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 . 
For example, anabolic steroids such as Testosterone have a tendency to promote water retention through their ability to be aromatized into Estrogen via the aromatase enzyme. While such an effect might not be a concern for a strength athlete or a powerlifter (such an effect might even be beneficial or desired in such sports), this is not a desired effect for athletes involved in sports that involve speed and swiftness, such as sprinting. Instead, a sprinter, for example, would more likely opt for an anabolic steroid such as Stanozolol ( Winstrol ) or Oxandrolone ( Anavar ), which are two anabolic steroids unable to convert into Estrogen and therefore the issue of water retention, and therefore the issue of added weight that would slow the athlete down is avoided. Many athletes may also elect to ‘stack’ anabolic steroids in a given cycle (stacking refers to the practice of combining more than one anabolic steroid in a cycle). In the case of cycle stacks, an individual might be able to increase the synergy and synergistic effects between the anabolic steroids to create a highly anabolic environment or to create a stack that might assist the user in favoring certain particular athletic or physique goals. These are some of the major reasons as to why the development of different types of steroids has been done.
As a non-aromatizing androgen, dihydrotestosterone is extremely potent. Aromatization refers to the conversion of testosterone or anabolic steroids into estrogen. High estrogenic activity causes bloating, acne, water retention and oily skin. As dihydrotestosterone does not aromatize even at high dosages, users do not face the aforementioned side-effects. Lack of water retention also has a hardening effect on muscle tissue, in bodybuilders. Being a powerful androgen, dihydrotestosterone is also responsible for a shift in the estrogen-testosterone ratio in the body. Due to its predominant androgenic component, the steroid has a stimulating effect on the adreno-pituitary functions, and causes neurological excitation in the ‘sexual orientation areas of the brain’. This in turn, spikes sex drive in males.