The effects of diminishing testosterone should be hardly noticeable for most men in their 30s. Once you hit 50, however, the effects may truly start to set in. Your sex drive may be reduced, you may have difficulty getting or maintaining an erection, your overall mood and energy may drop, and you may feel symptoms of irritability and even depression. Fortunately, the availability of testosterone replacement therapies for men over 50 means that you can break this cycle and regain at least some of the vitality you possessed as a young man.
Other significant adverse effects of testosterone supplementation include acceleration of pre-existing prostate cancer growth in individuals who have undergone androgen deprivation; increased hematocrit , which can require venipuncture in order to treat; and, exacerbation of sleep apnea .  Adverse effects may also include minor side-effects such as acne and oily skin, as well as, significant hair loss and/or thinning of the hair, which may be prevented with 5-alpha reductase inhibitors ordinarily used for the treatment of benign prostatic hyperplasia , such as finasteride .  Exogenous testosterone may also cause suppression of spermatogenesis , leading to, in some cases, infertility.  It is recommended that physicians screen for prostate cancer with a digital rectal exam and prostate-specific antigen (PSA) level before starting therapy, and monitor PSA and hematocrit levels closely during therapy.