What is classified as a 5-alpha reductase inhibitor?

Finasteride, known by the brand name Proscar, is classified as a 5-alpha reductase inhibitor because it specifically functions by inhibiting the enzyme 5-alpha reductase. This enzyme is responsible for converting testosterone into dihydrotestosterone (DHT), a more potent androgen that contributes to conditions such as benign prostatic hyperplasia (BPH) and androgenic alopecia. By reducing levels of DHT, finasteride effectively helps in shrinking the prostate gland in males and can also promote hair regrowth in patients with male pattern baldness.

The other medications listed serve different purposes and mechanisms of action. Terazosin and tamsulosin are alpha-1 adrenergic antagonists used primarily to treat BPH by relaxing smooth muscle in the prostate and bladder neck, thus improving urinary flow. Sildenafil, on the other hand, is a phosphodiesterase-5 (PDE5) inhibitor used in the treatment of erectile dysfunction by increasing blood flow to the penis. Each of these medications impacts the body in unique and specific ways, distinguishing finasteride as the only medication in this group that inhibits 5-alpha reductase.