Hi.. Thanks for the query.. Yes, Methadone can be a cause for decaying of teeth but as an indirect effect.. This is because Methadone can cause dry mouth leading to reduced saliva production. As saliva plays an important role in continuous cleaning of teeth therefore in absence of optimum amounts of saliva there can be increased decaying of teeth due to reduced flushing debris from the teeth.. Waiting for a week under antibiotic cover is fine and you need not to worry about it but you should get the cause ruled out and work on it.. If it is due to dry mouth then chew sugarfree chewing gums.. -Drink plenty of water. -Avoid regular mouthwash and use specialized mouthwash that are formulated for dry mouth patients like Biotene.. -You can also instil artificial saliva drops.. -Suck tart candies as it also stimulates saliva flow.. If does not improve consult an Oral Physician and get evaluated.. Hope this helps.. Regards.