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Depends on where you keep it and the type of medication.
There is a difference between expiration date of a medicine and shelf life.
A drug kept in a closed tight bottle does not suffer from the damaging effects produced by drastic changes in temperature or humidity and should be good for a long time pass the expiration date. But sadly, we tend to keep them in the worse place we can find for safe keeping (bathroom medicine cabinets) and shortens the shelf life way before its time.
Some medication like antibiotics have active ingredients that will lose potency and render them useless.
Other medications like ointments, eye or ear drops, is best to throw them out unless you are very sure they have not been contaminated and continue to be sterile.
Personally, I would take the flexeril because I'm sure it won't kill me, might not fix me but won't kill me. I take them all the time. A year and a half is kind of pushing it though, good luck! Hope you feel better soon.