Every longevity tool we've had for dogs was designed for something else. Rapamycin was built for organ transplants. Metformin for type 2 diabetes. LOY is different. It was made specifically for dogs, targeting the precise biological reason large breeds die so young.
There is a puzzle at the heart of dog biology that has been quietly bothering researchers for decades. In the animal kingdom, bigger generally means longer-lived. A blue whale outlives a sparrow. An elephant outlives a mouse. Size brings robustness, slower metabolism, more time. This rule holds across millions of years of evolutionary history.
But in dogs, it runs exactly backwards.
A Chihuahua lives 15 to 20 years. A Great Dane lives 8 to 10. An Irish Wolfhound — one of the largest breeds — often doesn't reach 7. The bigger the dog, the shorter the life. And this isn't a rough trend with lots of exceptions; it's one of the most consistent size-to-lifespan correlations ever documented within a single species.
Understanding why this happens turns out to be the key to doing something about it.
When researchers started examining what, biologically, sets large dogs apart from small ones, a consistent pattern emerged around a hormone called IGF-1 — insulin-like growth factor 1. Large dog breeds carry significantly higher levels of circulating IGF-1 than small breeds. And circulating IGF-1, as it turns out, is one of the most robust predictors of canine lifespan.
IGF-1 is a growth hormone — its job is to promote cellular growth and division. During puppyhood, it's essential. But sustained high levels throughout life appear to push cells into a state of constant activity rather than maintenance, accelerating the kind of cellular wear that shows up as aging. Studies have traced much of the size-variation between dog breeds to variants near the IGF1 gene itself, making the connection unusually direct: breed large, carry more IGF-1, age faster.
The size-lifespan reversal in dogs doesn't happen in natural populations, and the reason tells you something important. Wild animals were shaped by natural selection over millions of years, which tends to optimise lifespan. Dogs, by contrast, were selectively bred by humans for specific traits — particularly size and appearance — over just a few thousand years. That process optimised for what humans wanted, not for what the dogs' biology could sustain long-term. The result is a species where artificial selection has inadvertently created a built-in aging accelerant in large breeds.
Celine Halioua founded Loyal in San Francisco around 2019 with a specific ambition: develop drugs that extend healthy lifespan in dogs, going through proper regulatory channels rather than relying on off-label use of human medications.
LOY-001 is the first drug candidate to emerge from that effort. The premise is direct: if elevated IGF-1 is the specific mechanism driving accelerated aging in large breeds, then pharmacologically lowering it should slow that process. LOY-001 is designed to reduce circulating IGF-1 and growth hormone levels in large and giant breed dogs. It is administered as a monthly injection by a vet.
This is a meaningfully different approach from rapamycin. Rapamycin works by blocking the mTOR pathway — a cellular signalling system that governs how cells respond to nutrient availability and growth signals. It produces effects broadly similar to caloric restriction across multiple species. LOY-001 doesn't target mTOR. It targets the specific hormonal axis — IGF-1 and growth hormone — that research has tied directly to why large dogs age faster than small ones. These are distinct mechanisms acting on overlapping pathways.
An important distinction: LOY-001 is specifically for large and giant breeds. The IGF-1/size relationship it targets is a large-dog problem. Smaller dogs don't have elevated IGF-1 in the same way and wouldn't benefit from this specific drug. LOY-002 — Loyal's second candidate — is designed for all dog sizes through a different mechanism.
In late 2023, something happened that had never happened before in the history of drug regulation. The FDA issued a determination that LOY-001's scientific rationale met the threshold for a "reasonable expectation of effectiveness" — the first time any regulatory body had acknowledged sufficient evidence to move forward with a drug specifically targeting lifespan extension in any animal.
Loyal used the FDA's MUMS pathway — Minor Use Minor Species — which allows conditional approval while field effectiveness studies continue. The "technical section completeness" determination means the FDA reviewed the scientific package and agreed the evidence justified proceeding. This is not the same as full approval, and LOY-001 is not commercially available as a result of this milestone alone.
But the significance is real. The FDA has now established that "extends lifespan" can be a legitimate drug claim — that there is a regulatory pathway for this kind of medicine. Before this, no one had successfully made that case.
Loyal's second drug candidate, LOY-002, is designed as a daily oral tablet intended for dogs of all sizes. The mechanism is different from LOY-001 and targets longevity pathways more broadly — Loyal has described it as addressing the biology underlying aging across the canine lifespan rather than the specific IGF-1 elevation problem in large breeds.
As of this writing, LOY-002 is in clinical trials. Loyal hasn't published full mechanistic details, which is standard for drugs still in development. What's known publicly is that it's an oral pill and intended to be more universally applicable — if your dog is a medium or small breed, LOY-001 isn't the right drug, but LOY-002 may eventually be.
For anyone who read Part 2 of this series, a natural question is how LOY compares to rapamycin. The short answer is that they're aimed at different problems through different mechanisms, which means they're not really competing.
| Drug | Mechanism | Target dogs | Availability |
|---|---|---|---|
| Rapamycin | mTOR inhibition — mimics caloric restriction at cellular level | All breeds; cardiac benefits especially studied in medium/large dogs | Available now, off-label, through a vet willing to prescribe |
| LOY-001 | IGF-1 / GH axis reduction — targets large-dog-specific aging driver | Large and giant breeds specifically | Not yet available — pending full FDA approval |
| LOY-002 | Undisclosed longevity pathway — broader applicability | All dogs (intended) | In clinical trials as of this writing |
The practical implication is that if you have a large breed dog today and are exploring pharmacological longevity options, rapamycin is the conversation to have with your vet. LOY-001 is the thing to watch — and potentially more targeted for large breeds when it does become available, because it's addressing the specific biology that makes large dogs age faster in the first place.
The honest answer is: not much has changed at the level of what you can do this week. LOY-001 and LOY-002 are not on pharmacy shelves. Field studies are still running. Full FDA approval requires more data. If and when LOY-001 becomes commercially available, it will require veterinary administration. Nobody outside of Loyal and its trial participants has access to it today.
What has changed is the landscape. Before Loyal's FDA milestone, dog longevity pharmacology was entirely dependent on drugs designed for other purposes, applied off-label and studied in dogs as a secondary effort. Loyal is the first company to build drugs specifically for this problem, go through regulatory channels, and successfully move those drugs forward in the process. That matters for what becomes available five years from now.
The science behind LOY-001 is grounded. The IGF-1/large dog connection is well-documented research. The FDA milestone was genuine. The team is real, the company is funded, and the regulatory progress is farther along than anything that existed in this space before.
What isn't known yet: exactly how much healthy lifespan LOY-001 will add in practice, what the full safety profile looks like across years of use, and when (or whether) full FDA approval will come. Field studies take time, and time is the one thing dogs don't have a lot of.
For now, the most actionable things are the ones we've covered earlier in this series: keeping your dog lean, talking to your vet about rapamycin if that's appropriate for your dog, and feeding a diet that supports longevity at the cellular level. LOY is the most exciting thing on the horizon. But the horizon is still the horizon.
Loyal's drug development is ongoing. The information here reflects the state of publicly available knowledge up to the time of writing. Drug development timelines shift, study results get updated, and approval status can change. Always check current information through your vet and Loyal's own communications (loyal.vet) before making any decisions. Nothing here is veterinary advice.
Series Summary
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