The investigators want to prove that people WITHOUT advanced cancer who are taking opioid medications (for problems like back pain) can receive methylnaltrexone (MNTX) safely. Since the FDA has only approved MNTX for advanced cancer patients, the investigators' research is investigating how MNTX can work for NON-cancer patients. This research is being conducted to prove that MNTX can work for non-cancer patients with opioid related constipation.