Molnupiravir (EIDD-2801) for FIP: the oral antiviral, and how it compares to GS-441524
Molnupiravir is an oral antiviral used both as a primary FIP treatment and as salvage therapy when GS-441524 falls short. Head-to-head data put it on par with GS-441524. Here's what the studies show, and how EIDD-2801 and EIDD-1931 fit together.
Our Beloved Friends Research · August 21, 2026 · 9 min read
Short answer: molnupiravir (EIDD-2801) is an oral antiviral whose active form is EIDD-1931. In FIP it's used both as a primary treatment and as salvage therapy for cats that relapse or don't respond to GS-441524. Head-to-head data show it works about as well: in one comparison, 51 of 52 molnupiravir-treated cats reached remission, versus 48 of 48 on GS-441524.
What it is
Molnupiravir works differently from GS-441524. Rather than stalling the viral copier, its active metabolite is incorporated into new viral RNA and drives lethal mutagenesis — it makes the virus copy so many errors into itself that it can no longer function. Same goal, different mechanism, which is part of why it's useful when the virus has stopped responding to GS-441524.
The name situation confuses people, so plainly:
- EIDD-2801 = molnupiravir — the prodrug you actually administer (orally).
- EIDD-1931 — the active metabolite the body converts it into, which does the antiviral work.
They are the same therapy described at two stages, exactly as remdesivir and GS-441524 are.
What the evidence shows
Molnupiravir has moved quickly from salvage-only to a genuine primary option, on the back of real data:
- A head-to-head study found GS-441524 and molnupiravir similarly effective and similarly safe — 48/48 vs 51/52 cats into remission. Read on Frontiers
- A 2025 case series of 54 cats treated with oral molnupiravir reported good outcomes over a median 84-day course at a median dose around 13 mg/kg twice daily. Find on PubMed
- A 2025 study of the course of improvement on molnupiravir adds detail on how cats recover on it. Read on PMC
Where it fits
- Primary oral treatment — for owners and cats who do better with pills than injections, molnupiravir is now a defensible first choice on its own evidence.
- Salvage / relapse therapy — its different mechanism makes it valuable when GS-441524 stops working or a cat relapses. Switching mechanisms can succeed where more of the same would not.
- Combination — some protocols use it alongside GS-441524 in difficult cases.
The honest caveats
- Molnupiravir's mutagenic mechanism means dosing and veterinary supervision matter — it is not a supplement, and the twice-daily oral schedule has to be kept.
- As with GS-441524, much of the supply is unregulated, with the same product-quality risks.
- It is generally not used in pregnancy.
Use the dosage calculator to see how weight scales a dose — as an estimate to bring to your vet, not a prescription.
Not veterinary advice. Molnupiravir dosing and sourcing are decisions for a veterinarian who has examined your cat.
References
- GS-441524 and molnupiravir are similarly effective for the treatment of cats with FIP. Frontiers in Veterinary Science, 2024. Read on Frontiers
- Clark et al. Treatment of FIP in cats with molnupiravir: clinical observations and outcomes for 54 cases. Australian Veterinary Journal, 2025. Find on PubMed
- Evaluation of the course of improvement with molnupiravir treatment for FIP. 2025. Read on PMC
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