How does finasteride (generic name) help with hirsutism?

How Finasteride Helps with Hirsutism

Finasteride reduces hirsutism by blocking the conversion of testosterone to dihydrotestosterone (DHT) in hair follicles through competitive inhibition of type II 5α-reductase, the enzyme responsible for excessive hair growth in hirsute women. 1, 2

Mechanism of Action in Hirsutism

• Finasteride is a competitive and specific inhibitor of type II 5α-reductase, which metabolizes testosterone to DHT in the skin and hair follicles 1
• The drug slowly forms a stable enzyme complex with 5α-reductase, with extremely slow turnover (half-life ~30 days), providing sustained DHT suppression 1
• Enhanced 5α-reductase activity in the skin is considered the major mechanism of excessive hair growth in hirsute women, particularly those with idiopathic hirsutism (normal serum androgen levels) 2, 3
• By preventing testosterone conversion to DHT, finasteride directly targets the pathophysiologic mechanism driving hirsutism 2

Clinical Efficacy Data

Hirsutism Score Reduction

• The modified Ferriman-Gallwey score decreases by approximately 40-50% after 6-12 months of treatment 2, 4
• In one study of 17 women with idiopathic hirsutism, scores decreased from 11.7 ± 1.3 to 5.9 ± 0.6 after 6 months (P < 0.01) 2
• A larger study of 35 patients showed score reduction from 19.06 ± 6.12 to 11.31 ± 4.93 over 12 months, with 26 of 35 patients showing clinical improvement 4
• Percent reductions in hirsutism scores were 25.8% at 6 months and 41.3% at 12 months 4

Hair Diameter Changes

• Both finasteride and spironolactone produce significant decreases in anagen hair diameters of approximately 14% (finasteride: -14.0 ± 6.7%, P < 0.05) 3
• Topical finasteride gel 0.25% decreased mean hair thickness from 102.00 ± 9.58 µm to 86.4 ± 11.4 µm after 6 months (P < 0.05) 5

Hormonal Changes

• Serum DHT levels decrease significantly with finasteride treatment, confirming target engagement 2, 6
• 3α-androstanediol glucuronide (3α-diolG), a marker of peripheral 5α-reductase activity, decreases significantly (P < 0.05) 3
• Serum testosterone may increase by approximately 10-20% as DHT conversion is blocked, but remains within physiologic range 1, 3
• The 3α-diolG/testosterone ratio and 3α-diolG/androstenedione ratio both decrease, indicating effective 5α-reductase inhibition 3

Dosing and Treatment Duration

• The standard dose is 5 mg orally once daily 2, 4, 6
• Treatment duration should be at least 6 months to achieve meaningful clinical improvement, with continued benefit up to 12 months 4, 6
• Topical formulations (0.25% gel) applied once daily to affected areas show efficacy with potentially fewer systemic side effects 5

Guideline Recommendations

• The ACOG guidelines state that antiandrogen drugs including finasteride have been used for hirsutism in PCOS, though the primary treatment has not been definitively established 7
• Combined medical interventions (antiandrogen plus ovarian suppression agent) appear most effective, though the best specific antiandrogen is not known 7
• The FDA has labeled only topical eflornithine hydrochloride cream specifically for hirsutism treatment; additional benefits or risks for finasteride use in women with PCOS are unknown 7

Safety Profile in Women

• Finasteride is well tolerated in hirsute women with no significant adverse effects reported in multiple studies 2, 4, 6
• No serious side effects or pathological laboratory findings were observed in a 12-month study of 35 patients 4
• Topical gel application showed no side effects in a 6-month trial 5
• All patients treated with finasteride perceived a reduction in hirsutism after 6 months 6

Critical Caveats

• Finasteride is contraindicated in pregnancy due to potential effects on male fetal development (Category X) - this is critical for women of reproductive age 1
• Women must use reliable contraception during treatment and for at least 1 month after discontinuation 1
• Clinical improvement is gradual, requiring 3-6 months before significant changes are visible 4, 6
• Combination with oral contraceptives may produce slightly more pronounced clinical efficacy 2
• Finasteride shows similar clinical efficacy to spironolactone (100 mg daily) in head-to-head comparison, despite different hormonal effects 3