Testosterone therapy improves sex drive and erection capacity in men with low blood levels of the hormone (less than 300 ng/100 ml). A Harvard University study by Ernani Rhoden and Abraham Morgentaler showed that testosterone supplements increased erection capacity, sex drive, energy, and mood in nearly two-thirds of men during a 12-month study. The men received either testosterone injections or testosterone gel.
Testosterone did not improve sexual function in 26 percent of the men. Most middle-aged and older men with low testosterone levels benefit from testosterone supplements within three months and can tolerate treatment for at least a year. (Journal of Sexual Medicine, 7: 277-283, 2010)