"i have had demerol twice via injection during er visits and it worked very well. the first time it took 2 x 100 mg shots to knock out the severe pain caused by the neurotoxin in an atlantic box jelly fish sting - worst pain i&#039;ve ever felt! the second time it was a 75mg shot that pretty much dulled a uterus spasm i was having - felt like someone was ripping my insides out. i was given a shot of vistaril shortly after the demerol and it greatly enhanced the demerol&#039;s effectiveness. "