This is a list of 5α-reductase inhibitors (5α-RIs), drugs which inhibit one or more isoforms of the enzyme 5α-reductase. This enzyme is responsible for the conversion of the androgen hormone testosterone into the more potent dihydrotestosterone (DHT) and is essential for the production of neurosteroids like allopregnanolone, tetrahydrodeoxycorticosterone (THDOC), and 3α-androstanediol from progesterone, deoxycorticosterone, and DHT, respectively. 5α-Reductase inhibitors have medical applications in the treatment of benign prostatic hyperplasia, androgenic alopecia (pattern hair loss), and hirsutism (excessive hair growth).