Steroids can have long-lasting and sometimes irreversible side effects on the body. Anabolic steroids have been linked to increased cholesterol, stroke and blood clots, urinary and bowel problems, and problems with the musculoskeletal system. Since steroids are a hormone, much like testosterone, the effects on sex characteristics can be far reaching, causing a kind of hyper-masculinity in young men. They can also cause male-pattern baldness and shrinking of the testicles. The excess of testosterone can also have feminizing effects on young men, such as breast development. (Jerry Adler, 2004)
The most commonly used AAS in medicine are testosterone and its various esters (but most commonly testosterone undecanoate , testosterone enanthate , testosterone cypionate , and testosterone propionate ),  nandrolone esters (most commonly nandrolone decanoate and nandrolone phenylpropionate ), stanozolol , and metandienone (methandrostenolone).  Others also available and used commonly but to a lesser extent include methyltestosterone , oxandrolone , mesterolone , and oxymetholone , as well as drostanolone propionate , metenolone (methylandrostenolone), and fluoxymesterone .  Dihydrotestosterone (DHT; androstanolone, stanolone) and its esters are also notable, although they are not widely used in medicine.  Boldenone undecylenate and trenbolone acetate are used in veterinary medicine .