Here is some info on it.
Since mesterolone(Proviron) is not used for anabolic purposes for the most part, there must be other reasons why steroid users administer this compound. First, there is some evidence that by using this compound when cycling testosterone, it may actually increase it's potency. It appears that the mesterolone will attach to the sex hormone-binding globulin (SHBG) and albumin. This leaves a larger percentage of free testosterone to conduct anabolic actions.
Mesterolone can also be used for it's action as an anti-aromatase. It is because of this function that many users will use it when stacking other compounds that may partially convert to estrogen. Mesterolone will bind to the aromatase enzyme. This in turn does not allow other steroids to interact with the enzyme and form estrogen.