I cannot really find a legal case from what you described. What I would suggest is that you contact a local attorney who specializes in this area of law and see if they may know of a way you can legally ensure no issues arise for you.In general if the employer feels you are unable to perform your job they will have the right to fire you but again I do not want to jump the gun and cause you to be concerned for no reason. This is why I want you to speak with a local attorney. I would suggest that if you have no choice but to take the medication and you have doctors notes to that effect use that explain your situation explain its seasonal and try to protect yourself by addressing the problems outright rather than hope they go unnoticed. good luck