Mesterolone is used in the treatment of androgen deficiency in male hypogonadism, anemia, and to support male fertility among other indications. It has also been used to treat delayed puberty in boys. Because it lacks estrogenic effects, mesterolone may be indicated for treating cases of androgen deficiency in which breast tenderness or gynecomastia is also present. The drug is described as a relatively weak androgen with partial activity and is rarely used for the purpose of androgen replacement therapy, but is still widely used in medicine.
Like other AAS, mesterolone is an agonist of the androgen receptor (AR). It is not a substrate for 5α-reductase, as it is already 5α-reduced, and hence is not potentiated in so-called “androgenic” tissues such as the skin, hair follicles, and prostate gland. Mesterolone is described as a very poor anabolic agent due to inactivation by 3α-hydroxysteroid dehydrogenase (3α-HSD) in skeletal muscle tissue, similarly to DHT and mestanolone (17α-methyl-DHT). In contrast, testosterone is a very poor substrate for 3α-HSD, and so is not similarly inactivated in skeletal muscle. Because of its lack of potentiation by 5α-reductase in “androgenic” tissues and its inactivation by 3α-HSD in skeletal muscle, mesterolone is relatively low in both its androgenic potency and its anabolic potency. However, it does still show a greater ratio of anabolic activity to androgenic activity relative to testosterone.
Mesterolone is not a substrate for aromatase, and so cannot be converted into an estrogen. As such, it has no propensity for producing estrogenic side effects such as gynecomastia and fluid retention. It also has no progestogenic activity.
Because mesterolone is not 17α-alkylated, it has little or no potential for hepatotoxicity. However, its risk of deleterious effects on the cardiovascular system is comparable to that of several other oral AAS