Testosterone is converted to estrogen in the blood, and some researchers and healthcare providers are concerned that the long-term risks of estrogen therapy will also be seen in women on testosterone therapy. In fact, a study published in Archives of Internal Medicine found that the risk of breast cancer was nearly 2.5 times greater in postmenopausal women who took hormone pills combining estrogen and testosterone than in those who didn't take the medications. The researchers also reported that the risk of breast cancer was greater with estrogen-testosterone therapy than with estrogen alone or estrogen combined with progesterone. A study in The New England Journal of Medicine of 814 women who used testosterone patches or a placebo for a year documented four cases of breast cancer among women using testosterone patches, but no cases in the placebo group -- a difference that could be due to chance but is worrisome nonetheless. Clearly, more study is needed to determine how testosterone might influence the risk of breast cancer.