Are you indulging in sexual activities far less often than you used to? Are you also enjoying it far less? If you're a menopausal or pre-menopausal woman the answer might be testosterone. Ivanhoe's Medical Breakthrough states:

During his presentation at the annual meeting of the International Society for the Study of Women's Sexual Health in Orlando, Fla., Irwin Goldstein, M.D., Ph.D., a former professor of urology and gynecology at Boston University, encouraged physicians to use testosterone therapy to treat patients with low sex drive. In fact, in a recent study of 45 pre-menopausal women with low sex drive, researchers found women who received transdermal testosterone therapy were more likely to engage in sexual activity and enjoyed it more than women who did not receive the therapy. Women who received testosterone treatments also reported having more sexual fantasies and orgasms than their counterparts in the control group.

What, exactly, is testosterone? Wikepedia describes it this way:

It is the principal male sex hormone and an anabolic steroid. In both males and females, it plays key roles in health and well-being. Examples include enhanced libido, energy, immune function, and protection against osteoporosis. On average, the adult male body produces about twenty to thirty times the amount of testosterone that an adult female's body does.