5-alpha Reductase are enzymes involved in steroid metabolism. They participate in 3 metabolic pathways: bile acid biosynthesis, androgen and estrogen metabolism. There are three isoenzymes of 5α-reductase, which vary in different tissues with age.The mechanism of 5α reductase inhibition is complex, but involves the binding of NADPH to the enzyme followed by the substrate. 5α-reductase inhibitor drugs are used in benign prostatic hyperplasia, prostate cancer, pattern hair loss (androgenetic alopecia), and hormone replacement therapy for transgender women.
Inhibition of the enzyme can be classified into two categories: steroidal, which are irreversible, and nonsteroidal.