Statistical Methods/Data Analysis Primary endpoint of the study is the reduction of constipation symptoms severity as measured by the difference between BFI scores taken after one week of receiving combination of methadone and naloxone and after one week of receiving methadone only. Secondary endpoint of the study is the occurrence of opioid withdrawal symptoms as measured by the difference between the mean SOWS scores taken daily during each of two weeks of the study. Paired t-test will be used to compare BFI and mean number of CSBMs between the weeks of treatment with a combination of methadone and naloxone and methadone alone as well as to compare average SOWS scores between the weeks of treatment with a combination of methadone and naloxone and methadone alone.