 This is a Phase 1, open-label all people know the identity of the intervention, single-sequence study to evaluate the potential interaction of steady-state telaprevir on the steady-state pharmacokinetics of methadone. The study consists of 3 phases: the screening phase within 21 days prior to the start of supervised methadone intake on Day -14, treatment phase, and follow-up phase. Participants on stable methadone maintenance therapy will be enrolled in this study and their methadone doses will not be changed starting at Day -14. During the treatment phase, telaprevir will be added for 7 days from Day 1 to Day 7 to participants current methadone therapy. The methadone dosage will not to be changed from Day -14 until Day 7. All the intakes of methadone and telaprevir will be supervised and pharmacokinetic parameters will be measured during the study. Safety evaluations for adverse events, clinical laboratory tests, alcohol breath test, cardiovascular safety, and other evaluations physical examination, and pharmacodynamic assessments will be performed throughout the study. The total duration of the study for each participant is approximately 60 days. 