WASHINGTON -- Tens of thousands of aging men are trying testosterone shots, patches and gel in hopes of regaining youthful vigor and virility. A new report uncovers little evidence it works -- or that the therapy is even safe -- but recommends careful study to find out.

"Certainly there are people out there who say testosterone replacement therapy is just wonderful, and others who say it doesn't do any good at all. What we're saying is we just don't know," said Dr. Dan Blazer of Duke University Medical Center, who led an Institute of Medicine probe of the therapy.

The report spotlights a huge area of contention, the claim that because testosterone levels slowly drop as men age, they undergo a male version of menopause -- dubbed andro-pause by some doctors and drug companies -- that requires treatment.

In fact, many doctors don't agree that andropause is a real phenomenon. Nor is there evidence that symptoms many older men report -- such as decreased libido, frailty and fatigue -- really are connected to testosterone when they could be inevitable consequences of aging, Blazer said.

Rising demand

Demand for prescription testosterone treatment by middle-aged and older men whose natural hormone levels are near normal is rising rapidly, the study found. More than 1.75 million prescriptions for testosterone products were written in 2002 -- a 170 percent increase from 1999 -- for an estimated 800,000 patients.

The Food and Drug Administration has approved testosterone therapy for men with hypogonadism, where the body makes very little testosterone. An estimated 4 million to 5 million Americans have hypogonadism, but only about 5 percent are thought to be getting treatment.

That means a lot of men using testosterone are in a gray zone.

And specialists hope the report's recommendations will help avoid the confusion that has plagued menopausal women, who once considered hormone replacement therapy a rite of passage until a major study recently declared that treatment too risky.

To determine how it should study the male hormone question, the National Institutes of Health asked the independent Institute of Medicine -- which advises the government on scientific issues -- to analyze just how much is known about testosterone therapy in older men.

Not much, concludes the report, released Wednesday.

The NIH will do some sort of testosterone research, and hopes to begin studies by 2005, said Dr. Richard Hodes, director of NIH's National Institute on Aging.