Maintenance treatment with methadone can cause serious, potentially fatal adverse effects on the cardiac electrical conduction system leading to a prolonged QT interval and predisposing patients to arrhythmias. As such, cardiac conduction co-morbidity is a (relative) contraindication for the use of methadone as a maintenance treatment. Thus, OT could be an added treatment option to current available treatments such as buprenorphine or levomethadone for maintenance treatment of patients with cardiac conduction defects.