Your question is difficult to answer without some additional information. For example, what do you do for a living? Are you governed by any regulatory rules (for example, DOT) aside from the policies your employer requires adherence to? What type of drug is it? What does the doctor's authority (or lack of authority) to prescribe that current mediation have to do with your current dilemma? - Has the employer raised that as an issue? Have you lost your job? What is the current status of your employment? In providing additional detail, be cautious not to share specifics like employer name, name of drug, etc. -- Just a little more general factual detail like that indicated above would be helpful in responding to your question.
This does not constitute legal advice. You should consult an attorney who can assess the facts of your case and discuss your legal rights and obligations.