The main treatment phase of the study will last 52 weeks. Participants in Group 1 will receive BUP/NX under the tongue, at first daily and then three times a week for 52 weeks. Participants assigned to Group 1 will take part in a BUP/NX reduction phase, which will occur between Weeks 47 and 52. Participants in Group 2 will receive short-term BUP/NX; dosage and length of treatment will be determined by the investigator. Participants assigned to Group 2 will receive BUP/NX for a maximum of 18 days; detoxification may be repeated at Week 26 if the participant is still injecting opiates. After Week 4 of the safety phase and Weeks 26 and 52 of the overall study, participants will complete an intervention acceptability assessment.