The following is an excerpt from: http://www.medscape.com/viewarticle/460159
First, the expiration date, required by law in the United States, beginning in 1979, specifies only the date the manufacturer guarantees the full potency and safety of the drug -- it does not mean how long the drug is actually "good" or safe to use. Second, medical authorities uniformly say it is safe to take drugs past their expiration date -- no matter how "expired" the drugs purportedly are. Except for possibly the rarest of exceptions, you won't get hurt and you certainly won't get killed. A contested example of a rare exception is a case of renal tubular damage purportedly caused by expired tetracycline (reported by G. W. Frimpter and colleagues in JAMA, 1963;184:111). This outcome (disputed by other scientists) was supposedly caused by a chemical transformation of the active ingredient. Third, studies show that expired drugs may lose some of their potency over time, from as little as 5% or less to 50% or more (though usually much less than the latter). Even 10 years after the "expiration date," most drugs have a good deal of their original potency. So wisdom dictates that if your life does depend on an expired drug, and you must have 100% or so of its original strength, you should probably toss it and get a refill, in accordance with the cliché, "better safe than sorry." If your life does not depend on an expired drug -- such as that for headache, hay fever, or menstrual cramps -- take it and see what happens.
One of the largest studies ever conducted that supports the above points about "expired drug" labeling was done by the US military 15 years ago, according to a feature story in the Wall Street Journal (March 29, 2000), reported by Laurie P. Cohen. The military was sitting on a $1 billion stockpile of drugs and facing the daunting process of destroying and replacing its supply every 2 to 3 years, so it began a testing program to see if it could extend the life of its inventory. The testing, conducted by the US Food and Drug Administration (FDA), ultimately covered more than 100 drugs, prescription and over-the-counter. The results showed that about 90% of them were safe and effective as far as 15 years past their original expiration date.
I feel so vindicated in NOT tossing medications for all these years… I guess I will chuck them when they get close to 15 years old… nawh –maybe not. Never know when the end of the world will happen, right? Isn’t it supposed to end this December?? (http://www.december212012.com/) hahahahaha!