Dutasteride is used alone or with another medication (tamsulosin [Flomax]) to treat benign prostatic hyperplasia (BPH; enlargement of the prostate gland). Dutasteride is used to treat symptoms of BPH and may reduce the chance of developing acute urinary retention (sudden inability to urinate). Dutasteride may also decrease the chance that prostate surgery will be needed. Dutasteride is in a class of medications called 5-alpha reductase inhibitors.