The Mesterolone hormone is not estrogenic. It does not aromatize and it carries no progestin nature. As a result, the side effects of Proviron will not include any related effects such as gynecomastia or excess water retention. Such adverse effects are impossible with this steroid. This will also greatly reduce the risk of high blood pressure as high blood pressure associated with anabolic steroid use is often due to extreme water retention. In fact, Proviron should provide an anti-estrogenic effect by preventing testosterone to estrogen conversion or at least tremendously slow it down.
Like other AAS, drostanolone is an agonist of the androgen receptor (AR).  It is not a substrate for 5α-reductase and is a poor substrate for 3α-hydroxysteroid dehydrogenase (3α-HSD), and therefore shows a high ratio of anabolic to androgenic activity.  As a DHT derivative, drostanolone is not a substrate for aromatase and hence cannot be aromatized into estrogenic metabolites .  While no data are available on the progestogenic activity of drostanolone, it is thought to have low or no such activity similarly to other DHT derivatives.  Since the drug is not 17α-alkylated , it is not known to cause hepatotoxicity . 
Masteron is a highly androgenic injectable steroid that is derived from DHT (dihydrotestosterone). DHT does not aromatize to estrogen (and in fact may combat estrogenic sides), and as a result, there was no noted water retention during administration nor gynocomastia. Masteron is almost exclusively used during the last 4-5 weeks before a bodybuilding show at a dose of 100-mg every second day. Additionally, according to available literature, Masteron is not much of a mass drug, and it's always used for cutting, from what I've seen reported. Masteron has a receptor binding ability above that of testosterone, due to it's being DHT-derived, which should impart lypolytic (fat-burning) effects above that of testosterone, and also give it a nice strength building component.