The life of a pet dog follows a predictable trajectory. Over time, the floppy-eared puppy that keeps falling asleep in his food bowl will become a lanky-legged adolescent with an insatiable interest in squirrels — before eventually settling into adulthood as a canine creature of habit, with a carefully chosen napping location and a well-rehearsed greeting ritual.

But as the years progress, his joints will stiffen and his muzzle will gray. And one day, which will inevitably arrive too soon, his wagging tail will finally still.

“When you adopt a dog, you’re adopting future heartbreak,” said Emilie Adams, a New Yorker who owns three Rhodesian Ridgebacks. “It’s worth it over time because you just have so much love between now and when they go. But their life spans are shorter than ours.”

In recent years, scientists have been chasing after drugs that might stave off this heartbreak by extending the lives of our canine companions. On Tuesday, the biotech company Loyal announced that it had moved one step closer to bringing one such drug to market. “The data you provided are sufficient to show that there is a reasonable expectation of effectiveness,” an official at the U.S. Food and Drug Administration informed the company in a recent letter. (Loyal provided a copy of the letter to The Times.)

That means that the drug, which Loyal declined to identify for proprietary reasons, has met one of the requirements for “expanded conditional approval,” a fast-tracked authorization for animal drugs that fulfill unmet health needs and require difficult clinical trials. The drug is not available to pet owners yet, and the F.D.A. must still review the company’s safety and manufacturing data. But conditional approval, which Loyal hopes to receive in 2026, would allow the company to begin marketing the drug for canine life extension, even before a large clinical trial is complete.

“We’re going to be going for claiming at least one year of healthy life span extension,” said Celine Halioua, the founder and chief executive of Loyal.