There are potential side effects to testosterone replacement therapy. Men who may have breast cancer or prostate cancer should not undergo testosterone replacement therapy. There has been some controversy about whether the treatment may increase the risk of developing prostate cancer. In addition, it may increase your risk for cardiovascular events, obstructive sleep apnea, and excess red blood cell production. It’s worth having a conversation with your doctor about the latest research, as well as the risks and benefits of testosterone therapy.