I’m a retired nurse, a chronic pain pain who uses buprenorphine patches for pain and I’ll try to explain. The treatment for your OIC in my opinion would not be a medication, it would be fluids and foods not another drug that may be a tier 4. I have been on narcotics for pain since 1988 and control my potential GI issues with fluid and roughage in foods I eat. I also walk 2 miles a day which helps move food downward through your gastrointestinal system. Your GI system breaks down nutrients from food and liquids in the small intestines and the excess fluid that is left over aides at making your colon slippery. If you don’t have sufficient fluid in your system on an ongoing bases you become constipated.
I looked into Relistor and it’s indicated for “advanced disease” meaning constipation and for those who have failed a home laxative program. If you haven’t seen a GI doc it is recommended is you’ve done everything I’ve suggested. Also, a GI doc might have other recommendations too. Good luck.