Participants in Group 2 will take 1 dose of slow-release morphine and 1 dose of methadone by mouth every 12 hours, every day for 15 days (plus or minus 3 days). You will also have access to immediate-release morphine to be used, as needed, for pain. If you need more than 3 of these immediate-release doses in a 24-hour period, you should call your study doctor.