How does finasteride work?

Finasteride (Propecia) works by lowering the levels of dihydrotestosterone (DHT), a hormone that contributes to hair loss.

DHT is formed from testosterone by an enzyme (protein) called 5-alpha reductase. Finasteride blocks this enzyme, which lowers DHT levels and helps prevent hair loss.

This is why finasteride is referred to as a 5-alpha reductase inhibitor.