Masteron, being 5-alpha reduced, cannot form estrogen upon interaction with the aromatase enzyme yet still shows a very high affinity for it. Because it takes up so much of the aromatase enzyme, yet is refrained from actually using it by its structural make-up, it reduces the amount of estrogen formed from other steroids as well because there are less aromatase enzymes to be used by those compounds to form estrogen with (1). However most will find that drostanolone is too expensive to employ it for that reason alone, especially when compounds such as Proviron are available for the same purpose at much lower costs.
1. Hillier SG, van den Boogaard AM, Reichert LE Jr, van Hall EV.,Alterations in granulosa cell aromatase activity accompanying preovulatory follicular development in the rat ovary with evidence that 5alpha-reduced C19 steroids inhibit the aromatase reaction in vitro. J Endocrinol 1980 Mar;84(3):409-19