Inhibition of steroid 5 alpha-reductase with finasteride: sleep-related erections, potency, and libido in healthy men

To objectively measure the effects of a 5 alpha-reductase inhibitor on erectile function, we studied 20 sexually active men (aged 41-64 yr) during double blind, randomized administration of 5 mg/day finasteride (F) or placebo (P). Serum testosterone and dihydrotestosterone (DHT) were measured every 4 weeks. Sleep-related erections were assessed with comprehensive polysomnography for 2 nights before randomization (session 1) and at week 12 (session 2). Sexual function questionnaires were administered weekly. Serum DHT levels at week 0 were 1.47 +/- 0.11 and 1.16 +/- 0.27 nmol/L (P > 0.05) in the P and F groups, respectively. F group levels fell to 31% and 28% of control values at week 4 and 12. Penile tip peak tumescence time increased on second nights more in the P than the F group at 12 weeks, producing a session main effect (P < 0.02) and a group X session interaction (P < 0.05). No significant group X session interactions were found for any sleep erection measures in a best night analysis or for self-reported sexual activity. Thus, F did not consistently suppress sleep-related erections compared to P. F primarily inhibits type 2 5 alpha-reductase activity; however, type 1 5 alpha-reductase is the major enzyme in the central nervous system. Therefore, DHT involvement in the maintenance of libido and potency is not excluded. Nonetheless, these data support the feasibility of using a type 2 inhibitor to treat benign prostatic hyperplasia without impairing erectile function.