I can see a few options:
1. Go to court and get them declared imcompetant to make medical decisions for themselves and get yourself granted medical guardian. If you succeed have them sent to an inpatient treatment detox center as well as finding them new doctors to treat their ailments. The problem there is that they may end up going behind your back once they are back home and continuing to see their current quack.
2. Report this doctor to everyone you can think of. If he works as part of a clinic or hospital system contact the management. Also report what is going on to the state medical board, department of health and the state's licensing agency. Hopefully you can at least bring enough pressure on this doctor to get him to rethink his practices if not get his medical license revoked or at least suspended.
The web site below lists which agency in every state that is responsible for medical licensing.
www.ama-assn.org/ama/pub/education-careers/becoming-physician/medical-licensure/state-medical-boards.shtml3. In any case I think you need to take a tough love approach. Stop enabling them by doing things for them. Leave them to their own means until they agree to get help. You don't have to confront them and tell them you think they are drug addicts, but tell them you don't like what the medication is doing to them and you can't bear to see it any longer and until they get some help, which you will be happy to help them do, the best thing for you to do is concentrate on your own life and health. If you do this in person then have someone with you, otherwise a phone call would probably be best given your father's past reception to this.