Mesterolone, sold under the brand name Proviron among others, is an androgen and anabolic steroid (AAS) medication which is used mainly in the treatment of low testosterone levels. It has also been used to treat male infertility, although this use is controversial.
Mesterolone is an orally active, 1-methylated DHT. Like Masteron, but then actually delivered in an oral fashion. DHT is the conversion product of testosterone at the 5-alpha-reductase enzyme, the result being a hormone that is 3 to 4 times as androgenic and is structurally incapable of forming estrogen. One would imagine then that mesterolone would be a perfect drug to enhance strength and add small but completely lean gains to the frame. Unfortunately there is a control mechanism for DHT in the human body. When levels get too high, the 3alpha hydroxysteroid dehydrogenase enzyme converts it to a mostly inactive compound known as 3-alpha (5-alpha-androstan-3alpha,17beta-diol), a prohormone if you will. It can equally convert back to DHT by way of the same enzyme when low levels of DHT are detected. But it means that unless one uses ridiculously high amounts, most of what is administered is quite useless at the height of the androgen receptor in muscle tissue and thus mesterolone is not particularly suited, if at all, to promote muscle hypertrophy.