muscle spasm
"I pulled/strained a muscle in my upper back last week, and the pain was so bad, I could hardly breathe.  My doctor prescribed Flexeril which I took as soon as possible with a little food - around 2:00 PM.  About an hour later I was out COLD - asleep for the rest of the day, that night, and until late the next morning.  Good thing I took a couple of days off from work, because I felt drowsy and weird for at least 24 hours after I took it.  There's no way I could have driven - much less accomplished anything at work.  Having said all this, it really did kill the pain. It's as if I never injured my back.  But at what cost?  I think I'll take 800 mgs of ibuprofen instead if I ever have a similar injury."