A review of factors associated with methadone maintenance treatment adherence and retention in Vietnam

•In Vietnam, MMT adherence and retention rates have a downward trend over time and are lower in mountainous provinces.•Factors associated with adherence and retention are at individual, community, and institutional levels.•Important individual-level factors are age, education, awareness of MMT and HIV, co-occurring disorders, and comorbidities.•Stigma is the major community risk factor.•Suboptimal methadone doses, distance to clinic, and clinic service hours are the three most important institutional factors.