Men over 65 with low levels of testosterone saw improved sexual function, mood and walking ability after being treated with the hormone, according to a study published in the New England Journal of Medicine.
Testosterone levels decrease as men age, but prior studies on the effects of testosterone treatment on older men had been inconclusive.
The new study involved seven trials and 790 men who had low testosterone and met other criteria.
They were split into two groups: one treated daily with a testosterone gel, while the other received a placebo.
The study found that testosterone treatment improved all aspects of sexual function, including activity, desire and the ability to have an erection.
Distance walked in six minutes improved when taking into account all the men in the trials, not just those in the physical function trial.
The treatment improved mood and depressive symptoms, but not energy.
The gel treatment increased blood testosterone levels in the participants to a "mid-normal" level for young men, the study said.
"While these initial findings are encouraging for men with low testosterone levels, the (trials) will continue to analyze the data to determine whether testosterone treatment improves cognitive function, bone density, cardiovascular health and anemia, as well as the risks of testosterone treatment," Ronald Swerdloff, a lead researcher at Los Angeles Biomedical Research Institute, said in a statement.
Initial results indicate adverse effects such as heart attack, stroke and prostate conditions were similar in men who received testosterone and those who received the placebo. But the number of men in the trial was too small to draw conclusions about the risks of testosterone treatment.
The study reported the results of the first three trials, which tested sexual function, physical function and vitality.
Testosterone treatment efficacy was evaluated at three, six, nine and 12 months.
Sexual function, mood and vitality were evaluated using questionnaires. Physical function was measured by questionnaires and the distance walked in six minutes.