Anna Nicole Smith and Human Growth Hormone

When the Broward County, Fla., medical examiner performed an autopsy on Anna Nicole Smith's body, he zeroed in on her left buttock. There, he found evidence of repeated needle injections that had produced a "deep-seated" abscess filled with yellow-green pus. It was likely, he wrote in his report, that bacteria from that infection entered her bloodstream, sending her temperature soaring to 105 degrees and prompting her to respond with an overdose of medication. What was Smith injecting herself with? According to the medical examiner, it was a cocktail of anti-aging drugs including human growth hormone, or HGH. Smith's "repeated intramuscular injections," he wrote, were "self-treatment for longevity and weight control."

That revelation shines a spotlight on the hot—and sometimes murky—market for HGH. Touted as a wonder drug that builds muscle, sheds fat and restores youth, HGH has grown into a multibillion-dollar worldwide business, by some estimates. The only problem is, it's not always a legal one. Though growth hormone has certain uses approved by the Food and Drug Administration, these don't include anti-aging therapy, bodybuilding or athletic enhancement. Just last month, Sylvester Stallone was charged by Australian authorities with illegally importing 48 vials of the stuff. (Stallone's lawyers are expected to enter a plea on his behalf on April 24.) And the district attorney in Albany County, N.Y., recently disclosed an ongoing, multistate investigation that has allegedly uncovered an illegal drug ring involving, among other things, HGH. "Millions of dollars are spent on it by people in their 40s and 50s, and there's no indication I can think of that's legal for [them]," says Dr. Tom Perls, an associate professor at Boston University School of Medicine.

The body naturally makes human growth hormone. A product of the pituitary—a small pea-sized gland at the base of the brain—HGH is critical for normal childhood development. But by the time people reach their 30s, their bodies begin to produce progressively less of it. A man in his 70s, for instance, may generate only one third to one half of the HGH he used to generate in his 20s. Doctors may prescribe growth hormone injections for patients with abnormally low HGH levels, like people who suffer from dwarfism or chronic wasting disease. But for the most part, declining HGH production is simply a part of aging—and not necessarily a bad thing, medical experts say.

The current boom in HGH use traces its roots to a 1990 article in the New England Journal of Medicine. In it, researchers presented the results of a study in which 12 men over the age of 60 received regular HGH injections and demonstrated increases in lean body mass and bone density. The authors didn't claim that the treatment reversed the aging process, and an editorial note warned against widespread use of growth hormone. Yet, the study helped spawn a bevy of anti-aging clinics and a surge in HGH sales. According to a 2005 study in the Journal of the American Medical Association, between 20,000 and 30,000 people used growth hormone as an anti-aging therapy in 2004—a tenfold increase from the mid-1990s. Another researcher, Dr. Mary Lee Vance of the University of Virginia, has estimated that 30 percent of growth hormone prescriptions in the U.S. are for reasons not approved by the FDA. The price tag for such treatment is hefty: often more than $1,000 per month.

One of the many companies profiting from the anti-aging trend is Las Vegas-based Cenegenics Medical Institute, which has been featured on "60 Minutes" and in the pages of GQ. The clinic administers HGH and other hormone supplements as part of what it calls "a proven age-management medical system with predictable results." It treats around 2,000 HGH-"deficient" patients with the growth hormone, according to Cenegenics CEO Dr. Alan Mintz. "We do very careful evaluations first," he says. "Used appropriately, there is actually no risk associated with this. It actually reduces risk of heart disease. … The side effects are minor, dose-related and totally reversible."

But many medical experts challenge such claims. Some studies have demonstrated that HGH increases muscle mass, but not necessarily muscle strength or function. Moreover, some researchers have detected worrisome side effects, like diabetes, arthritis and hypertension. "Ironically, people who take [HGH] for vanity may find it hurts their looks because people with high levels of growth hormone … can become disfigured," says Perls. He notes that there's even evidence that it could reduce life span. All of which led him to start a Web site, antiagingquackery.com, to challenge what he claims are dangerous myths about the drug.

Law enforcement is evidently concerned as well. As part of the Albany County D.A.'s investigation, authorities have arrested doctors, pharmacists and clinic operators in New York and Florida—all part of what prosecutors describe as an elaborate network to sell HGH and steroids illegally on the Internet. The investigation, which is still ongoing, will likely gain more attention when the D.A. reveals the names of professional athletes and celebrities who were alleged customers (HGH is popular among some athletes because it's hard to detect in drug tests). In response, New York Sen. Charles Schumer is proposing a bill that would make HGH a controlled substance and give the federal government the power—which it doesn't currently have—to prosecute the illegal sale of prescriptions. "Certainly what's happened with Anna Nicole Smith will help" authorities crack down on unauthorized HGH use, says Perls. "The more attention, the more pressure will be brought to do something about this."