You hold the ultimate trump card. You are the patient. Without a court order, the physician may only recommend a medication, not force you to accept it.
I have had the same behavior from several of my physicians. I use the following responses in order:
1) I don't tolerate that medication well [physicians understand a patient's "informed consent" and the word "tolerate," so it will shift the conversation in your favor].
2) I have tried that medication before and had side effects such as ___ [legally, physicians try to avoid admitted side effects, so it will continue to shift the conversation in your favor].
3) I have personal or religious reasons to avoid that medication [this response usually shuts them up because most states have laws that require a patient's exemption from medical care based on personal and religious reasons unless a court orders the patient to comply].
Good luck!