Dr. Ron Livesey was fat, tired and out of shape. At 49, he felt that his best years were behind him. So one day seven years ago, on his way to a meeting, he stopped at a doctor's office in Palm Springs, Calif., for his first hormone injections.

Early the next morning, Livesey was at the medical meeting, sitting in a darkened auditorium watching slides of technical data. To his surprise, he found himself alert, taking everything in. He continued the hormone treatments.

"People started commenting that I had so much more bounce and energy," he said. He lost 50 pounds - thanks, he said, to diet changes and exercise made possible by the increased vigor.

So Livesey, then an internist in New Hampshire, decided to go into business for himself. With a colleague, Dr. Joseph Raffaele, who went on a similar regimen, he founded Anti-Aging Medicine Associates, a clinic on Central Park South in Manhattan. They are part of a growing movement among doctors to offer a hormone replacement therapy that claims to restore the energy of youth.

It is expensive: $1,000 a month for the panoply of drugs and dietary supplements, including human growth hormone and testosterone for men and women, estrogen and progesterone for women (the doctors say their "bioidentical" hormones are safe), melatonin, DHEA, vitamins and antioxidants. But beyond the cost lies a conundrum of modern medicine.

Until recently, most scientists considered anti-aging treatments to be little more than snake oil, provided by hucksters. Now, few doubt that growth hormone and testosterone can reshape aging bodies, potentially making them more youthful.

But whether they counteract aging is unknown. And their long-term risks are ill defined. So medical experts ask whether it is right to regard aging as a disease, as fierce as a malignant cancer, to be fought with any and all means, tested or not.

"How much are you willing to pay for a treatment that is not proven?" asked Dr. Huber Warner, an associate director at the National Institute on Aging. "How much risk are you willing to take?"

Dr. Ronald Klatz of Chicago, the founder and director of the American Academy of Anti-Aging Medicine, says patients cannot wait for long-term studies, which are not even in planning stages and would take years or decades to complete.

Clearly, many are not waiting. The academy, which began with 12 doctors in 1993, now has 8,000 physician members in the United States, Klatz said.

The unlikely hero of today's anti-aging movement was Dr. Daniel Rudman, an academic researcher at the Medical College of Wisconsin who asked if he could reverse the effects of aging by giving growth hormone to elderly men.

Aging people, he noted, lose muscle and put on fat, their skin thins and their bones weaken. At the same time, growth hormone levels steadily decline. He observed that the effects of aging also appeared in young people who lacked growth hormone for medical reasons.

He gave growth hormone to 12 elderly men for six months, reporting that they gained muscle and lost fat. Nine men who served as controls for the experiment had no such body changes. In his paper, published July 5, 1990, in The New England Journal of Medicine, Rudman concluded with this sentence: "The effects of six months of growth hormone on lean body mass and adipose-tissue mass were equivalent in magnitude to the changes incurred during 10 to 20 years of aging."

Human growth hormone has been approved by the Food and Drug Administration for use by people with medical deficiencies, and once a drug is on the market, doctors can legally prescribe it for any reason.

The most recent major paper was published Nov. 13 in The Journal of the American Medical Association. In it, Dr. S. Mitchell Harman of the Kronos Longevity Research Institute in Phoenix and Dr. Marc Blackman of the National Center for Complementary and Alternative Medicine, part of the National Institutes of Health, reported that older men and women taking growth hormone lost fat and gained lean body mass without dieting or exercising.

But many had the same side effects that afflicted the subjects of a California doctor's tests in 1996. In the 1996 tests, those taking growth hormone did slightly worse on memory tests. They also suffered swollen legs and feet and achy joints.