Mesterolone (Proviron)

The 1α-methyl group makes it orally active, and because it is already 5α-reduced (a DHT analog) it is NON-AROMATIZABLE — it cannot be converted to estradiol.

Pharmacologically it is an unusual androgen: in receptor-binding studies it binds the androgen receptor in muscle and prostate relatively weakly (a weak anabolic), yet binds sex-hormone-binding globulin (SHBG) extremely avidly — about four times more strongly than DHT itself.

That SHBG affinity is the mechanistic basis for its main off-label rationale in the bodybuilding community: by occupying SHBG it can transiently raise the free (unbound) fraction of a person's own or co-administered testosterone, and its DHT-like signaling is credited anecdotally with libido and a 'hardening'/anti-estrogenic feel.

None of those bodybuilding uses are well evidenced in controlled trials. The premise was that a 'stimulatory' androgen given at a dose low enough not to suppress pituitary gonadotrophins might directly support spermatogenesis or sperm maturation in the epididymis. That premise largely failed in controlled testing.

A large WHO Task Force multicentre randomized double-blind trial of 75 or 150 mg/day mesterolone vs placebo in idiopathic male infertility found no significant difference in pregnancy rate or semen quality.

A Belgian double-blind placebo-controlled trial of high-dose (150 mg/day) mesterolone for 12 months actually saw a higher pregnancy rate in the placebo arm, with the authors concluding the data 'cast doubt' on its usefulness.

A randomized comparison against clomiphene found mesterolone (like placebo) did not significantly raise sperm concentration or pregnancies, while clomiphene raised sperm concentration.

A Cochrane systematic review of androgens (including mesterolone) for idiopathic oligo/asthenospermia, since withdrawn by Cochrane, found pooled pregnancy odds ratios crossing 1.0 and concluded there is not enough evidence to evaluate their use.

So the marketed fertility indication is, on the controlled evidence, weak-to-absent.

Some smaller and older studies report softer benefits — improved subjective potency/'aging male' symptom scores and lower-urinary-tract quality of life, and the predictable endocrine signature of an exogenous androgen (suppressed total testosterone, estradiol, and SHBG, with a rise in the free-testosterone fraction).

The safety side is the side of a real androgen: as a 5α-reduced DHT derivative it can drive androgenic effects (acne, oily skin, accelerated male-pattern hair loss, prostate stimulation), it can adversely shift lipids (lower HDL) like other oral AAS, in-vitro work groups it with anabolic steroids that disturb adrenal steroidogenesis (a proposed mineralocorticoid/cardiovascular concern), and at higher doses any androgen can suppress the hypothalamic-pituitary-testicular axis.

It is a prescription anabolic-androgenic steroid and a doping agent on the WADA list — not a dietary supplement, not a longevity drug.

The overall picture: a genuine, mechanistically-distinct androgen (non-aromatizable, SHBG-binding, weakly anabolic) whose headline marketed use — improving male fertility — is not supported by controlled trials, whose bodybuilding uses are poorly evidenced, and which carries the androgenic risks of its class.

This is a hormonal compound for harm-reduction information, not a recommendation.