5α-reductase inhibitor

Finasteride blocks type 2 5α-reductase, the enzyme that turns testosterone into the more potent androgen dihydrotestosterone (DHT). At 1 mg a day it slows male pattern hair loss and regrows some hair, with benefits that held over 5 years of placebo-controlled trials. At 5 mg a day it shrinks an enlarged prostate and halves the risk of urinary retention and surgery, and in a trial of 18,882 men it cut prostate cancer diagnoses by about a quarter, while slightly raising the rate of high-grade tumours found. Sexual side effects are more common than on placebo, and reports of depression and persistent symptoms after stopping remain contested.