Prostate cancer thrives on testosterone. Sometimes doctors castrate patients to reduce the testosterone that will drive a prostate cancer. It seems harsh but it might be a case of would you rather be castrated or die?

Alternate to castration they sometimes use testosterone repression drugs to slow down hormones to castrate levels. All these attempts to fight testosterone are very difficult as any pure drug will have limited success. This happens because the body is a complex and sophisticated feedback mechanism. It measures testosterone levels at all times. So using a pure anti-androgen to reduce the effect of testosterone essentially tells the feedback loop "hey, we need more testosterone, we are not getting enough". The feedback loop then determines the testicles to start producing more testosterone. So the testicles start churning out nearly unlimited testosterone, and eventually kind of overcome the effect of any "pure" anti-androgen, by simply flooding the body with ever more testosterone.

This ongoing process can be overcome by things like GNRh agonists like Lupron. They basically sneakily interrupt that whole feedback loop and tell the body to shut down testosterone production. They essentially "trick" it into thinking testosterone levels are too high in the body. They can reduce testosterone to castrate levels basically as long as you take them. Sometimes to reduce testosterone even further an additional pure anti-a such as Casodex might be used on top of them.