One day last November, my dog, Forrest, sat on the cold marble steps of the Smithsonian’s natural-history museum in Washington, D.C., ready to meet Celine Halioua, a woman who may one day add a tail-wagging year or so to his life, and also the lives of millions of other dogs. In 2019, Halioua founded a company called Loyal, and in February 2025, a pill that she developed for dogs was deemed likely to be effective by the FDA. If the company ticks a few remaining boxes, the drug could soon be on sale, kick-starting a new era of longevity medicine that could eventually also lengthen humans’ lives.
More than 10,000 years ago, dogs made a farseeing bet on humans. They padded carefully up to our campfires, ate scraps, and kept watch, hitching their fates to a species that would soon bestride the planet. They have since become the fourth-most-populous large land mammal, trailing only sheep, cows, and goats, which all lead less pampered lives. Now we’re trying to keep our best animal friends around longer too.
If only I could have explained all of this to Forrest before our walk with Halioua. As a Portuguese water dog, he hails from a clever breed, but he doesn’t understand advanced pharmacology, so I worried that he might be indifferent to her, or even rude. But Halioua, who is 31, had arrived with a plan. She stooped down, squealed his name, and opened her hand, revealing a treat that he promptly devoured.
Halioua was 18 years old when the cold fact of death blew through her. She was working at a neuro-oncology lab and couldn’t unsee the cosmic unfairness of a brain-cancer diagnosis, the way it constricted the possibilities of a person’s life and cut short their closest relationships. Death had an important role to play back when life was single-celled and simple, Halioua told me. It helped evolution iterate rapidly and build up more complicated organisms. But now that natural selection has created complex, intelligent animals—namely, us—we should stretch out the good, healthy part of our lifespan as well as that of our dogs, too. With extra decades, she said, we might even become more forward-looking, and less likely to wreck a world that we will have to keep living in.
After college, Halioua enrolled in a doctoral program in genetics at Oxford and worked in life-extension research during her time off school. The field has never been uniformly rigorous in its approach to research. Over the years, Halioua has developed an aversion to what she described as longevity “bro science.” She’s not into the translucent-skinned gurus who primarily experiment on themselves and post their physiological data, including the duration of their sleeping erections, on X. She’s not trying to gain eternal life through obsessively healthy living, she once told me over a tray of french fries.
Halioua got her big break from Laura Deming, a former child prodigy who was accepted to MIT at age 14 and later co-founded the world’s largest venture fund for longevity research. Halioua was only 23 when she started interning at Deming’s offices in San Francisco, but after two weeks, Deming hired her, and eventually promoted her to chief of staff. Early on, she gave Halioua a blunt pep talk. “I didn’t really speak the Silicon Valley language,” Halioua told me. Deming told Halioua that she didn’t sound smart. That made Halioua self-conscious, but she was grateful and resolved to assimilate by listening to every last episode of the Y Combinator Startup Podcast. It wouldn’t make for the most cinematic training montage if there were ever a movie about her life, but it helped her pick up the local lingo and speak at a more rapid clip.
At Deming’s fund, Halioua sat in on start-up pitches and broadened her view of the longevity industry. She saw that serious money was flowing into it. Investors have dropped more than $10 billion on life-extension companies in just the past five years. Most of that has gone to long-term bets on radical life-extension projects for humans, some intended to defeat aging altogether. Jeff Bezos and Sam Altman have both bankrolled new efforts to wind back the internal clocks of our bodies’ cells using epigenetic techniques that have already extended the lives of mice. But the companies that they have invested in—Altos and Retro Biosciences—are focused on preclinical or early-phase work. The same goes for Calico, Alphabet’s secretive life-extension company. It may be decades before we know if those bets have paid off.
Halioua wanted to move faster, and she had personal reasons for focusing first on dogs. She grew up on Austin’s semirural outskirts, the lone daughter of immigrant parents from Germany and Morocco, and like many children—especially only children—she formed intense relationships with animals. Her family had 15 cats and several dogs, most of them strays. In middle school, she started visiting an old cowboy who lived in a run-down house nearby. She began taking his retired racehorse, Ziggy, on walks, and her family later bought the horse. Years later, when the horse died, she got a tattoo of its racing name next to her heart.
Halioua has made a habit of adopting senior dogs, and that means she more frequently has to experience the heartbreak that she’s trying to forestall for dog owners. Four years ago, she brought home a 10-year-old rottweiler named Della that had been found wandering Oakland’s streets. In 2024, I visited Halioua in San Francisco, and Della came with us almost everywhere we went—to the local coffee shop, to the stables where Halioua keeps a dressage horse. Shortly before I saw Halioua in D.C., I learned that she’d had to put Della down. On the plane, she’d made the mistake of scrolling through old pictures. She spotted Della in one and felt her vision blur with hot tears. “Della would literally spoon me at night,” she said. “They’re such pure souls.”
