Glossary

Finasteride

A 5α-reductase type 2 inhibitor that lowers scalp DHT by 64 to 70%. FDA-approved at 1 mg/day for male pattern hair loss. The standard first-line oral treatment for male AGA.

Also: Propecia, Proscar

Finasteride is a selective inhibitor of type 2 5α-reductase, the enzyme that converts testosterone to DHT in hair follicles, prostate, and beard tissue.

Doses and indications:

1 mg/day (Propecia): FDA-approved for androgenetic alopecia in men 18 and over. Lowers scalp DHT by approximately 64 to 70% and serum DHT by approximately 70%.
5 mg/day (Proscar): FDA-approved for benign prostatic hyperplasia (BPH); occasionally used off-label for hair loss but does not produce additional hair benefit over 1 mg.
Topical 0.25% spray, once daily: investigational/compounded formulation that achieves around 70% scalp DHT reduction with around 25 to 30% serum reduction (lower systemic exposure than oral).

Timeline: stabilisation of further loss by 3 to 6 months; visible regrowth at 6 to 12 months; maximum effect at 18 to 24 months. Stopping reverses gains within 6 to 12 months.

Side effects: sexual side effects (decreased libido, erectile difficulty, reduced ejaculate volume) occur in roughly 1 to 4% of users above placebo rates. Most resolve on stopping. Persistent symptoms after stopping (post-finasteride syndrome) are reported but uncommon and contested in mechanism.

Not used in women of reproductive age because of risk of male foetus feminisation. Used off-label in postmenopausal women for FPHL.