I did some research on this very thing about a year ago when I stumbled upon the very same situation- only percocet. Apparently, the drug may increase or decrease in potency after that 1 year mark, but is safe to take for a while after (i.e. your meds are prolly still good).

Take care,
G

P.S. If you don't get something more specific (I think I recall it saying shelf-life around 4 years, but to be honest, I don't remember if that's right or just what I think I recall, lol) and you want something more specific- do a google or some other engine search on the shelf-life for the drug and you'll find what you're looking for. That's how I found it.