Hyoscyamine
"I have severe IBS and am frequently suffering from either constipation or diarrhea. The pain from either one (but especially the diarrhea) is excruciating at times and can be debilitating. After a hospitalization for dehydration due to the diarrhea and for pain management, my PCP prescribed a regular dose 1x a day as well as a sublingual PRN dose. The PRN in particular has been an absolute miracle drug. It does cause some drowsiness, but it is worth it to me to get so much pain relief so quickly. I usually feel better within just a couple of minutes of taking it. No other medication has ever helped me so much or so quickly."