can an employer require, as part of keeping youre employment, a provision that you must stop taking your doctor prescribed pain medication?

Your employer cannot tell you what medical treatment or meds you can/should take. However, if your job involves some duty that makes it dangerous for you to take certain pain killers, then the employer can possibly limit you from performing those duties if it would be unsafe to do so with that medication.  If your doctor is fully aware of what your job duties are and believes that you can perform those duties with the medication, then have the doctor prepare a statement to that effect.