Reply to da bomb...
It's an interesting question to ponder. The simplest reason to say 0.5mg/day finasteride. is effective is that 1mg/day is really more than you need. But the story might be a bit more complicated than that...
Think of it this way: Let's just argue that 1mg finasteride/day is the minimum dose necessary to knock out 100% of the type-II 5AR. Let's also say the dose response to finasteride. is linear, meaning that 0.5mg finasteride/day knocked out 50% of the type-II 5AR, and so on. To make the math easy, let's further assume that it takes ten days to restore full type-II 5AR activity after a 1mg dose of finasteride; your body can synthesize 10% of its full complement of type-II 5AR per day, and in ten days you're back to normal.
OK, assuming those conditions, taking 0.5mg finasteride. would reduce the type-II 5AR in your body by 50%. If you didn't take another dose, next day you would have 60% of normal type-II 5AR, and so on. What if you do take another dose, though? Well, then on day two, you might start with a maximum of 60% of your type-II 5AR, and you'd knock it down to 10%. On day three you'd start with a maximum of 20% of your type-II 5AR, and, oops, you've maxed out your response: now you're down to zero. The next day you start with maxium 10% of the type-II 5AR, and you continue from that point on to knock it down to zero.
Granted, this is a grossly oversimplified model of what's going on in your body, and there are a number of pharmacokinetic considerations that I've completely ignored in the thought experiment above. However, you can get a basic idea of what's going on with repeated finasteride. dosing using the above line of thinking. I would guess that with 1mg/day dosing of finasteride., not only do you destroy all the type-II 5AR in your body, there's enough finasteride. around at any given time that you essentially never get any back. Even 24hr after a 1mg dose of finasteride., there's likely to be enough around to kill any new type-II 5AR that gets synthesized, and then you hit your body with another dose, totally destroying anything that may have escaped by the laws of chance.