Introduction: Methadone maintenance therapy (MMT) is a multiple therapy used for opiate dependence. It is combined with psychological therapies that aim at supporting individuals, families, and communities in improving their health, reducing criminal activities, and minimising risky behaviours for human immunodeficiency virus (HIV) transmission. Retention in treatment is associated with a successful outcome of MMT. Material and methods:Provided since 2008 in Vietnam, MMT has been implemented in Can Tho province since 2010. However, its dropout rate is high: nearly 40% from 2010 to 2015. Evidence is insufficient to understand the factors that influence the drop out rates among MMT patients in Vietnam, and more especially in Can Tho province. There is an urgent need to better understand the reasons and the context in which MMT dropouts occur, in order to inform the development of policies and programmes that can improve adherence to MMT in Can Tho.Results: Qualitative interviews have been conducted with 17 methadone clients (dropouts and current), 4 health workers and 3 family members of clients. Barriers to retention in treatment included individual factors, community factors and institutional factors. Conclusions:The factors influencing the decision to drop out among MMT clients could be at individual, community, or institutional levels. This study suggests that increasing meaningful involvement of the clients' families and strengthening counselling sessions on adherence for clients can improve their retention in treatment and thus the MMT outcomes.