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 . 
When taken for more than 4-6 weeks, it can increase body mass and protein anabolism. However, for the body to build muscle one also has to eat a diet rich in calories. Most bodybuilders also take a combination of other hormones at the same time. Because testosterone is converted to estrogen in the fatty tissues, there is always the chance of developing gynecomastia and water retention. To prevent this, most bodybuilders add Arimidex or tamoxifen during the off cycle.
During testosterone injections, the natural body testosterone release is inhibited by feedback mechanism through inhibition of pituitary LH. At large doses, even spermatogenesis can be impaired. To allow for the hypothalamic-pituitary axis to recover, it is highly recommended that the drug be tapered off slowly over a period of a few weeks. Sudden discontinuation of the drug can lead to extreme fatigue, weight loss and susceptibility to fractures.