One reason for this confusion is the lack of a generally accepted term for the condition. "The term 'male menopause' is biologically silly and anatomically improbable," says Harvard Medical School professor Dr. Richard F. Spark, who, in a 1980 Journal of the American Medical Association article, was among the first to suggest that impotence might have physiological rather than psychological causes.

The word menopause means a pause, or cessation, of the menstrual cycle. But since men don't have menstrual cycles, they can't stop having them. Other terms sometimes enlisted to describe the condition include hypogonadism, viropause, "puberty in reverse," and ADAM, for Androgen Decline in Aging Males. (Androgens are a group of hormones, of which testosterone is the most powerful.)

One of the primary difficulties in identifying and diagnosing male menopause is that the decline in testosterone production is so gradual and drawn out that most men fail to notice the subtle changes they're undergoing. And if they do notice the symptoms - which can include mood swings, depression, anxiety, palpitations, memory loss, even hot flashes - they often blame them simply on getting older. Since testosterone plays a vital role in sexual functioning, this drop-off also causes a decline in libido and erectile function.

"Usually a patient will come in about other issues and when I ask about sexual functioning he'll say, 'Oh yeah, that's a little off, too,'" says Jerome S. Fischer, an endocrinologist at the Diabetes & Glandular Disease Clinic in San Antonio. And while a prescription for Viagra might counteract the more obvious symptom of male menopause, it's important to look into the varied causes - including lower testosterone levels - behind erectile dysfunction before making a choice on treatments. While a decline in testosterone production is a natural part of the aging process, not all men become clinically deficient. Anywhere from 15 to 20 percent of otherwise healthy men between the ages of 40 and 60 can be said to suffer male menopause, and that number increases with each succeeding decade.