The study is an open label methadone- doltegravir (DTG, GSK1349572) drug interaction study consist of 2 periods. Subjects must be opiate-dependent and are HIV sero-negative and are on stable dose of methadone 14 days prior to pre-screening. After meeting all eligible criterions, subjects will remain on their individual methadone dose for 3 days in period 1. In period 2, subjects will continue on their individual stable dose of methadone and will receive 50 mg DTG every 12 hours for 5 days. Pharmacokinetics will be collected on Day 3 of period 1 and Day 5 of period 2. Safety measurements will be routine chemistry, haematology and urinalysis. Pharmacodynamic parameters for pupillometry and opioid symptom questionnaire will be collected at the same timepoints as for the pharmacokinetics. Subjects will be discharged after the last PK sample has been collected. A follow-up visit will occur 7-14 days after the last dose of study drug. This study will be conducted at one centre in Canada.