Don't know if it can affect the test results (vaguely remember reading or hearing something saying it can, but not at all sure about that). But if you are taking mestinon when you no longer should be, it would be having an adverse effect on you. If you are doing well, and not having problems with how you feel and function, or with the meds or the regime you are following, why on earth would a doctor want to mess it up?

(and here's where the snide comments about increasing their billing and patient load comes into play....)