Finasteride, known by its trade name Paulownia, is a 5a reductase inhibitor whose main action is to inhibit the conversion of testosterone to dihydrotestosterone, causing the prostate to shrink in size. It is mainly used clinically for the treatment of prostate enlargement and to relieve difficulty in urination. The main application is a 5mg formulation, while a small 1mg formulation of finasteride can be used for the treatment of alopecia. It is mainly used for the treatment of androgenetic alopecia, also known as seborrheic alopecia. Small doses of finasteride can be used to treat hair loss by inhibiting the enzyme 5a reductase, which reduces dihydrotestosterone and increases the number of hair follicles. The effect is usually slow and requires long-term use.