The most commonly used 5-alpha-reductase inhibitor is Proscar (finasteride). Proscar is marketed as a selective inhibitor of 5-alpha-reductase, the enzyme that blocks the conversion of testosterone to dihydrotestosterone (DHT). DHT is one of the culprits in stimulating the growth of the prostate gland.

In some cases, finasteride lowers the DHT concentration in prostatic tissue by up to 85 percent. It is prescribed at 5 mg once a day. Although Proscar is the most popular medicine used to treat prostate enlargement, a number of studies comparing it to placebo have shown mixed results. Some of these studies have shown the drug to have clear benefits, while others have shown no statistically significant improvement in the symptoms of patients. However, Proscar has shown clear benefits when studies have restricted its use to those who have very large prostates. Although the fastest decrease in prostate size occurs in the first three months, it's possible the full benefits of Proscar may not be evident right away and could continue up to a year after initiation. Most doctors are now starting to recommend Proscar only for men who have very large prostate glands.

Proscar will not, by itself, be extremely effective. Prostate enlargement has many causes, not just the influence of DHT. Just blocking the conversion of testosterone to DHT is not going to solve the whole problem.

As a rule, the benefits of Proscar therapy are maintained as long as the medicine is continued. It is unclear whether the use of Proscar for many years will induce enough shrinkage to provide continued symptom relief after discontinuing the medicine. Most of the time Proscar is well tolerated by patients. Unfortunately, it does have an unpopular side effect: A small percentage of men lose their sex drive.