This is what I would do. *Assuming you have not solved this already.
First, it might be illegal for them to drop you because you could have major health problems from stopping some medications cold-turkey.
I would tell them up front in writing that you have called another practice to be seen for a second opinion *I would do this when you have a fresh bottle of pills, and several refills pre-approved, if possible. If there is any flack at all, I would in writing inform them that you are filing a complaint to the Medical Board and I would do so. Perhaps I would call the Medical Board first and ask them how to handle it.
The rationale of this practice is that they can say they are protecting themselves and their patients to prevent patients going to multiple physicians to obtain multiple prescriptions of pills to either take or to sell. I would think that if you tell them up front, they would not be able to drop you. How could they?
This makes sense but I am fearful that some employee will jump the gun. I think it is safer to put the risk on them (the first doctor). She will think twice before she drops you if she knows that you have been above board with her, and she knows she will get an ethics complaint on her record.
If you have a paper trail, I believe you can protect yourself.