They would likely act the same way they did when they were just on heroin. Methadone is supposed to block the activity of heroin and other opiates in the body but it doesn't do that great a job all the time. That said, Methadone's high lasts a long time and a person can act the same way on either drug. Some people on Methadone will also take Xanax with their Methadone, and the combination is remarkably close to the feeling of heroin.
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