I kept having my Provigil rx refilled when I had insurance (and the co-pay was only $10). I don't take it everyday so I have a pretty good stash of it saved up. Does it lose it's effectiveness the longer you keep it?

I remember looking at my drug printout that the pharmacy attaches with each rx you get filled and it would say something like "your insurance saved you $769. I couldn't believe one rx could cost so much.