Halioua had another reason for starting with dogs, beyond her connection to them: Federal approval for animal drugs is easier to come by than it is for human drugs. And because dogs tend to live only a decade or so, she can quickly tell whether a life-extension drug is working in them. Her end goal is to lengthen human lives. For thousands of years, dogs have gone out ahead of humans as wilderness scouts. They have ventured into buildings to sniff out explosives. Some even got killed rocketing into space before us. Now they’re entering another new frontier that may be fraught with its own unforeseeable dangers.
Scientists have already dramatically lengthened the lives of many animals, but they’ve mostly been tiny ones that people don’t care much about. By the early 1990s, a molecular biologist named Cynthia Kenyon had for years been arguing that aging is not simply a matter of accumulated wear and tear. To Kenyon, who now serves as the vice president of aging research at Calico, the fact that animals have such a wide range of lifespans was evidence that the aging process is directed by genes.
In 1993, Kenyon doubled the lifespan of a roundworm, Caenorhabditis elegans, with the tweak of a single gene that targeted its insulin receptors. Scientists have since used similar genetic tricks to substantially extend the lives of flies. Most tantalizing, they have done it in adult mice.
Dogs are nearer to us, genetically, than mice are, and they age in many of the ways that we do. We suffer from some of the same cancers and use some of the same chemotherapies to treat them. The human brain’s neurons experience similar modes of decay, and so, too, do our downstream behaviors. Dogs can lose control of their bladder in old age. They can forget faces, become more grumpy, and bump into walls. Kenyon told me that although the mice research is encouraging, a drug pathway that successfully extends a dog’s life will generate more enthusiasm among scientists who hope to try similar treatments on humans.
Forrest is four years old, and although not a dot of gray appears in his glossy black muzzle, he is already showing some signs of aging. He catches fewer cases of the zoomies, and before he leaps up onto the bed, he takes its measure. As we walked on the National Mall, Halioua explained that inside him—as in humans of every age—cancer cells are constantly popping up. His immune system still has lots of ways of zapping cancer cells out of existence before they multiply into tumors. But by the time Forrest reaches age 10, when dogs become eligible for Loyal’s pill, those defenses will more often misfire and fail to stop not just cancer but also other life-abbreviating ailments.
On her tricep, Halioua has another tattoo, this one of a Labrador retriever, but it doesn’t pay tribute to a previous pet. It’s a tribute to the 48 Labrador puppies who participated in a 14-year study by scientists at the dog-food company Purina. They split the puppies into two groups and fed one group 25 percent less than the other. After tracking the dogs for the rest of their lives, they found that those who were fed smaller bowls of kibble lived nearly two years longer, on average. By making one crude shift to the dogs’ metabolism, the scientists had extended their lives by more than 15 percent.
Several recent biomedical findings suggest that a profound link exists between an animal’s metabolism and its lifespan. The potential life-extending effects of Ozempic and other GLP-1s have been especially intriguing on this score. The drugs treat obesity by slowing the movement of food through the gastrointestinal system and suppressing appetite, but they also seem to trigger a cascade of other unexpected benefits all across the body. They appear to improve the condition of people’s kidneys, liver, heart, and even their brain. Some longevity researchers now argue that GLP-1s are the first de facto anti-aging drugs because they slow so many of the life-shortening processes that operate inside of us.
[From the December 2022 issue: Sarah Zhang on how much a cat’s life is worth]
But GLP-1s are blunt instruments. The longevity benefits they confer are a byproduct of appetite suppression. Loyal’s drug is designed to trigger some of the same effects, without a dog having to face any of the deprivations experienced by GLP-1 users or the Labradors in the Purina study.
Scientists who studied the Purina data noted that the calorie-restricted dogs were less awash in insulin, a metabolic hormone that is known to accelerate certain aging processes, in excess. The exact mechanism of Loyal’s drug is still proprietary, but scientists at the company told me that the pill tinkers with a dog’s insulin sensitivity.
Every time Forrest eats, his small intestine breaks down the resulting slurry of kibble into glucose and other compounds. His pancreas then produces a tiny pulse of insulin, a chemical whisper that echoes all through his bloodstream, telling different kinds of tissues to absorb the glucose. In a young, healthy dog, this system is precise; it requires only that whisper. But like us, dogs become less sensitive to insulin as they get older and require more of a clanging cymbal. The resulting glut of insulin can inflame tissues all across the body, and over time, this can weaken the immune system and contribute to all manner of chronic diseases, including cancer and heart disease. When a dog or a person becomes less sensitive to insulin, brain decay can set in quicker, and the operations of neurons can be scrambled.
Loyal’s daily pill is intended to restore a dog’s insulin sensitivity. It dissolves into particles that travel throughout the body, like little Paul Reveres, telling tissue systems to be on the lookout for insulin and to respond quickly when it reaches them. That way, the glucose doesn’t linger, and the pancreas doesn’t keep flooding the blood with the hormone.
Federal regulators have generally preferred drugs that target specific diseases, classified by organ systems, an approach that may simply miss certain whole-body aging processes. That the FDA has taken the unusual step of allowing Loyal to develop a drug specifically for life extension, as opposed to for some particular ailment, suggests a shifting approach. After reviewing Loyal’s early data, the agency found that by operating across different organ systems, the drug was reasonably likely to extend a dog’s life. Whether it works or not, this new openness on the part of the FDA is exciting all by itself.
