They're actually related. Flexeril is a muscle relaxant and elavil or Amytriptyline is classified as an antidepressant. Like many antidepressants it's also often used to treat pain as well as migraine headaches. Flexeril may sound like a radically different drug but structurally it's related to the tricyclic class of antidepressants that Amytriptyline is in. So while they're distinct drugs they do share some important commonalities.
http://en.wikipedia.org/wiki/Flexeril
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