FIP Dosing Now: 10-15 mg/kg for 84 Days

FIP is no longer an automatic death sentence, but the number printed on a vial label is not the number your cat needs. Specialist practice for GS-441524 now sits at 10 to 15 mg/kg once daily for 84 days, far above the 2 to 4 mg/kg in the first field reports from 2019. GS-441524 is a prescription-only antiviral: a veterinarian diagnoses, prescribes and adjusts it, and nothing here replaces an exam or promises a cure.

Why the Number Rose From 4 to 15 mg/kg

The early protocols were built on the smallest dose that worked in a colony of sick cats, because drug was scarce and expensive: 2 mg/kg, then 4 mg/kg, then 5 to 6 mg/kg as relapses appeared. Compounded oral oil changed the arithmetic. Absorption of the oral form is variable, so a 4 mg/kg oral dose can deliver less drug than 4 mg/kg injected, and clinicians responded by raising the target to 10 to 15 mg/kg rather than guessing at a conversion factor.

The disagreement is real. Some internists argue that 10 mg/kg injected is over-treatment for a cat with plain effusive FIP, that it doubles cost with no proven benefit, and that dose escalation should be reserved for neuro and ocular cases. Others point at the relapse pattern: cats treated at 4 to 6 mg/kg relapse more often than cats treated at 10 mg/kg, and a second 84-day course costs far more than the extra milligrams did. A reasonable default is 10 mg/kg injected, 15 mg/kg oral, with 15 to 20 mg/kg for neurological or ocular disease.

What the 84 Days Is Actually Based On

Twelve weeks, 84 doses, is the shortest course that held in the field: shorter courses of 4 to 8 weeks produced a wave of relapses 2 to 6 weeks after the last dose, when owners had already stopped watching. The 84-day figure is not a law of pharmacology. It is the point where relapse risk in treated cats fell low enough that stopping became defensible, and it stays fixed because a fixed end date is easier to hold than a symptom-based one.

Two rules follow. The clock starts on the first dose, not on the day the cat improves, and it does not pause for a missed injection. A cat that misses 2 days adds 2 days at the end. Fever usually breaks in 24 to 72 hours and appetite returns in 3 to 7 days, which is exactly why owners stop early around week 6 and exactly why relapses cluster there.

Dose by Weight, and the 84-Day Total

Cat weight	10 mg/kg/day	15 mg/kg/day	Total mg over 84 days
2.0 kg	20 mg	30 mg	1,680 mg at 10 mg/kg
3.0 kg	30 mg	45 mg	2,520 mg at 10 mg/kg
4.0 kg	40 mg	60 mg	3,360 mg at 10 mg/kg
5.0 kg	50 mg	75 mg	4,200 mg at 10 mg/kg

Weigh weekly on the same scale, because a kitten gaining 100 to 200 g a week outgrows a day-1 dose inside 3 weeks and underdosing is the most common cause of relapse. Recalculate the volume every 7 days; never stretch a vial by shaving the dose.

Relapse Rates and What They Look Like

Relapse is reported in roughly 5 to 15% of treated cats, most often 2 to 8 weeks after the last dose, and the risk climbs with three factors: neuro or ocular involvement at diagnosis, a course cut short below 84 days, and a dose below 10 mg/kg. A relapse is not a failed case. A second course of 84 days at a higher dose, usually 15 mg/kg, resolves most of them, which is why owners should budget for one repeat course before starting.

Oral Oil Versus Injection

The route matters as much as the number. Oral GS-441524 in oil crosses the gut, peaks lower and later, and loses part of the dose to first-pass metabolism, so many clinicians add 30 to 50% to an injected protocol when switching to oral. That is why 15 mg/kg oral and 10 mg/kg injected are treated as the same plan. Oral suits owners who cannot inject: 1 dose a day mixed into 3 to 5 g of food, no needles, no injection-site sores. Injected drug peaks higher and is preferred when the cat vomits, carries effusion, or shows neurological signs. Neither route holds a license in most countries; both are compounded, so check the concentration on the label, because 15 mg/mL versus 20 mg/mL changes the volume you draw by a third.

The Post-Treatment Monitoring Calendar

Week after last dose	Day count	Bloodwork	Record at home
Week 4	Day 28	CBC, chemistry, albumin:globulin ratio	Weight, appetite, temperature 2x weekly
Week 8	Day 56	A:G ratio, repeat if it fell	Weight, activity, any fever above 39.5 C
Week 12	Day 84	Full panel, compare to day 0	Weight curve over 12 weeks
Month 6	Day 182	Chemistry, A:G ratio	Weight, appetite, eyes checked
Month 12	Day 365	Chemistry, A:G ratio	Weight, appetite, eyes checked

Red Flags That Mean Call Today

Fever above 39.5 C for more than 48 hours, refusal to eat for 24 hours, labored breathing, a distended abdomen that refills, sudden wobbly walking, or an eye that turns cloudy or holds a fixed pupil. Any of those during or after treatment means the plan needs review, not a bigger dose at home. Injection-site sores appear in a minority of cats after 20 to 40 injections; rotate sites across 4 zones and report any hard lump that lasts beyond 7 days.

What to Do Before Day 1

• Get the diagnosis in writing: effusion analysis, A:G ratio, and imaging, not a guess from a fever.
• Fix the dose and the route with your vet, then buy for 84 days, not 30.
• Book bloodwork at day 0, day 28, day 56 and day 84 so nobody improvises.
• Weigh weekly and log it, since 200 g of growth changes the dose.
• Plan for one repeat course financially. The 12-week treatment timeline covers the daily routine; cancer treatment costs show how a 3-month drug budget compares with other chronic plans, and the cat age tool tells you how much of that budget a 14-year-old cat has left.

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