Rapamycin vs metformin for dogs and cats: two longevity drugs, one has the pet evidence
Both are borrowed from human medicine and floated for pet ageing. Only one has a randomized trial in dogs and robust lifespan data in mammals — and it isn't the cheap, familiar one.
Our Beloved Friends Research · July 23, 2026 · 9 min read
Short answer: rapamycin is the only one of the two with a published randomized trial in pet dogs and repeatable lifespan extension in mammals; metformin has neither, but it carries a decades-long human safety record and costs almost nothing. They are not two versions of the same bet. One has the aging evidence, the other has the track record — and for a pet, those pull in different directions.
Both drugs reach the pet-longevity conversation the same way: a human medicine, cheap and long-used, that did something interesting in a lab animal. That is where the similarity ends.
What each drug actually is
Rapamycin inhibits mTOR, the nutrient-sensing pathway at the centre of nearly every longevity result in animals. In the NIA Interventions Testing Program, rapamycin extended lifespan in mice — repeatedly, in both sexes, across labs, even when started late in life. It is the single most reproducible result in the field.
Metformin is a diabetes drug that lowers blood glucose and mildly activates AMPK. It is the safest, most-prescribed drug of its class in human medicine, which is exactly why it became a longevity candidate. The catch: when the same testing program tried metformin on its own, it did not extend mouse lifespan. The human trial built to test it for aging, TAME, is about people — not pets.
Side by side
| Rapamycin | Metformin | |
|---|---|---|
| Mechanism | mTOR inhibition | AMPK activation / glucose lowering |
| Mouse lifespan data | Robust, repeated, both sexes | Null on its own in the ITP |
| Randomized trial in pet dogs | Yes — 24 dogs, 2017 | None |
| Ongoing large pet trial | TRIAD, hundreds of dogs | None |
| Human safety record | Transplant use since the 1990s | 60+ years, extremely well understood |
| Prescription needed | Yes | Yes |
| Rough cost | Higher | Very low |
| Main risks | Immunosuppression, metabolic shifts | GI upset, B12 depletion, rare lactic acidosis |
Where metformin genuinely wins
This is not a page where our preferred molecule sweeps every row. Metformin beats rapamycin on the things that matter to a cautious owner:
- Human track record. Sixty years and hundreds of millions of patients. We understand metformin's risks better than almost any drug on earth.
- Cost. It is one of the cheapest drugs in existence.
- Gentler failure mode. Metformin's common side effect is an upset stomach. Rapamycin's is a suppressed immune system — a more serious thing to get wrong in an old animal.
If your only goal is "something low-risk and cheap that might help metabolic health," metformin is the more conservative choice.
Where rapamycin wins
On the actual question — is there evidence this slows aging in a dog or cat? — rapamycin is not close to metformin. It is the only one with a randomized, placebo-controlled trial in companion dogs, the only one with a large longevity trial under way, and the only one that reliably extended lifespan in the mouse program that failed metformin. There is even a dedicated feline tolerability trial; metformin has nothing comparable in cats.
The honest ceiling, for both
Neither drug has been shown to extend the lifespan of a dog or a cat. TRIAD has not reported. Every number you have seen about "adding years" comes from mice, and cats in particular are not small dogs — their drug metabolism is genuinely different. Anyone quoting you a percentage for your pet is quoting mouse data with a pet's photo next to it.
The verdict, by owner
- You want the option with the most aging evidence, and you'll accept a prescription drug's risks: rapamycin. Read how the dose question actually works first, and understand that it is an immunosuppressant.
- You want the lowest-risk, cheapest thing and are comfortable that the aging evidence is weak: metformin, discussed with your vet.
- You are comparing both because a website made aging drugs sound settled: neither, yet. Start with the five things that have better evidence than any longevity drug.
Whichever way you lean, this is a prescription decision, not a checkout decision. Your own vet is the first call. If they haven't read this literature, the veterinarians at our sister brand Rapavie will — and they will decline when the answer for your animal is no. Start with the quiz; it flags the conditions that should stop the conversation before cost ever enters it.
Not veterinary advice. This compares what the published evidence shows; whether either drug suits your animal is a decision for a veterinarian who has examined them.
References
- Urfer S.R. et al. A randomized controlled trial to establish effects of short-term rapamycin treatment in 24 middle-aged companion dogs. GeroScience, 2017. Find on PubMed
- Harrison D.E. et al. Rapamycin fed late in life extends lifespan in genetically heterogeneous mice. Nature, 2009. Find on PubMed
- Strong R. et al. Longer lifespan in male mice treated with… and the null result for metformin alone (NIA Interventions Testing Program). Find on PubMed
- Dog Aging Project — TRIAD. dogagingproject.org
Compounds in this article
- Rapamycin (sirolimus)Emerging evidenceStudied for healthy aging, cardiac function, and immune resilience. A prescription drug — veterinary supervision is not optional.
The content on this site is for informational purposes only and is not veterinary advice. These statements have not been evaluated by a food or drug regulatory authority and are not intended to diagnose, treat, cure, or prevent any disease. Always consult a licensed veterinarian about your pet's health.