Asking your doctor or pharmacist to tell you that it's okay to take expired medication, um, isn't going to get you a truthful answer. They're just going to tell you to throw it away. They want nothing to do with you next week, when you walk into their office and tell them you have a rash or a cough or a headache and think it was because you took expired meds.
I work for a pharmaceutical company and my boss is a PharmD - Doctor of Pharmacology - she has told us many times not to worry about expiration dates - that any increase or decrease in strength is negligible and that even with strict manufacturing guidelines, strengths vary from batch to batch - again, in an extremely negligible amount.
Here's an excerpt from an article at Medscape.com on expired drug - I've included the link below - it's pretty interesting and pretty much says that the expiration date is the date the pharmaceutical company guarantees the drug is good by, it is untested as to the actual length of time the drug will maintain it's potency.
"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."
The article states that over 100 different drugs were tested, but I had read elsewhere that 312 different drugs were tested. Expired Tetracycline seems to be the one drug that has proved dangerous if taken - although it could have been the patient's health or the number of year's expired.