Legal side of DAs and OCDs

I have started this so that we can have a forum to discuss the way forward when it comes to tackling the Great BIG Pharmaceutical Companies (GBPC)! This is because this subject keeps cropping up in other threads that were originally meant for something else.
I don't think you can start legal proceedings against a doctor if the drugs he prescibes are ineffective, or if they have side effects. If this is the case, when is a doctor or organisation liable?