A pharmaceutical version is approved for treating children who fail to grow for various reasons, for AIDS patients with muscle wasting syndrome and for adults with legitimate growth hormone deficiency caused, for example, by surgery or radiation.

Other uses are illegal, including to turn back the clock on aging. The FDA says it is investigating violations of the law _ the Food, Drug and Cosmetic Act as amended in 1988 and 1990 _ and has sent warning letters to companies selling HGH over the Internet for other uses.

University of Illinois-Chicago epidemiologist Jay Olshansky, who co-authored a paper published last year in the Journal of the American Medical Association on legal issues surrounding HGH, said anti-aging doctors were surprised to learn they were on shaky legal ground.

"They've been administering growth hormone as an anti-aging intervention for a long time. They haven't been hiding it at all," Olshansky said. "Now they're trying to redefine it as a treatment for growth hormone deficiency."

Most prescriptions for HGH should go to children, according to Dr. Thomas Perls of Boston Medical Center, but 74 percent in 2004 went to people age 20 and older.

"In my opinion, that suggests a large amount of illegal distribution," said Perls, a co-author with Olshansky of the JAMA paper and director of a project that studies people who live to age 100.

Weisman, the Las Vegas car dealer, said he pays $1,000 a month for supplements and hormones, and a twice-yearly blood test costs him $650 each time. Plus, he paid $2,000 for an initial evaluation with Cenegenics Medical Institute in Las Vegas, the clinic that designed his program and was featured recently in GQ magazine.

In 2003, a placebo-controlled, randomized study of people 65 and older found that growth hormone increased lean body mass and decreased fat mass, but the study subjects experienced frequent side effects including diabetes and glucose intolerance.

Weisman said he's never experienced side effects, although his doctor informed him about them. Doctors interviewed for this article who prescribe HGH said side effects can be avoided by initial low doses or by giving smaller doses if side effects occur. These doctors insist they prescribe HGH only to patients whose blood tests show a deficiency.

Dr. Evan Hadley of the National Institute on Aging said legitimate research on growth hormone has been blown out of proportion by wishful thinkers and marketers.

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