Diazepam
"I've tried most of the muscle relaxers - flexeril (cannot tolerate), skelaxa (unfortunately cannot tolerate) and Robaxin (not quite strong enough) for a tight and unrelenting  piriformis.  5mg of good old fashioned Valium takes care of it in the evening. Too bad it's sedating otherwise it would be my go to med during the day for this tight muscle - that despite loads of PT, massage and stretching I am unable to get to let go. I really appreciate Valium for this!"