Masteron is an injectable preparation containing the steroid drostanolone propionate. Drostanolone is a derivative of dihydrotestosterone, most specifically 2alpha-methyldihydrotestosterone. As a result, the structure of this steroid is that of a moderate anabolic/potent androgen which does not aromatize to estrogen. Water retention and gynecomastia therefore do not come into play with this drug. Masteron may in fact exhibit anti-estrogenic activity in the body, competing with other substrates for binding to aromatase. This would reduce the conversion rate of other steroids, Masteron acting in the same way as the oral steroid Proviron.