FIP Treatment: 12 Weeks of GS-441524

Feline infectious peritonitis (FIP) used to end the conversation. Antiviral treatment with GS-441524 changed that for many cats and is now routine practice in a growing number of countries. The course is 12 weeks of daily injection, usually 4 to 6 mg/kg subcutaneously, rising to 8 to 10 mg/kg for neurological or ocular forms. GS-441524 is a prescription drug: a veterinarian must prescribe and supervise it, the dose comes from your vet after examination and bloodwork, and nothing here promises a cure for an individual cat.

What 84 Days Actually Costs You

Treatment is daily for 84 days, and the count matters: stopping at day 70 because the cat looks well leaves virus behind. Most owners inject at home after 2 to 4 teaching visits, so the rhythm is a 5 minute procedure, a weekly weight, and bloodwork at fixed points. Budget the course before day 1: a 4 kg cat at 5 mg/kg needs 20 mg a day, which is 1,680 mg over 84 days, and prices vary by country.

Dose: 4 to 6 mg/kg, 8 to 10 for Neuro and Ocular

The common range is 4 to 6 mg/kg once daily subcutaneously. Many vets start at 5 mg/kg for a cat with effusion and move to 6 mg/kg for one that has not improved by day 7. Neurological FIP (seizures, ataxia, head tilt, nystagmus) and ocular FIP (uveitis, uneven pupils, haze in the eye) need drug across the blood-brain and blood-eye barriers, so the range rises to 8 to 10 mg/kg, sometimes higher under specialist guidance.

Weight sets the volume, so weigh weekly on the same scale. A kitten gaining 100 to 200 g a week outgrows a day-1 dose within 3 weeks, and an underdosed cat relapses. Doses are recalculated for weight and reviewed against bloodwork, never split to stretch a vial.

The First 7 Days Decide the Next 77

Temperature is the clearest single number: a fever of 39.5 to 40.5 C that drops below 39.0 C within 48 to 72 hours is a good sign, and a temperature still above 39.5 C at day 7 usually means the dose or the diagnosis needs review. Appetite is second: 25% of normal intake on day 1, 50% by day 3 and 75% by day 7 is on track. A cat that eats nothing for 48 hours needs supportive care, not a higher dose.

The Monitoring Calendar

Timepoint	Dose (mg/kg/day)	Bloodwork	Record at home
Day 0	4 to 6	CBC, chemistry, albumin:globulin ratio	Weight, temperature, appetite score
Day 3 to 7	4 to 6, reviewed at day 7	None unless worse	Temperature twice daily, grams eaten
Week 4	4 to 6, recalculated	CBC, chemistry, A:G ratio	Weight, appetite, activity
Week 8	4 to 6, recalculated	CBC, chemistry, A:G ratio	Weight, injection sites
Week 12	Last dose on day 84	CBC, chemistry, A:G ratio, imaging if effusion	Final weight
Week 16 and 24	0 (off drug)	Recheck 4 and 12 weeks after the last dose	Weight, any new symptom

Injection Technique and Site Rotation

Use the needle your vet supplies, commonly 21 to 23 gauge. A 4 kg cat at 5 mg/kg of a 15 mg/ml solution is 1.3 ml, a small volume that needs no long needle. Warm the vial in your hand for 2 to 3 minutes, never in hot water, then inject into the scruff or flank with the tent technique. Rotate 5 to 6 sites, moving at least 3 cm each time, and log date, site, volume and reaction.

Site reactions appear in roughly 1 in 10 cats: a firm 1 to 2 cm lump, soreness for 24 to 48 hours, or a scab. Warm compresses for 5 minutes twice a day help. A lump past 3 cm, one that opens, or one still there at 2 weeks needs a vet, because a wound that will not heal means changing technique. Never inject through dirty skin, and never reuse a needle.

Relapse, and What the Numbers Mean

Relapse after a full course is uncommon but real. Clinics commonly quote figures from the single digits to about 10%, with most appearing within 12 weeks of the last dose. That is why the observation window exists, and why stopping early is the biggest avoidable risk: 8 weeks in a cat that looked fine at week 6 carries a clearly higher relapse rate than 84 days. A relapse gets a second course, usually at a higher dose and for longer, with the workup repeated.

The 12 Week Observation Window

From day 85 to day 168 the cat is off drug but not off watch. Weigh every 2 weeks, book rechecks at 4 and 12 weeks after the last dose, and call for any of three things: fever above 39.5 C for more than 48 hours, appetite below 75% of normal for 3 days, or a 5% weight drop in 2 weeks. Keep the cat indoors and away from unknown cats, and do not re-vaccinate or schedule elective surgery inside the window without asking the vet.

Prescription, Sourcing, and the Law

GS-441524 is not a licensed veterinary medicine in every country. Where it is unavailable, vets may prescribe it through a compounding pharmacy or a regulated import route. What does not change: it is a prescription drug, and unregulated powders sold online have no assay, no sterility guarantee and no recourse. Ask for concentration in mg/ml, batch number and receipt. A product with no mg/ml on the label cannot be dosed at 4 to 6 mg/kg, and guessing is how cats run 6 weeks underdosed and relapse.

Support Care, Fleas, and Vaccines

Eighty-four days of under-eating costs muscle, so aim for roughly 60 kcal per kg per day in 4 to 6 small meals, and ask about a feeding tube if intake stays under half for 3 days. Fluids, anti-nausea drugs and pain relief are prescribed case by case. Keep parasite control current: the method in applying topical flea treatment matters more when the cat is thin and sore, and kitten flea safety applies under 6 months. Hold vaccines until the window closes, then rebuild with the vaccine calendar.

Call the Vet the Same Day For

Open-mouth breathing, a temperature above 40.5 C, a seizure, sudden hind-leg weakness, no food for 48 hours, vomiting more than 3 times in a day, or a cat that cannot stand. Also call if the cat is no better at day 7, if fever returns after 5 afebrile days, or if an injection site opens. FIP is diagnosed by a vet from age and history, effusion analysis, imaging and bloodwork, and it can coexist with another disease; treatment started on a guess may be treating the wrong thing.

1. Get the diagnosis, the dose and the vial concentration in writing before day 1.
2. Weigh weekly and recalculate at every 100 to 200 g of gain.
3. Log temperature twice a day for 7 days, then any day the cat feels warm.
4. Book bloodwork at day 0, weeks 4, 8 and 12, then weeks 16 and 24.
5. Budget all 84 days and the 12 week window before the first injection.

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