5α-reductase inhibitors (5-ARIs) are a class of drugs with antiandrogen effects, used primarily in the treatment of benign prostatic hyperplasia (BPH) and androgenic alopecia.

These agents inhibit the enzyme 5α-reductase, which is involved in the metabolic transformations of a variety of endogenous steroids. 5α-reductase inhibition is most known for preventing conversion of testosterone, the major androgen sex hormone, to the more potent dihydrotestosterone (DHT), in androgen-associated disorders.


Inhibition of 5α-reductase results in decreased conversion of testosterone to DHT by reducing the Δ4,5 double-bond. This, in turn, results in slight elevations in testosterone and estradiol levels. Gynecomastia, sexual dysfunction, and depression, are some possible side effects of 5α-reductase inhibition.

Other enzymes compensate to a degree for the absent conversion, specifically with local expression at the skin of reductive 17β-hydroxysteroid dehydrogenase, and oxidative 3α-hydroxysteroid dehydrogenase and 3β-hydroxysteroid dehydrogenase enzymes.[11]

In BPH, DHT acts as a potent cellular androgen and promotes prostate growth; therefore, it inhibits and alleviates symptoms of BPH. In alopecia, male and female-pattern baldness is an effect of androgenic receptor activation, so reducing levels of DHT also reduces hair loss.

http://wikipedia.org/wiki/5-alpha-reductase_inhibitor