Testosterone is essential to sexual function, through effects on both the brain and sexual organs. Studies demonstrate a reduction in frequency, size, and stiffness of erection in men with low testosterone. Testosterone also affects energy levels, which may contribute to a decreased interest in sex when energy is low.

Testosterone makes male sex organs grow when boys are developing. Sexual excitement is caused in part by a rise in testosterone as levels rise and fall throughout the day. Some men notice they are more easily excited when testosterone is high, which is typically in the morning.

While low testosterone is a common cause of low sex drive in men, there is a variety of other possible causes. Psychological causes can contribute or cause low libido in men. These include anxiety, depression, stress, or relationship problems. In addition to low testosterone, there is also a variety of other physical causes that can result in a decreased sex drive. They may include being overweight, having a chronic illness, or taking medications such as opiates, beta-blockers, or antidepressants.

In the case of a low sex drive due to low levels of testosterone, increased testosterone levels from the hormone replacement therapy can often increase sex drive and performance. One may notice an immediate change in sex drive as testosterone levels are maintained at a balanced level.