Mesterolone (also known as Proviron) is an orally effective modified form of dihydrotestosterone (DHT) that differs by the addition of a methyl group at the carbon C1 position, which helps protect the hormone from hepatic metabolism during oral administration. Similar to dihydrotestosterone, mesterolone is a strong androgen with weak anabolic activity. Schering reportedly developed Proviron in 1934, making this an extremely old medication as far as steroids. In clinical practice, it is used to treat sexual dysfunctions of various kinds, which are most often caused by low levels of endogenous androgens. Bodybuilders use mesterolone to increase muscle hardness and density. The main benefit of mesterolone for bodybuilding purposes is that it significantly reduces sex hormone-binding globulin (SHBG); in fact, studies have shown that mesterolone is capable of binding to SHBG much stronger than any other steroid. Reduction of SHBG levels increases the amount of free (bioavailable) testosterone and other steroids, which greatly increases the efficiency of a stack. Athletes primarily use mesterolone to increase androgen levels when dieting or preparing for a competition. 

ADVICE: Some athletes reportedly use Proviron as a PCT drug, claiming that it allegedly helps them recover faster and feel better; this is a disastrous mistake. Proviron is a DHT-derivative steroid; using it during PCT is no different than using other oral steroids and is counterproductive to the goal of restoring the HPTA. Proviron does not stimulate endogenous production of testosterone; instead, it simply replaces the lacking hormone and provides similar effects. Therefore, mental and sexual function may feel improved, but only while using the exogenous steroid. The fact is that Proviron will continue to suppress HPTA function (maybe not as severely as some ASS, but still suppressed nonetheless). Using Proviron during PCT will only further delay HPTA restoration through the mechanism known as negative feedback inhibition. In order to completely and quickly recover after a cycle, do not use Proviron during PCT!