5-alpha-reductase inhibitors are a group of medicines that block the action of 5-alpha-reductase, the enzyme that converts testosterone into dihydrotestosterone. This results in increased levels of testosterone and decreased levels of dihydrotestosterone; an overabundance of dihydrotestosterone has been implicated in benign prostatic hyperplasia (BPH) and prostate cancer. The scalp of men with androgenetic alopecia (male-pattern baldness) has also been found to contain increased amounts of dihydrotestosterone and miniaturized hair follicles compared with men who have a lot of hair.
Currently, two 5-alpha-reductase inhibitors are available, finasteride and dutasteride. The 5-alpha-reductase enzyme exists in two forms: type 1 and type 2. Finasteride inhibits type 2 only, whereas dutasteride inhibits both. Although dutasteride provides greater suppression of dihydrotestosterone, it is not known if this provides a significant advantage clinically. Rates of side effects such as impotence, decreased libido, ejaculation disorder, and gynecomastia are similar.
5-alpha-reductase inhibitors may be used in the treatment of benign prostatic hyperplasia (enlarged prostate gland) and male-pattern hair loss (androgenic alopecia). They are not approved for the prevention of prostate cancer.
