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 . 
This is the scenario: a guy, say age 21, becomes serious about gaining muscle. He’s 5′ 10″, 7″ wrists, 9″ ankles, average genetics for muscle size-and-proportioned. He’s played sports, but never done more than an occasional resistance workout. Now, he begins a good training-eating-and-resting program. With his genetics, he has the potential for naturally gaining 45 pounds of lean mass if he stays consistent with progressive training/proper eating for a continuous 3 to 4 years.
But, about three months after beginning his training, he starts taking steroids. He does three steroid cycles in the following 18 months, and includes proper post-cycle therapy. That entire time, he’s continuing to consistently train and eat properly. Before the end of two years, he’s gained 45 pounds of lean mass (which with steroids, by the way, is not necessarily typical but neither improbable). At that point, he permanently quits using steroids, but he does continue properly training and eating for another two years. At the end of four years, he carries the same 45 pounds of lean mass.