Testosterone is the key hormone (chemical messenger) in men. It is vital for to the development and maintenance of men’s sexual characteristics and function, including sex drive (libido). Normal active testosterone levels help to maintain normal sex drive and fertility, as well as energy levels and healthy mood. It is also increasingly recognised as being very important to wider male health in a range of other areas, such as contributing to maintaining energy levels and healthy bones. Whilst best known as a male hormone, testosterone is also made, albeit in much lower quantities, in the female body.
Information from the Nurses' Health Study indicated that the combination of estrogen and androgen used to treat hypoandrogenism could increase breast cancer risk. However, other studies indicated androgens may decrease breast cancer risk. Follow-up studies on the Women's Health Initiative found women who received estrogen and no progestogen showed a significant decrease in cardiovascular disease (CVD) and breast cancer. This has caused a reconsideration of androgens added to estrogens. Still, the FDA requires demonstration of CVD and breast cancer safety for any product containing androgens or estrogen plus an androgen; that has not been done.