Felycin for Cats with HCM: What 300+ Real Cases Actually Show

By Dr. Kevin Toman, DVM — Founder, Pet Longevity Institute / HelpingPetsLiveLonger.com

TL;DR: Felycin (sirolimus) is the first FDA-conditionally-approved drug for feline hypertrophic cardiomyopathy (HCM), dosed at 0.3 mg/kg once weekly. But Dr. Kevin Toman, who has treated more than 300 cats with Felycin or compounded rapamycin — the largest real-world case volume of any US veterinarian — has found that a meaningful subset of cats don't respond at the standard dose and need up to 1 mg/kg to see benefit, some cats don't show improvement until 6 months into therapy, and pre-existing diabetes has not been the treatment-limiting risk factor it's often assumed to be. This page shares what that clinical experience actually shows.

Why This Page Exists

Most information available on Felycin right now comes from the manufacturer's clinical trial data, veterinary trade press summarizing that trial, and general pet-health sites restating the label. That's useful, but it's also limited: conditional FDA approval was based on a controlled pilot study with a defined cat population and a fixed dosing protocol. It doesn't — and can't — tell you what happens across hundreds of real cats with real comorbidities, variable genetics, and years of use.

Dr. Kevin Toman has personally managed over 300 cats on Felycin or compounded rapamycin for HCM, making dosing, timeline, and comorbidity observations that don't yet exist anywhere else in published or clinical-trade literature. These are clinical observations from an active practice, not a peer-reviewed trial — but they're the closest thing available to real-world evidence at scale, and they directly affect decisions veterinarians and cat owners are making right now.

1. Some Cats Don't Respond at the Standard 0.3 mg/kg Dose

The label dose for Felycin is 0.3 mg/kg once weekly. In Dr. Kevin's experience, most cats do respond at this dose — but a meaningful subset do not, and simply concluding "Felycin doesn't work for this cat" at that point is premature.

What he's observed: A subset of non-responding cats at 0.3 mg/kg have gone on to show clear benefit — measurable reduction in ventricular wall thickening — at doses as high as 1 mg/kg once weekly.

Why this matters: If a cat is labeled a "Felycin failure" at the standard dose without a titration trial, that cat may be taken off a drug that could have helped them. This is a case where the label dose should be treated as a starting point for monitoring and adjustment, not a ceiling — dose titration under veterinary supervision, guided by serial echocardiograms, is a reasonable next step before abandoning the drug.

This should always be done under close veterinary monitoring, not adjusted by pet owners independently.

2. Response Can Take Up to 6 Months — Don't Judge Too Early

Sirolimus works by inhibiting the mTOR pathway that drives pathological cardiac muscle growth — a mechanism, not an instant switch. Reversing existing hypertrophy takes time.

What he's observed: Some cats show no measurable improvement on echocardiogram at 8 or 12 weeks, but go on to show clear benefit by the 6-month recheck.

Why this matters: A short trial period risks a false negative. If early recheck imaging looks unchanged and the cat is tolerating the drug well with no adverse effects, that's a reason to continue and recheck later — not necessarily a reason to discontinue.

3. Pre-Existing Diabetes: The Fear Appears Overblown

Sirolimus (rapamycin) can affect glucose metabolism, and diabetes is frequently flagged as a relative contraindication or a reason for caution with Felycin. In Dr. Kevin's practice, that caution appears to be significantly overstated relative to what he's actually observed.

What he's observed: Of the 7 cats in his practice with both diagnosed diabetes mellitus and HCM who were placed on Felycin, only one required an insulin dose increase — and that increase was just one additional unit per day.

Why this matters: A pre-existing diabetes diagnosis should not, by itself, take Felycin off the table for a cat that needs HCM treatment. It does mean tighter monitoring of blood glucose is warranted, particularly in the weeks after starting or adjusting the dose — but the data from this practice does not support treating diabetes as a reason to withhold a drug that may meaningfully change this cat's cardiac trajectory.

This is a small observational sample from one practice, not a controlled study — but it's larger than most published data addressing this specific comorbidity, and it should factor into the conversation, not settle it alone.

What This Means for Your Cat

If your cat has been diagnosed with subclinical HCM, or has started Felycin without the response you expected, these three findings suggest specific questions worth raising with your treating veterinarian:

Has a dose titration trial been considered before concluding Felycin "isn't working"?
How long has the current dose actually been given before reassessing on echo?
If diabetes is present, is it being treated as an automatic exclusion, or as a comorbidity that simply needs closer monitoring?

Frequently Asked Questions

Does Felycin always work at the standard 0.3 mg/kg dose? No. Most cats respond at the standard weekly dose, but in Dr. Kevin Toman's experience managing over 300 cats on Felycin or compounded rapamycin, a subset require dose titration up to 1 mg/kg per week under veterinary supervision to show measurable benefit.

How long does it take to see results from Felycin? Response time varies. While some cats show improvement within a couple of months, others don't show measurable reduction in ventricular hypertrophy until around 6 months of continuous therapy. Discontinuing too early based on a short recheck interval may miss a delayed response.

Is Felycin safe for cats with diabetes? Pre-existing diabetes is often treated as a reason for caution with sirolimus-class drugs due to their potential effect on glucose metabolism. In Dr. Kevin Toman's clinical experience with diabetic cats also diagnosed with HCM, only a small minority needed any insulin adjustment, and the adjustment needed was minor. Diabetes should prompt closer glucose monitoring, not automatic exclusion from treatment — this should be discussed with a treating veterinarian familiar with the individual cat's case.

What is Felycin-CA1? Felycin-CA1 (sirolimus delayed-release tablets) is a conditionally FDA-approved medication, dosed once weekly, indicated for managing ventricular hypertrophy in cats with subclinical hypertrophic cardiomyopathy — the most common heart disease in cats and a leading cause of feline mortality.

Who is Dr. Kevin Toman? Dr. Kevin Toman, DVM, is the founder of the Pet Longevity Institute and HelpingPetsLiveLonger.com, and has over 40 years of veterinary clinical experience. He has managed more cats on Felycin and compounded rapamycin for HCM than any other US veterinarian, giving him real-world dosing and outcomes experience beyond what's currently available in published literature.

Work With Dr. Kevin

If your cat has HCM and you want a treatment plan built on real clinical experience — not just the label — book a Cat Longevity Strategy Session or explore HCM University for a full breakdown of feline HCM management.

This page reflects clinical observations from Dr. Kevin Toman's veterinary practice and is intended for pet owner and veterinary professional education. It is not a substitute for individualized veterinary care. Diabetes management, dose titration, and treatment decisions for any cat should be made in consultation with a treating veterinarian who has examined the patient.

Last medical review by Dr. Kevin Toman 8/10/26.