finasteride

Finasteride inhibits type 2 (and to a lesser extent type 1) 5-alpha-reductase, the enzyme responsible for converting testosterone into dihydrotestosterone (DHT), a more potent androgen. DHT is critical for prostate growth and androgenic hair follicle miniaturization. By lowering DHT levels (typically by 60-70%), finasteride shrinks prostate volume in BPH, improving urinary symptoms, and can slow or partially reverse androgenetic alopecia in men. It is also used off-label in combination with alpha-blockers for BPH and has a role in reducing risk of prostate cancer in some studies (PCPT trial), though it may be associated with a slightly increased risk of high-grade prostate cancer, which remains debated. Key adverse effects include decreased libido, erectile dysfunction, gynecomastia, and decreased ejaculate volume ('sexual side effects'). Importantly, finasteride falsely lowers PSA levels by approximately 50%, so PSA values should be doubled when interpreting results in men on this medication for prostate cancer screening. It is a teratogen (pregnancy category X) — pregnant women should avoid handling crushed or broken tablets due to risk of external genitalia abnormalities in a male fetus. Dutasteride is a related drug that inhibits both type 1 and type 2 5-alpha-reductase isoenzymes.