Methadone-Dolutegravir (DTG - GSK1349572) Drug Interaction Study.

Inclusion Criteria: * Healthy as determined by a responsible and experienced physician, based on a medical evaluation including medical history, physical examination, laboratory tests and cardiac monitoring. A subject with a clinical abnormality or laboratory parameters outside the reference range for the population being studied may be included only if the Investigator feels that the finding is unlikely to introduce additional risk factors and will not interfere with the study procedures. Subjects with positive serology for hepatitis B or C may be entered at the discretion of the investigator, if there is no evidence of active disease or hepatic impairment and it is not a new diagnosis. * Male or female between 18 and 65 years of age inclusive, at the time of signing the informed consent. * Subject is enrolled in a methadone maintenance program for at least 12 weeks prior to Period 1, Day 1, and is expected to continue in the program though the duration of this study. * Subject is receiving a methadone QD regimen that has remained unchanged for 14 days prior to Pre-Screening Visit and the dose is ≤200mg daily. * A female subject is eligible to participate if she is of: Non-childbearing potential defined as pre-menopausal females with a documented (or self reported history) tubal ligation or hysterectomy; or postmenopausal defined as 12 months of spontaneous amenorrhea \[in questionable cases a blood sample with simultaneous follicle stimulating hormone (FSH) \> 40 MlU/ml and estradiol \< 40 pg/ml (\<147 pmol/L) is confirmatory\]. Child-bearing potential and agrees to use one of the contraception methods listed in Section 8.1 for an appropriate period of time (as determined by the product label or investigator) prior to the start of dosing to sufficiently minimize the risk of pregnancy at that point. Female subjects must agree to use contraception until the follow up visit. * Body weight \>/=50 kg for males and \>/=45kg for females and BMI within the range 18.0 - 33.0 kg/m2 (inclusive). * ALT, alkaline phosphatase and bilirubin \</=1.5xULN (isolated bilirubin \>1.5xULN is acceptable if bilirubin is fractionated and direct bilirubin \<35%). * Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form. Exclusion Criteria: * The subject has a positive pre-study drug screen. Drugs that will be screened for include amphetamines, barbiturates, cocaine, and PCP. * The subject has a positive pre-study alcohol screen. * A positive test for HIV antibody. * Current use of alcohol, which in the investigator's opinion, would compromise subject's safety and/or compliance with the trial procedures, including subjects who are likely to experience withdrawal symptoms upon abstinence. * The subject has participated in a clinical trial and has received an investigational product within the following time period prior to the first dosing day in the current study: 30 days, 5 half-lives or twice the duration of the biological effect of the investigational product (whichever is longer). * Subjects using any concurrent prohibited medication. (see Section 9.2, for details on prohibited medications). Use of common medications which are permitted during the study are listed in Section 9.1 (permitted medications). * History of sensitivity to any of the study medications, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GSK Medical Monitor, contraindicates their participation. * Where participation in the study would result in donation of blood or blood products in excess of 500 mL within a 56 day period. * Pregnant females as determined by positive serum or urine hCG test at screening or prior to dosing. * Lactating females. * Unwillingness or inability to follow the procedures outlined in the protocol. * Subject is mentally or legally incapacitated. * History of sensitivity to heparin or heparin-induced thrombocytopenia. * Consumption of red wine, seville oranges, grapefruit or grapefruit juice and/or pomelos, exotic citrus fruits, grapefruit hybrids or fruit juices from 10 days prior to the first dose of study medication. * Subjects with a pre-existing condition interfering with normal gastrointestinal anatomy or motility, hepatic and/or renal function, that could interfere with the absorption, metabolism, and/or or excretion of the study drugs. Subjects with a history of cholecystectomy, peptic ulceration, lower or upper GI bleeding, inflammatory bowel disease or pancreatitis should be excluded. * Subject has inadequate venous access. * History of Gilbert's disease. * The subject's systolic blood pressure at screening visit is outside the range of 90-140mmHg, or diastolic blood pressure is outside the range of 45-90mmHg or heart rate is outside the range of 50-100bpm for female subjects or 45-100 bpm for male subjects. * Exclusion criteria for screening ECG as listed in the protocol. * Evidence of previous myocardial infarction (Does not include ST segment changes associated with repolarization). * Any clinically significant arrhythmia, any clinically significant conduction abnormality (including but not specific to left or right complete bundle branch block, AV block \[2nd degree or higher\], Wolf Parkinson White \[WPW\] syndrome), sinus pauses\>3 seconds, and non-sustained or sustained ventricular tachycardia. * Heavy smokers (e.g., \>20 cigarettes/day) who are unable to abstain for at least 8 hours as required by the protocol.