Finasteride hair loss drug molecule, computer model. Atoms are represented as spheres and are colour-coded; carbon (green), hydrogen (white), oxygen (red) and nitrogen (blue). Finasteride (Propecia) belongs to a class of drugs called antiandrogens. It works by inhibiting the enzyme 5-alpha reductase, which converts the male sex hormone testosterone to dihydrotestosterone (DHT). DHT is thought to cause hair loss in men and to promote the malignant growth of prostate cells. Finasteride is used in low doses to treat male pattern hair loss (androgenetic alopecia) and in higher doses to treat enlargement of the prostate (benign prostatic hyperplasia) and prostate cancer.