Like other opioids – heroin, Vicodin®, oxycontin®, Vicodin® or fentanyl – too much methadone can be harmful. Unlike these drugs, methadone effects start slow and take time to reach full effect. Methadone stays in the body much longer than other opioids. MAT clinical staff will start a patient on a low dose of methadone and will gradually increase the dose over time. This increases the total amount accumulated in the body.