• To evaluate the effect of steady-state TVR 750 mg every 8hr on the steady-state pharmacokinetics of R- and S-methadone (2 forms of methadone which have a different structure). [ Time Frame: On the day before starting TVR and on day 7. ] [ Designated as safety issue: No ]
  

• The potential effect of TVR on the pharmacodynamic effects of methadone therapy will be evaluated. [ Time Frame: The 'Short Opiate Withdrawal scale' and the 'Desires for Drugs Questionnaire' will have to be completed 10 times in a period of 14 days. Also the diameter of the pupil of the eye will be measured 14 times in a period of 14 days. ] [ Designated as safety issue: No ]
  
• The short-term safety and tolerability of coadministration of TVR and methadone on stable methadone maintenance therapy will be evaluated. [ Time Frame: Safety and tolerability will be followed during 1 month. ] [ Designated as safety issue: Yes ]
  

This is a Phase I, open-label (both participant and investigator know the name of the assigned medication), single-sequence (all participants receive treatments in the same order) drug-drug interaction trial for participants on stable methadone maintenance therapy, to investigate the potential interaction between telaprevir (TVR) 750 mg every 8 hours and methadone. The trial population will consist of 16 participants on stable methadone maintenance therapy. Participants will receive TVR 750 mg every 8 hours for 7 days added to their current methadone therapy. The current methadone dosage for the participants is not to be changed from screening until Day 7 inclusive, unless warranted by investigator's assessment of the safety of the participant. During 24 hours, several blood samples will be taken to determine the pharmacokinetic profiles of methadone on the day before starting TVR (while taking methadone alone) and on Day 7 (taking methadone + TVR). On Day 7 (taking methadone + TVR), several blood samples will be taken during 8 hours to determine the pharmacokinetic profile of TVR. The potential effect of TVR on the pharmacodynamic effects (what the drug does to the body) of methadone therapy will be evaluated. The 'Short Opiate Withdrawal scale' and the 'Desires for Drugs Questionnaire' will have to be completed 10 times throughout the study by the patients. Also the diameter of the pupil of the eye will be measured 14 times during the study as changes to the size of the pupils could indicate that TVR increases or decreases the levels of methadone in the blood. Safety and tolerability of coadministration of TVR and methadone will be evaluated. Safety blood samples will be taken at screening, 14 days before starting TVR, the day before starting TVR, day 1, day 3, day 5, day 7, day 8 and at the 2 follow-up visits. A urine sample will be collected at screening, 14 and 7 days before starting TVR, daily from 4 days before starting TVR to day 8 and at the 2 follow-up visits. 3 ECG's (a painless tracing of the electrical activity of the heart) will be recorded at screening, twice on the day before starting TVR, day 1, day 3, day 5, twice on day 7, day 8 and at the first follow-up visit. Vital signs will be taken at screening, 14 days before starting TVR, 3 times on the day before starting TVR, day 1, twice on day 2, day 3, twice on day 4, day 5, 3 times on day 7, day 8 and at the 2 follow-up visits. Illnesses and side effects will be checked from screening until the last trial related visit. A physical examination will be done at screening, 2 days before starting TVR, day 1, day 6 and at the 2 follow-up visits. An alcohol breath test will be done at screening and at 2 days before starting TVR. TVR will be given from day 1 to day 7, every 8 hours as 2 oral tablets. Methadone will be provided as a commercially available formulation (oral tablets or solution) at an individualized dose between 30 and 130 mg daily.