It does not appear that anyone has answered this longstanding “open” post. I will endeavor to provide a response for the Pastor and for others in similar situation.
As a general matter, your employer, in this case the Church, can compel you to take your medication under threat of discipline, including discharge, if you are an at-will employee. In contrast, if you have a contract for employment -- as many ministers do -- then you only have to adhere to the terms and conditions set forth in the agreement.
As an aside, irrespective of whether the Church has grounds to compel you to take the medication, you may nonetheless want to consult with your physician to explore whether he or she can prescribe you a different medication or a different dosage. While you do not perceive anything as being awry, the Church Elders (and probably many of your other congregants) view your behavior as being erratic or otherwise somewhat off. You should not brush aside their perceptions. See Matthew 7:1-5.