Sirolimus vs Rapamycin: They Are the Same Molecule

Sirolimus and rapamycin are the same molecule. If your veterinarian wrote "sirolimus" on a prescription and you searched for rapamycin, or the reverse, nothing has gone wrong — you are looking at one drug with two names. This page explains where each name comes from, the places the distinction does matter, and what it changes when you are buying.

Why one drug has two names

The compound was isolated in 1972 from a soil bacterium found on Rapa Nui (Easter Island) and named rapamycin after the island. When it was developed as a medicine it received sirolimus as its international non-proprietary name — the generic name regulators use. Pfizer then sold it for human transplant medicine as Rapamune.

So:

• Rapamycin — the original, and the name almost all the research uses.
• Sirolimus — the same substance under its official generic name. This is what appears on prescriptions, pharmacy labels and regulatory approvals.
• Rapamune — a human brand of sirolimus.
• Felycin-CA1 — a feline sirolimus product, conditionally FDA-approved in March 2025.

One molecule. Same mechanism: it quiets mTOR, a growth pathway that stays switched on more than it should as an animal ages.

Where the names actually differ

The molecule is identical, so anything that differs is about the product, not the drug:

• Formulation. A delayed-release tablet and an immediate-release one release the drug differently, which changes blood levels even at the same milligrams. Products are not interchangeable milligram-for-milligram unless a veterinarian says so.
• Strength. Human sirolimus tablets are made for human bodies. A 1 mg human tablet is an enormous dose for a 4 kg cat, which is why splitting a human product at home is a genuinely dangerous idea rather than a thrifty one.
• Label. Rapamune is labelled for human transplant patients. Felycin-CA1 is labelled for cats with subclinical HCM. Everything else in animals is off-label, which is legal where a licensed vet judges it appropriate — but it is a different regulatory footing, and worth knowing which one you are on.

Does the name change what you should buy?

Only in one direction. If a seller uses the names as if they were different drugs — "our rapamycin, not that sirolimus" or the reverse — they either do not know the subject or are counting on you not to. That is the single most useful thing this distinction tells you.

What should decide your purchase is the same either way: is a veterinarian reviewing your animal, is the dose set by weight, is the formulation appropriate for a cat or a dog, and can the supplier say no. We set out how that works for dogs and for cats.

Dosing is by species and weight, not by name

Whichever name is on the box, companion-animal protocols are weight-based and weekly rather than daily — roughly 0.3 mg/kg once weekly in cats and 0.15 mg/kg once weekly in dogs in the published work, confirmed for the individual animal by the prescribing vet. You can see what your animal’s weight points to with our dose calculator.

Human dosing is not a useful guide, and neither is a dose that worked for someone else’s animal of a different size.

What is established, and what is not

Rapamycin extends healthy lifespan in laboratory mammals — that is solid. In dogs it is being studied through the NIH-funded TRIAD trial, which has not reported its primary results. In cats, sirolimus reached conditional FDA approval for one cardiac indication, which is a real regulatory milestone and also a narrow one: conditional approval means a reasonable expectation of effectiveness, not the substantial evidence full approval requires.

Anyone telling you the lifespan question is settled in cats or dogs is ahead of the evidence, whichever name they use for the drug.

For the research laid out study by study, with nothing to sell you, see the evidence library at Our Beloved Friends.

If a veterinarian has raised sirolimus for your animal, our therapies start with a veterinary review — and if your pet is not a candidate, you pay nothing.