Finasteride

Finasteride is a 5-alpha-reductase inhibitor, a type of antiandrogen medication, developed by Merck and introduced as the first drug of its kind ^[1]. Chemically it is a synthetic 4-azasteroid, structurally related to testosterone and its derivative dihydrotestosterone (DHT) ^[1]. It received approval in the United States in 1992 for benign prostatic hyperplasia, marketed at a higher strength as Proscar, and in 1997 for male pattern hair loss at a lower strength as Propecia ^[1].

Its two main uses both follow from its ability to lower DHT ^[1]. In benign prostatic hyperplasia, an enlargement of the prostate that obstructs urine flow, finasteride shrinks the gland and can ease urinary symptoms over months of use ^[1]. In androgenetic alopecia, or male pattern baldness, the drug slows the progressive miniaturization of scalp hair follicles; a large placebo-controlled study found that daily finasteride increased scalp hair counts and slowed further loss over two years in affected men ^[2]. It is also used off-label for hirsutism and as part of hormone therapy in some transfeminine individuals ^[1].

Finasteride is generally well tolerated, but it is associated with sexual side effects, including reduced libido, erectile dysfunction and ejaculatory problems, in a minority of users ^[1][3]. Some men report that sexual, and sometimes mood-related, symptoms persist after stopping the drug, a contested cluster of complaints described as post-finasteride syndrome; whether it constitutes a distinct medical condition remains debated ^[3]. Analyses have also examined a possible association with depression and suicidal ideation, and prescribing information notes mood changes as a potential effect ^[4].

Finasteride is a prescription-only medicine available as a low-cost generic in the United States, the United Kingdom and elsewhere ^[1]. It is not approved for use in women who are or may become pregnant, because blocking DHT can interfere with the normal development of a male fetus ^[1].

A minority of users experience sexual side effects (reduced libido, erectile or ejaculatory difficulty); these usually resolve after stopping, though persistent post-discontinuation symptoms ("post-finasteride syndrome") are reported and remain debated. Mood changes have also been reported. It lowers PSA, which affects prostate-cancer screening, and must not be handled by women who are or may become pregnant (risk to a male fetus).

Finasteride is metabolized by hepatic cytochrome P450 enzymes and excreted renally; however, its pharmacokinetics are not meaningfully altered by food or by most drugs. ^[22] Unlike alpha-1 adrenergic receptor antagonists commonly used in benign prostatic hyperplasia (such as tamsulosin or alfuzosin), finasteride does not interact with grapefruit juice or alcohol in clinically significant ways. Finasteride can be administered without regard to meals. Coadministration with other 5-alpha reductase inhibitors provides no additional therapeutic benefit and increases the risk of adverse effects; such combinations are not recommended. In elderly patients taking multiple medications, finasteride has a favorable interaction profile and does not require dose adjustment based on concomitant drug therapy.

Checking a whole stack? Run it through interactions + stacks.

Finasteride was developed by Merck & Co and became the first 5-alpha-reductase inhibitor brought to market. Its scientific rationale traced back to the pioneering studies of Julianne Imperato-McGinley, who investigated a population in the Dominican Republic with a congenital deficiency of 5-alpha-reductase type 2; these individuals produced little dihydrotestosterone and, tellingly, developed neither male-pattern baldness nor prostate enlargement. That natural experiment revealed DHT as the key androgen behind both conditions and pointed directly at the enzyme as a drug target. The Food and Drug Administration approved finasteride in 1992 at a 5 mg dose under the brand name Proscar for benign prostatic hyperplasia. In 1997 a lower 1 mg dose was approved as Propecia for male pattern hair loss, giving the same molecule two distinct therapeutic identities.

Finasteride carries a strong, evidence-backed reputation as a foundational treatment for both benign prostatic hyperplasia and male pattern hair loss, with decades of clinical use behind it. Dermatologists and urologists value its targeted mechanism; by lowering DHT while leaving testosterone largely intact, it addresses the specific androgen driving follicle miniaturization and prostate growth. For men with early to moderate hair loss it remains one of the most effective oral options available, often slowing shedding and enabling partial regrowth. Its once-daily oral dosing and low cost as a generic add to its practicality, and a topical formulation is now being explored to concentrate its effect at the scalp. Candor requires acknowledging that a minority of users report sexual or mood-related side effects, so its powerful hormonal action deserves an informed and considered decision.