Proscar is a DHT inhibitor. An inhibitor usually prevents something from being manufactured or converted, rather than simply blocking its receptor-sites. In the case of Proscar, the drug inhibits 5-alfa reductase enzyme which is the enzyme responsible for testosterone (nortestosterone converts to NOR-DHT) conversion to DHT (dihydrotestosterone). As you know by now, DHT is one of the hormones associated with accelerated balding and prostate enlargement. It is also responsible for many of the virilizing effects from androgenic steroids. (Like it also hardens musculature) Proscar is a prescription drug utilized for preventing hair loss. About 83% of users reported a halt to hair loss. Unfortunately, some also report a reduced libido too. (Most sport wooded if the wind blew during AAS cycles). Since Proscar inhibits, or atleast reduces, the conversion of testosterones to DHT, theoretically there would be a greater anabolic effect possible during AAS cycles. But there was an obvious loss of hardness to the physique reported as well.
*Bodybuilders who were chemically assisted did not have to be concerned with the libido issue.