I pretty much ignore experiation dates on medicine. mainly because i've taken pills where the expiration date is 6 months after I get them yet I ingest them 2 years after that and they still work like they did when they were new.
my mom, who's a nurse and works with multiple pharmaceutical reps, has told me the same thing-- they're there primarily for the doctor's liability, so some asshole doesn't get a headache, get prescribed something, and then use it again in a year if they get another headache. it's so they go back to the doc, cuz it might be important if it turns chronic. stuff like that. plus, it doesn't hurt the bottom line of pharmaceutical companies if people come in and get new pills every six months or so. I'm not trying to say some "conspiracy theory" but business is business and if Pfizer doesn't make money off of HMO's constantly paying them money, they don't have money to spend on researching important things, like cancer or erectile dysfunction.
however, that's with prescription drugs, not over the counter stuff. so I have no idea.