R., & Evans, S. J. M. (2022). Unlicensed Molnupiravir is an Effective Rescue Treatment Following Failure of Unlicensed GS-441524-like Therapy for Cats with Suspected Feline Infectious Peritonitis. Pathogens (Basel, Switzerland), 11(10), 1209. https://doi.org/10.3390/pathogens11101209

Sase O. (2023). Molnupiravir treatment of 18 cats with feline infectious peritonitis: A case series. Journal of veterinary internal medicine, 37(5), 1876–1880. https://doi.org/10.1111/jvim.16832

Sase, O., Tomoko, I., Sasaki, T. & Sanao, T. (2024). GS-441524 and molnupiravir are similarly effective for the treatment of cats with feline infectious peritonitis. Frontiers in Veterinary Science, 11 .https://doi.org/10.3389/fvets.2024.1422408

Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S. (2025). icatcare_fipupdate_July25. https://icatcare.org/resources/icatcare_fipupdate_july25.pdf

Teli, D., Balar, P., Patel, K., Sharma, A., Chavda, V., & Vora, L. (2023). Molnupiravir: A Versatile Prodrug against SARS-CoV-2 Variants. Metabolites, 13(2), 309. https://doi.org/10.3390/metabo13020309

Zhou, S., Hill, C. S., Sarkar, S., Tse, L. V., Woodburn, B. M. D., Schinazi, R. F., Sheahan, T. P., Baric, R. S., Heise, M. T., & Swanstrom, R. (2021). β-d-N4-hydroxycytidine Inhibits SARS-CoV-2 Through Lethal Mutagenesis But Is Also Mutagenic To Mammalian Cells. The Journal of infectious diseases, 224(3), 415–419. https://doi.org/10.1093/infdis/jiab247