BOSTON -- Women aren't the only ones suffering through "hormonal difficulties" as they get older. It happens to guys, too. As men get older, testosterone production declines, leading to a host of symptoms that include reduced sex drive, erectile dysfunction, loss of muscle and energy, even depression. The medical term for this condition is hypogonadism, but it can also be known as male menopause. (Or how about "manopause?") Researchers believe that 19 million men in the U.S. suffer from testosterone deficiency, but right now, only 10% of them are diagnosed and treated, according to a report compiled by ThinkEquity Partners, which held a conference on reproductive health (including male sexual health issues) last week in New York. Still, this relatively small patient population accounts for about $500 million to $600 million in annual sales of testosterone replacement products -- gels, patches and injectables. And as U.S. males get older and more of them seek out diagnosis and treatment, this testosterone replacement market is going to grow strongly. With that in mind, here are three "male menopause" stock plays to consider: Indevus Pharmaceuticals ( IDEV): The current testosterone replacement market is dominated by quick-drying gels, which men must rub on their bodies (usually a shoulder) daily. The gels become part of a daily routine, but still, the hassle of applying a gel to the body every day, waiting for it to dry before dressing, etc., can be a chore.