Proviron, also know as Masterolone, is a agonist of the androgen receptor. It is not a substrate for 5a-reductase, as it is already 5a-reduced, and hence is not potentiated in so-called “androgenic” tissues such as the skin, hair follicles and prostate gland. Proviron has been used to improve physique and performance and because is not 17a-alkylated, it has little or no potential for hepatotoxicity. It has strong androgenic effects an week anabolic effects, which make it useful for producing masculinization. The drug has no estrogenic effects.