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 . 
Estrogen, which is the main enemy of breast cancer patients, is curbed through SERM and AI. Master also shares these characteristics and is capable of preventing estrogen buildup in the body. As DHT derivative, it does not allow aromatization and displays strong anti-aromatase properties. For this reason, Masteron has been used in combination with other drugs to treat breast cancer patients. The role of Masteron then was to block the amount of estrogens in the body while other drugs, such as Tamoxifen-citrate were used to block the binding of estrogen at the breast receptors. Today, many other more powerful aromatase inhibitors have replaced this breast cancer treatment to achieve even greater success in the fight against breast cancer.
Since estrogen is not an issue, side effects are generally mild with this steroid. As discussed earlier, gynecomastia, water retention, and high blood pressureare not a problem. Masteron is also not liver toxic, so there is little concern stress will be placed on this organ, even during longer cycles. The only real side effects would be from the basic androgenic properties of dihydrotestosterone. These includes oily skin, acne, body/facial hair growth, aggression and accelerated hair loss. Since this compound is already a synthetic DHT, Proscar would have no impact on the level of androgenic effects. Men with a receding hairline or those with a predisposition for baldness may therefore wish to stay away from Masteron completely, as the potent androgenic effect of this steroid can easily accelerate this condition.