5-alpha reductase inhibitor (5-AL-fuh ree-DUK-tays in-HIH-bih-ter)
A substance that blocks an enzyme needed by the body to make dihydrotestosterone (a male sex hormone made from testosterone). Dihydrotestosterone can cause the prostate to grow. 5-alpha reductase inhibitors are used to shrink an enlarged prostate gland and to improve the flow of urine in a condition called benign prostatic hyperplasia (BPH). A 5-alpha reductase inhibitor is a type of enzyme inhibitor.