BACKGROUND Lersivirine is a next-generation non-nucleoside reverse transcriptase inhibitor under development for the treatment of HIV-1 infection. HIV-1-infected patients receiving methadone may have a limited choice of antiretroviral agents due to drug-drug interactions. As methadone is metabolized by CYP3A4 and lersivirine is a weak CYP3A4 inducer, it is possible that lersivirine may decrease methadone concentrations. This study evaluated the effect of lersivirine on the pharmacokinetics (PK) of R- and S-methadone enantiomers. METHODS An open-label, single-sequence study was performed in 13 HIV-negative volunteers receiving stable methadone maintenance therapy (MMT) (50-150 mg QD) for ≥3 months. Healthy volunteers received their methadone to steady-state on day 1 and lersivirine (1000 mg QD) plus their same methadone dose on Days 2-11. Assessments included PK, safety, short opiate withdrawal scale (SOWS), desires for drugs questionnaire (DDQ) and pupillary diameter measurements (PDMs). RESULTS Following administration of methadone alone or in combination with lersivirine, R- and S-methadone concentrations did not appear different (ratios of adjusted geometric means for PK parameters: 95-104%). Following co-administration of lersivirine and methadone, adverse events (AEs) were generally mild to moderate in severity. One patient discontinued due to nausea. An examination of objective (vital signs, AEs, PDM), subjective (SOWS and DDQ scores) and PK data suggested that subjects did not experience opioid withdrawal during the study. CONCLUSIONS Co-administration of lersivirine (1000 mg QD) with methadone did not result in clinically relevant changes in R-/S-methadone concentrations or opioid withdrawal symptoms. No methadone dose adjustment is required when lersivirine is administered alongside MMT.