This February 18, 2016 NEJM article is part of a $50 Million US Government research project to look at the safety and efficacy of testosterone. The study concluded that testosterone gel significantly increased sexual desire and erectile function, decreased depression and improved mood. Also those who did not have mobility problems improved their 6 minute walking distance.
With regard to safety, although no long term effects could be concluded, there was no increase in cardiovascular events, no increase in prostate cancer. This study was supported by the NIH
Original Article from The New England Journal of Medicine — Effects of Testosterone Treatment in Older Men