Can HCM Be Reversed in Cats?
By Dr. Kevin Toman, DVM · Medically reviewed November 1, 2026 · 40+ years in clinical practice · 300+ cats with HCM treated
Short answer: It depends on the cause. If the heart wall is thick because of high blood pressure or an overactive thyroid, treating that problem can help the heart improve. True HCM has no cure yet. But in a 2023 trial, a weekly Felycin-CA1 (rapamycin) tablet stopped the wall from getting thicker over 6 months in cats with early HCM. That's a real step forward. It is not the same as a cure, and Felycin must be considered a lifetime medication.
Three words that sound alike but aren't
People often mix these up. Knowing the difference will help you judge any claim you read online.
| Word | What it means | Is it possible today? |
|---|---|---|
| Fixing a cause | Treating a problem that made the heart thick, like high blood pressure | Yes, for secondary thickening |
| Improving a measurement | The heart wall measures thinner on ultrasound, or stops getting thicker | In a minority of cats with sirolimus in early HCM |
| Curing HCM | The disease is gone for good | No, not yet |
Step 1: Rule out the "fixable" causes
Not every thick heart is true HCM. Two common problems can make the heart wall thicken:
- High blood pressure (hypertension). The heart has to push harder, so the muscle grows.
- Overactive thyroid (hyperthyroidism). Extra thyroid hormone makes the heart work too hard.
When these are treated, the heart wall often gets thinner again. That's why every cat with a thick heart wall should have a blood pressure check and a thyroid test.
From Dr. Kevin's 400+ Cats with HCM:
Many vets rely only upon echo exams to diagnose HCM, and those that do are missing common diagnoses -- like hypertension or hyperthyroidism-- that are FIXABLE.
Make sure your vet checks for these concerns, and -- while they are at it -- have them get a urinalysis and SDMA blood test to check your cat's kidney function.
Step 2: Understand true HCM
In primary HCM, the heart muscle itself is the problem. In some breeds, like Maine Coons and Ragdolls, a gene change is known to raise the risk. In many cats, no single cause is found.
For decades, vets could only treat what HCM causes: blood clots, fluid in the lungs and heart rhythm problems. Nothing targeted the thickening itself.
Step 3: The new science, Felycin-CA1 (sirolimus)
Sirolimus is also called rapamycin. It slows down a "grow" signal inside cells, called mTOR. In HCM, that signal seems to help drive the heart muscle to keep thickening.
Scientists asked: if we turn that signal down, will the heart stop thickening?
What the RAPACAT trial found
- Who: Cats with HCM and no symptoms (subclinical HCM).
- What: A delayed-release form of sirolimus given once a week, or a placebo (a sugar pill).
- How long: 6 months.
- Result: At 6 months, cats on the low dose had a thinner maximum heart wall than cats on the placebo. In the treated cats, the wall stopped getting thicker. In the placebo cats, it kept growing.
- Safety: Side effects were similar between the groups during the trial.
What the trial did not show
- It did not show that cats lived longer.
- It did not show fewer cases of heart failure or blood clots.
- It did not study cats that already had heart failure.
- It lasted only 6 months.
A much bigger trial, the HALT study, is testing this in about 300 cats for 12 months at more than 20 US clinics. Results are pending.
Felycin-CA1: the first approved drug for HCM in cats
Based on this research, the US FDA conditionally approved Felycin-CA1 in March 2025. It is a cat-specific, delayed-release sirolimus tablet. It is approved to manage heart wall thickening in cats with subclinical HCM (no symptoms yet).
"Conditional" approval means the FDA found the drug safe and found a reasonable expectation that it works. The company must still finish the bigger study for full approval.
Read our full guide to HCM here
Does a thinner heart wall mean HCM is gone?
No. A thinner wall on ultrasound is good news. But your cat still has HCM and still needs checkups. Vets also look at:
- Left atrium size and squeeze
- How well the heart relaxes and fills
- Heart rhythm
- Signs of fluid or clots
- Quality of life
- Side effects of any treatment
Be careful with any website or seller that promises to "reverse" or "cure" HCM.
7 questions to ask your vet
- Is this true HCM, or could high blood pressure or thyroid disease be part of it?
- What stage is my cat (A, B1, B2, C or D)?
- Is the left atrium enlarged?
- What is my cat's risk of a blood clot?
- Is my cat a candidate for Felycin?
- Does my cat have any liver disease or diabetes?
- When should we repeat the ultrasound?
Not sure your cat qualifies? Take the 3-minute Felycin candidate check → [DR. KEVIN: add link]
Outside the US?
We ship Felycin routinely to the US, UK, and Canada. You start by sending your vet's prescription, or by enrolling your cat in our Cat Longevity Protocol. You can purchase Felycin from us and get guaranteed delivery, although it sometimes takes a few days.
If you live in the EU, Taiwan, Hong Kong, or China you can purchase Felycin from us. You start by enrolling your cat in our Cat Longevity Protocol. You provide the shipping, and we cannot guarantee delivery but we can absolutely send it your way.
Frequently asked questions
Can a cat's heart wall go back to normal thickness?
Sometimes, when the thickening was caused by high blood pressure or thyroid disease and that problem is treated. True HCM is different and has no cure yet.
Can rapamycin reverse HCM in cats?
In a trial, it stopped the heart wall from getting thicker in cats with early HCM. That is not the same as reversing or curing the disease.
Is Felycin a cure for HCM?
No. It is FDA conditionally approved to manage heart wall thickening in cats with HCM and no symptoms. It should not be called a cure.
Can high blood pressure cause HCM?
High blood pressure can thicken the heart wall and look like HCM. That's why every cat with a thick heart should have its blood pressure checked.
Does diet reverse HCM?
No diet reverses true HCM. Cats do need enough taurine, because too little can cause a different heart disease.
Your next step
Want an honest opinion on your cat's options? Send Dr. Kevin your cat's echo report →
Read what Dr. Kevin has learned from treating 300+ cats: What 300 Real Cases Actually Show
References
- Kaplan JL, et al. Delayed-release rapamycin halts progression of left ventricular hypertrophy in subclinical feline HCM: results of the RAPACAT trial. J Am Vet Med Assoc. 2023;261(11):1628–1637.
- US FDA. FDA conditionally approves drug for management of ventricular hypertrophy in cats. March 2025.
- Luis Fuentes V, et al. ACVIM consensus statement on cardiomyopathies in cats. J Vet Intern Med. 2020;34(3):1062–1077.
- European Medicines Agency. Sirolimus TriviumVet: withdrawal of application. March 2025.
This article is for education only. It does not replace an exam, diagnosis or treatment plan from your own veterinarian. Prices and regulations change; confirm locally.