The investigators will enroll 20 patients receiving MMT and suffering from opioid-induced constipation. The study has a crossover design - all patients will receive one week of their regular methadone doses and one week of their regular methadone doses with naloxone added. Normal saline will be added to methadone-only formulations as placebo. Order of the weeks will be chosen randomly. Both subjects and investigators will be blinded to the study condition (i.e. whether naloxone or normal saline is added to methadone preparation on a given week).