Mesterolone, sold under the brand name Proviron among others, is an androgen and anabolic steroid (AAS) medication which is used mainly in the treatment of low testosterone levels. It has also been used to treat male infertility, although this use is controversial. It is taken by mouth.
Side effects of mesterolone include symptoms of masculinization like acne, increased hair growth, voice changes, and increased sexual desire. It has no risk of liver damage. The drug is a synthetic androgen and anabolic steroid and hence is an agonist of the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT). It has strong androgenic effects and weak anabolic effects, which make it useful for producing masculinization.The drug has no estrogenic effects.
Mesterolone was discovered and introduced for medical use in 1934.In addition to its medical use, mesterolone has been used to improve physique and performance, although it is not commonly used for such purposes due to its weak anabolic effects. The drug is a controlled substance in many countries and so non-medical use is generally illicit
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 androgenc 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.
The C1α methyl group of mesterolone inhibits its hepatic metabolism and thereby confers significant oral activity, although its oral bioavailability is still much lower than that of 17α-alkylated AAS. In any case, mesterolone is one of the few non-17α-alkylated AAS that is active with oral ingestion. Uniquely among AAS, mesterolone has very high affinity for human serum sex hormone-binding globulin (SHBG), about 440% that of DHT in one study and 82% of that of DHT in another study. As a result, it may displace endogenous testosterone from SHBG and thereby increase free testosterone concentrations, which may in part be involved in its effects.