Proscar lowers levels of a key hormone called DHT (dihydrotestosterone), which is a major cause of prostate growth. Lowering DHT leads to shrinkage of the enlarged prostate gland in most men. This can lead to gradual improvement in urine flow and symptoms over the next several months. Proscar will help reduce the risk of developing a sudden inability to pass urine and the need for surgery. However, since each case of BPH is different, you should know that: Even though the prostate shrinks, you may NOT notice an improvement in urine flow or symptoms. You may need to take Proscar for six (6) months or more to see whether it improves your symptoms. Therapy with Proscar may reduce your risk for a sudden inability to pass urine and the need for surgery.