Felycin or Rapamycin For Your Cat?
Which is Best?
Felycin or Rapamycin for Your Cat — A Quick Take
Same Active Ingredient, Different Formulations.
Felycin®-CA1 (sirolimus delayed-release tablets) and our RapaCat rapamycin (sirolimus) tablets are the same drug—sirolimus—delivered in delayed release tablet form for cats. The only differences are tablet strength and brand name.
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Felycin®-CA1 is a delayed-release formulation of sirolimus that received FDA conditional approval for managing ventricular hypertrophy in cats with subclinical hypertrophic cardiomyopathy (HCM).
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Our generic Rapacat tablets also contain sirolimus and are also considered delayed release. Like Felycin, they are designed to be given to cats once weekly
Bottom line: both Felycin and our generic RapaCat tablets contain the same drug delivered in delayed-release tablets. However, the tablet strengths differ and we should try and match them up with your cat’s body weight for best results.
What Causes Hypertrophic Cardiomyopathy in Cats
Hypertrophic Cardiomyopathy (HCM) is the single most common cardiac disease for cats, affecting about 1 in every 6 cats. It can just be bad luck, or it can be bad genes — especially in Sphynx and Maine Coon kitties.
HCM is typically found when your veterinarian hears a heart murmur during your cat’s exam. Another common
Science Shows that Rapamycin Slows/Stops HCM Progression
The Trivium clinical trial studied delayed-release (DR) rapamycin in cats with subclinical HCM. Published in JAVMA (2023), the study reported that DR rapamycin was well tolerated and may prevent or delay progressive left ventricular hypertrophy—the hallmark structural change in HCM. In practical terms, a majority of treated cats showed stability or improvement in key measures of heart muscle thickness over the study period compared with expected progression.
Why this matters for your cat: untreated subclinical HCM tends to worsen as the ventricular walls thicken and heart function declines. Rapamycin, whether Felycin or generic, targets the mTOR pathway to slow or even reverse structural thickening before heart failure develops, offering cat parents the first way to directly help their cats with HCM.
How Are Felycin and Rapamycin dosed for Cats with HCM?
Both Felycin and our generic rapamycin tablets are intended for once-weekly administration at a dose of 0.3 mg/kg. Although higher doses have been shown to be immunosupressive in humans, these low doses are not thought to cause immune suppression in our cats.
Generic Delayed Release Rapamycin Tablet Strengths
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0.5 mg for ~3.5 lb cats
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1.0 mg for ~7 lb cats
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2.0 mg for ~14 lb cats
Felycin®-CA1 Delayed Release Tablet Strengths
- 0.4 mg for ~3.3 lb cats
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1.2 mg for ~8.8 lb cats
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2.4 mg for ~17.6 lb cats
The significance of different strengths of generic Rapamycin and Felycin: Because these tablets are acid-resistant, they cannot be split or crushed. Thus, from a practical perspective, you should use the tablet, or combination of tablets, that gets you closest to your cat’s current weight.
What are the Side Effects of Rapamycin or Felycin for Your Cat?
In a word or two, not much.
In both our hands and in clinical studies, rapamycin has been very well tolerated in cats. The most common mild issue we see with either Felycin or generic rapamycin is temporary soft stool in about 3–5% of cases, which typically resolves on its own without treatment. (This aligns with published notes that GI signs can occur but are generally mild and transient.)
Just as with any medication, if you see anything that worries you just stop the med and call your local vet.
Vaccine response & immunity: At the recommended cat dosage of 0.3 mg/kg given once weekly, neither Felycin nor Rapamycin is considered immune suppressive. In fact, there are studies showing that low dose rapamycin actually BENEFITS the immune system by improving cancer surveillance.
Hello Dr. Toman –
Good news with Logan’s echo! The thickening completely resolved! Let it be noted that Logan’s condition is probably secondary to the mitral valve dysplasia, but for it to happen this fast I am sure the rapamycin helped! Next echo will be in 6 months instead of 3… Hoping for this outcome to stick. Thank you so much!
(Note that, medicine being medicine, I cannot guarantee any outcome. So far we are 4/5 for improvement in HCM kitties on rapamycin. I will update this as we get more rechecks. — Dr. Kevin)
FAQs on Felycin and Rapamycin For Your Cat
Is Felycin a different drug than rapamycin?
No. Felycin and generic rapamycin are both sirolimus. They are both Delayed Release. The primary difference is simply labeling.
What is the dose for Felycin or Rapamycin in Cats?
The Trivium study above found that a dose of 0.3 mg/kg given once weekly was most effective in cats with subclinical HCM.
How soon will I see changes in my cat’s HCM?
Structural heart changes are tracked over months, not days. The Trivium study showed that many cats do not respond for up to six months of rapamycin, so DO NOT QUIT on your cat!!
Do Rapamycin or Felycin work for Every Cat with HCM?
No. There are no drugs in human or veterinary medicine that ALWAYS work for ALL patients. In my experience, our generic Rapamycin works in about 80% of cases to slow or stop HCM progression.
Can I give Felycin or rapamycin with other heart meds?
Yes—Felycin and Rapamycin are safe to use with the other medications required by HCM kitties, like amlodipine, clopidogrel, , pimobendan and friends.
Do I need ongoing monitoring?
Absolutely. Although many cats on Rapamycin or Felycin show increased activity and affection, there is no way to really KNOW how much they are helping your cat except with echo exams or BNP blood tests.
About Me
Hey, there. Dr. Kevin here. I am a veterinarian who has been in clinical practice treating cats like yours for over 30 years. I live on the Central Coast of California, and have a passion for helping pets live longer. I am particularly interested in longevity drugs like rapamycin, dental care, and cancer therapy.
