Is it illegal to reuse an "outdated" prescription?

So for the past year I've gone through trial and errors with my doctors trying out different prescription (ie. Paxi, Prozac, Klonopin, Xanax, etc.). About 8 months ago I asked my doctor to switch me from Klonopin back to Xanax (something I had taken previously) because I figured that worked better. I still have my Klonopin prescription bottle with some pills in it, if I were to finish off that bottle, would that be illegal? It still has about 10 pills in it, so it'd take me about 5 months to use it all up.
Source: Cystic Fibrosis Nutrition Forum - Category: Nutrition Authors: Tags: Nutrition Source Type: forums