Mesterolone - Proviron is a steroid, it is an androgenic agent, which is used due to antiestrogenic properties. Unlike tamoxifen, which only blocks the estrogen receptors, mesterolone prevents direct aromatization and thus suppresses the formation of estrogen. When this medicine also highlight some of those muscle gains, even if minimal, is pretty much the muscle hardness and density. Only slightly hepatotoxic to the liver and kidneys, as this is not a C17-alpha-alkylated material. Mesterolone affect the production of its own testosterone.