Methadone addiction effects?

so i have this friend i havent seen in about a year. even though i didn't know but last time i saw her a year ago she was doing vicodin at least 3 pills a day and now she has quit vicodin and for the past 7 months has been doing methadone and you can tell when she takes it most of the time because she gets all twitchy and hyper or of she isn't talking she will zone out and close her eyes sometimes. its really scary and she is going to a 5 day rehab just to get her family off her back but she plans on continuing using after her detox. i keep telling her how she has a problem because she does anywhere from 3-10 pills a day!!!! but she keeps telling me its no big deals doctors prescribes these pills to people all the time. and i have been googling methadone trying to find bad effects it can cause people and the only thing i have found is tooth decay. so i was wondering is methadone even as bad as i think?? and if anyone can tell me if it does any harm to people in the long run besides being addicted to a pill and tooth decay because i want to try and scare her to show her thisisn'tt healthy and she needs to stops but i need to know if it even does any true harm besides dependency on a pill