It depends on the drug and how you store it. Medication should always be stored in a cool, dry, dark area (like a medicine cabinet). Factors such as light, moisture, and heat can result in decreased (or zero) efficacy. With drugs like pain medication, this is not such a big deal, and I've seen people (and clinics) use medication up to 6-12 months past the expiration date. I wouldn't try it with birth control though...

I have some unfinished prescriptions from a few years ago that I never threw out. Just out of curiousity, how long do prescription drugs keep their potency?