Loyal hopes to be able to start selling its drug next year, for about $100 a month for dogs of most sizes. (The pill has met FDA requirements for an expectation of safety and efficacy, but it needs to meet a core manufacturing requirement before the company can receive conditional approval to market the drug.) The unpublished study that the FDA reviewed was relatively small, involving about 50 dogs whose aging-related biomarkers were tracked for three months. There were clear improvements, but improvements in biomarkers are not enough to know that a drug works. As part of its campaign to secure full approval from the FDA, the company has launched a roughly five-year clinical trial to know, with greater certainty, whether and how much it extends a dog’s life.
As late as the 1980s, animal-health divisions at pharmaceutical companies were lightly staffed backwaters. At Merck, “you only worked in that part of the company if you weren’t very ambitious,” Linda Rhodes, an industry veteran, told me. Back then, the sector focused on drugs that helped cattle and swine survive long enough to make slaughter weight. Dog owners were regarded as a niche market inside a niche market.
In the ’90s, the blockbuster sales of two flea-and-tick medications, Frontline and Advantage, demonstrated untapped demand, and then intensified that demand by enabling new levels of indoor intimacy between dogs and people. Dogs have been bed warmers since the time of ancient Egypt, at least, but many more of them were invited to sleep with us after they were reliably rendered tick-and-flea-free.
Even just decades ago, fewer people described their dogs as family members. Now seniors spend as much on gifts for their dogs as they do for their grandchildren. People buy their dogs health insurance, take them in for regular dental visits, and sign them up for memberships at concierge-style veterinary clinics modeled after One Medical. Families are willing to go into debt to finance a surgery if doing so means saving a beloved dog’s life.
If dogs start living longer, these familial feelings of obligation may intensify. People may feel guilty if they can’t afford a daily pill that keeps their dog alive longer. Above a certain socioeconomic threshold, not spending an extra $1,000 a year or more in the hopes of doing so could seem neglectful. Elderly people may think twice about adopting dogs that have the potential to live much longer. Euthanasia decisions are already brutalizing for dog owners, and those decisions may become even more fraught.
The fundamental sadness of loving a dog is knowing that you are more likely to lose them than vice versa, because their lifespan is easily contained by yours. There’s every reason to try to keep them around longer, especially if the extra years are healthy ones. But our relationship with them may change if we succeed, perhaps in some ways that we don’t expect.
Loyal’s clinical trial, which the company says is the largest one ever run on an animal drug, began in December 2023. More than 1,300 dogs enrolled, all at least 10 years old and weighing at least 14 pounds, and representing many different breeds. Age verification at times proved difficult; some owners submitted screenshots of Facebook posts they’d made back when the dogs were puppies. Most clinical trials for dogs last a month or two, but the owners of these dogs have committed to keeping at it for at least half a decade. They don’t even know whether their pet might be taking a placebo, as half of the test population is. The FDA expects the pill to be safe, but no dog has yet had it in their system for five years.
By the time the trial finishes, Halioua may already have had the drug on the market for years. She recently assembled a focus group of 20 taste-tester dogs to get the flavor just right at launch. One day of taste-testing wasn’t enough, because many dogs will inhale just about anything with gusto the first time they eat it, and this is a pill they’ll have to take for the rest of their life. Compared with cats, dogs tend to chomp down on pills easily, but some of Halioua’s taste-testers were quite discerning; one spat out a disagreeable flavor variant and then, to underscore his verdict, peed on it.
Halioua is trying to anticipate other ways that her business might fail, even if the science proves out. In 2007, a much-hyped appetite suppressant for overweight dogs flopped spectacularly, not because it tasted bad or didn’t work but rather, in part, because it removed a dog’s great relish for tasting things in general. The human-dog bond has been food-based since its earliest campfire beginnings. Halioua explained that people didn’t like it when they couldn’t use treats to motivate their pets.
Halioua wants the pill to sell well so that she can build up a war chest. That way, the company could fund its own clinical trials for a human-longevity drug without having to sell out to “Big Pharma Daddy,” as she put it. Halioua doesn’t want to forfeit control of the process to a larger, more risk-averse, and possibly slower-moving company.
She has plenty of years left to see her plan through. I asked her if she wants to live for centuries, or even forever. “It’s not an obvious yes,” she told me. No one knows how much a human’s life can be extended, but whether it’s a few years or decades or more, Halioua said she wouldn’t want to keep living just for the sake of it.
She’d want to continually remake herself—“update,” in the founder parlance that she has adopted—try lots of new things. When I’d seen her in D.C., she was on the verge of one such change. I’d asked her if she would keep adopting older dogs. She told me that part of her felt like she had an ethical mandate to. “But also part of me is like, ‘Holy fuck, I don’t think I can sign up to do this again,’” she said. A few months later, Halioua did get another rottweiler, a rescue named Squish. She will most likely get to spend more time with her than she did with Della, whether the pill works or not. Squish is not even two years old.
