Natural testosterone and its esters, even when applied in supraphysiological doses, rarely produce side-effects. Via a negative feedback mechanism, exogenous testosterone suppresses the production of lutenizing hormone and follicle stimulating hormone, and leads to reduced testicular sperm production and, consequently, reduced testicular volume. The main concerns for the potential adverse effects of testosterone treatment are the prostate and the cardiovascular system. Androgens play a permissive role in the development of prostate cancer and benign prostate hyperplasia; however, there are no data to indicate that testosterone administration can lead to the progression of pre-clinical or clinical prostate cancer. Whether the effects of testosterone treatment on lipid metabolism are clinically relevant is as yet undetermined. The effects of testosterone on behaviour, especially on aggression, have not been firmly established. Some androgen effects, such as virilization and coarsening of the voice, considered normal in adult men are inappropriate in women and children.