At least 30% of patients receiving methadone maintenance therapy (MMT) are suffering from constipation that often affects effectiveness of MMT and increases its impact on health care system. Existing treatments include several over-the-counter medications which do not target the pathobiological basis of opioid-induced constipation and have limited effectiveness. At the same time well-known medication, naloxone, was already shown to help with constipation in patients receiving methadone for chronic pain, but was never tried in patients receiving methadone for opioid dependence. This study is aimed to try naloxone for treatment of opioid-induced constipation in MMT settings.