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 . 
Prohormones are legal dietary supplements that are available to buy over the counter. They are best defined as steroid precursors, meaning that they aren't technically as steroid, but are converted into one by the body upon ingestion... Prohormones are legal dietary supplements that are available to buy over the counter. They are best defined as steroid precursors, meaning that they aren't technically as steroid, but are converted into one by the body upon ingestion. There are many different types of these compounds available with each displaying a various set of pros and cons for their use, making them available for a wide array of applications.
Although many companies say that the minimum age to use these products is 18, we strongly recommend that no one under the age of 21 years old use them due to the risks associated with the development of the endocrine system.
There are many prohormones on the market today, and they are often categorized by class. Class 1 pro hormones are non-methylated (meaning liver friendly), they have low risk of side effects, and the gains are easy to maintain making the overall cycle easier to plan. These compounds are often used as part of a stack with higher class PH's by advanced users. Generally an over the counter PCT will suffice for cycles containing just a Class 1 prohormone. Compounds in class I include Furazadrol, Propadrol, 11-OXO and Bold.
The next stage is Class 2 prohormones. These compounds are methylated therefore liver protection is required. Side effects are much more likely to occur than class 1 compounds, however gains are often better with class 2 compounds. Prescription grade PCT regiments are recommended for class 2 compounds, however in some cases over the counter will suffice. Compounds in class 2 include H-Drol, Promagnon and Protodrol.
The next level of prohormones are those in class 3. These compounds are for advanced users that have previously completed 2-3 successful cycles. The side effects are more common than class 2 compounds and gains at this level are harder to maintain post cycle and require prescription grade post cycle therapy regiments. Cycles at this stage are also shorter in length due to the occurrence of side effects. Compounds include Tren, Epistane, M-1, 4ADD, Max LMG and DMZ.
Finally, the most intense level of prohormones are those in the class 4 category. These compounds are recommended only for advanced users as they will produce rapid gains but they come with significant side effect risks. Compounds in this class include Pheraplex, Superdrol and Mass Tabs.