Despite the increasing use of testosterone to stoke waning sexual desire, our consultants say that it's rarely worthwhile, for several reasons. There is little or no correlation between libido, sexual performance, and the normal age-related decline in testosterone. There's little evidence of the therapy's long-term efficacy and safety. And many other factors—from stress and lack of sleep to changed feelings about a partner—can contribute to decreased sex drive.

When low testosterone levels are well below normal, sexuality and performance may be affected. But those declines often stem from correctable health problems, notably pituitary or testicular disorders.

Even when those causes have been ruled out, taking testosterone is still a gamble. In men, while it can boost libido in those who have clearly low levels, side effects include breast enlargement and decreased sperm production. And it may speed the growth of prostate cancer and increase the risk of blood clots.

In women, some research suggests that restoring testosterone to a high normal level can increase sexual desire, at least temporarily. But it can also cause hair growth and acne, and may raise the risk of breast cancer. And other possible risks are largely unknown because of lack of long-term studies.

Moreover, the only testosterone product (EstraTest) approved for women in this country uses a combination that can reduce HDL (good) cholesterol. And while some doctors and pharmacists now compound special testosterone formulations for women, or use creams or gels intended for men, the safety and efficacy of such products or such use is even less well understood.

The reservations about testosterone apply doubly to DHEA, which is still allowed to be sold as a dietary supplement. That's because there's even less evidence of its safety and benefits and because it may increase both estrogen and testosterone levels. Further, dietary supplements are largely unregulated, so you don't know if what's on the label is in the bottle. And your use of this potentially potent hormone is unlikely to be monitored by a physician.

What to do: Our consultants advise avoiding DHEA entirely, and taking testosterone only after evaluation by an endocrinologist, a specialist in hormone therapy. Even then it should be used with extreme caution. (See "Testosterone Treatment in Men and Women," below).

Other measures—treating underlying disorders, adjusting drug dosages, reducing stress, or addressing problems in your relationship with your partner, with a therapist's help if necessary—should generally be tried first. The therapist could be a traditional one, a marriage counselor, or a sex therapist who focuses on negative sexual attitudes or beliefs. And remember that a lack of sexual desire is only a problem if you think it is. A recent study published in the November 2008 issue of Obstetrics & Gynecology found, for example, that while 43 percent of women report sexual problems, most commonly reduced libido, only about 12 percent were actually bothered by it.