US Govt. did a study on the military's massive drug reserves (MASSIVE), with the intent to decide if throwing away 'expired' meds was needed, or a waste of our money. Turned out that drugs, typically stored in warehouses in less than perfect environments that were 10+ years old (I think that was the age of them) were 90%+ effective....so I'd not worry about it.
I've had the same bottle of tylenol for ever (literally, forever)...my wife wants to chuck it, but I refuse, I ask her "what did it turn into?" and she says 'old tylenol'...and then I say 'thats right, as I put it back on the shelf'