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 . 
There is a myth being spread around both the net and the magazines today regarding which steroids are more likely to cause hair loss, but before we get to that, we must first gain an understanding of the subject of steroid “families”. Every AAS sold on the market today technically belongs to one of three classes (or families) of steroids. These are the testosterone-based, 19 nor-based, and DHT-based families of steroids. Each AAS is defined as belonging to one of these three classes of steroids, based on which of the three molecules it was originally derived from. For example, since the steroid Anavar is an alteration of the DHT molecule, it is classified as belonging o the DHT family of steroids. Since Dianabol is an alteration of the testosterone molecule, it is classified as belonging to the testosterone family of steroids. In reality, “all” steroids are derived from the testosterone molecule itself, but we utilize these 3 classes of AAS as basic sub-categories, in order to further categorize steroids into more specific groups.