The most ideal treatment is to make the prostate enlarged and then reduced in size for the cause of the disease. The development of prostate enlargement is related to endocrine factors, especially with dihydrotestosterone, an androgen that is closely related. In the body, dihydrotestosterone is converted from testosterone by substances of the enzyme 5α-reductase. Finasteride, a 5α-reductase inhibitor, can inhibit the action of 5α-reductase and prevent the enzyme from converting testosterone into dihydrotestosterone, thus stopping or reversing prostate enlargement and giving the possibility to treat prostate enlargement. Clinical studies suggest that daily oral finasteride can significantly reduce the concentration of dihydrotestosterone in the prostate. After 3 months of treatment the enlarged prostate begins to shrink in size, the maximum urinary flow rate increases, lower urinary tract symptoms improve, and there are few side effects and no adverse effects such as feminization due to the use of estrogen and other drugs. Since the drug does not affect plasma testosterone levels, it has no significant effect on normal sexual activity or libido.