A few years ago, Marc Provissiero, a movie producer approaching his mid-50s, started contemplating the next decades of his life. He joined a high-end longevity clinic for a few hundred dollars a month and began monitoring his blood panels, eating better and improving his sleep. He wanted to stay active as he aged — playing tennis, surfing, hiking and spending time with his young daughter.

But during Mr. Provissiero’s annual physical at the longevity clinic, his physician, Dr. Darshan Shah, gave him a sobering warning. Despite his healthy habits, Mr. Provissiero’s blood sugar was nearing prediabetic levels. A family history heightened his risk of developing diabetes, which can shave years off a person’s life expectancy. When a stricter diet and exercise regimen didn’t move the needle, Dr. Shah prescribed metformin.

Scientists have known for decades that metformin lowers blood sugar. It’s approved by the Food and Drug Administration to treat Type 2 diabetes and is also commonly prescribed for weight loss and polycystic ovary syndrome. But some doctors, including Mr. Provissiero’s, are betting this same drug can head off a number of chronic diseases before they even develop, to help otherwise healthy people live longer and better.

“The hope is that this is going to lead to a greater number of healthy years,” said Mr. Provissiero, who lives in Malibu, Calif. After one year on metformin, he said he feels less bloated, more energetic and more in control of his weight and blood sugar.

Research on metformin’s impact on the human life span is scant. The existing research, much of which has been done on mice, is “all over the place,” showing varying effects based on the animals’ sex and species, said Rafael de Cabo, a senior investigator at the National Institute on Aging who has studied metformin in mice for over two decades. Human studies, which have largely focused on diabetes patients, turn up conflicting findings on metformin’s benefits for healthy people, he said.

Still, interest in metformin’s anti-aging potential has remained high as scientists find early but promising evidence that it could also manage or delay other conditions like cardiovascular disease, cancer and dementia. “Metformin may be targeting the root cause” of several other diseases whose risks increase with age, potentially because it regulates hormones, repairs tissues and eases immune system inflammation, Dr. de Cabo said.