UNTIL THE PAST FEW DECADES,  THERE WASN’T MUCH TO BE DONE when a pet had a chronic condition or got very sick. Diseases such as cancer were rarely treated, and when they were, it was with chemotherapy drugs designed for humans. That has been steadily changing over the past few years, with a rapid increase in approvals for veterinary drugs. In a 2017 report, Transparency Market Research forecasts that the global veterinary-drug market will reach $26.7 billion by the end of 2024, up from $17.2 billion in 2015.

The boom is partly a reflection of a new generation of European and North American animal owners who have been increasingly willing to spend money to treat their pets. But for humans, the benefits of these drugs may not stop at added years of nuzzles and wagging tails. A new focus on animal health—whether for cattle or cats—may have tangible benefits for human well-being, too.

A number of the new veterinary drugs come from repurposing existing human drugs and modifying them to create more animal-friendly formulations. That is the tack taken by Kindred Biosciences, a Silicon Valley–based company, which has several human-to-veterinary drugs in clinical trials and more than a dozen others in earlier stages of development. Kindred’s founder and CEO, Richard Chin, says that starting with a human drug allows the company to bring a product to market in half the time and at a fraction of the cost that would be needed to design a new veterinary treatment.

Studying repurposed human drugs in veterinary medicine invariably deepens the understanding of both the disease and the drug’s effectiveness in treating it. “Many conditions work in very similar ways in people and other mammals,” says Chin, who previously led clinical biotherapeutics research at Genentech, headed an oncology drug startup and was CEO of OneWorld Health, a Gates Foundation–funded nonprofit developing drugs for impoverished populations in developing countries.

In late 2017 or early 2018, Kindred expects approval from the Food and Drug Administration for Zimeta, an anti-inflammatory drug to manage fever in horses, and Mirataz, a transdermal ointment to counteract weight loss in cats—the latter a problem that affects millions of animals but has no approved veterinary drug treatment. Mirataz is a repurposed form of a human antidepressant, mirtazapine, which causes weight gain as a side effect; Zimeta is a version of dipyrone, a human anti-inflammatory.

Other companies pursuing this angle include Zoetis, formerly a unit of Pfizer. In 2013, Zoetis got approval for Apoquel, a canine-dermatitis drug developed by tweaking a compound that Pfizer scientists were investigating to treat human rheumatoid arthritis. And animal-health startup VetDC received conditional FDA approval in 2017 for Tanovea-CA1, a lymphoma treatment adapted from a drug for the disease that caused unexpected side effects in humans.

While repurposing animal drugs for humans has been far less common than moving from people to dogs and cats, Chin beleives that the approach could “address the biggest problem with the development of human drugs: a success rate of 5% to 10%.”

Most human drugs are tried first in mice, rats and other laboratory animals. But critics argue that low success rates in human trials result from the fact that those animals are infected with genetically engineered or artificially induced versions of a disease. Companion animals such as dogs and cats, however, naturally come down with arthritis, diabetes, kidney disease and many types of cancer that are often virtually identical to those found in humans. Studying the effect of drugs on these ailments might provide more accurate predictions of how a drug will perform in humans.

Recent examples of vet-to-human translational successes include a new topical drug, lifitegrast, for dry-eye disease—an ocular inflammatory disease that affects dogs as well as about 7 million people in the United States over age 40. Research in dogs has also sped development of a new lymphoma drug (ibrutinib), as well as drugs and surgical methods for patients with bone cancer. This fall, researchers at Cornell and Tufts received a $2.5 million grant from the National Institutes of Health to use dogs as a model for studying cancer immunotherapies. At University of California, Davis, scientists from the medical and veterinary schools recently received a National Cancer Institute grant to study immunotherapies for osteosarcoma and melanoma in dogs.

Meanwhile, scientists at the Scripps Research Institute in San Diego are taking aim at C. difficile, an antibiotic-resistant bacterium that causes hundreds of thousands of severe, sometimes fatal intestinal infections in the United States every year. The treatment they’re developing is a cocktail of deworming drugs that are in every veterinarian’s arsenal. Other cross-species studies are looking at drugs for longevity and vaccines against bacterial infection. Many of these projects are coordinated through One Health, an international initiative to promote collaborations among researchers in veterinary and human health.

Getting the last bit of effectiveness out of a drug, through off-label use and novel applications, has long been the standard in human medicine. Recent progress suggests that going beyond the species barrier may bring relief and longer lives to both humans and their animal companions.