I have severe asthma and have been on Advair for over five years with no problems. You do need to make sure to rinse your mouth out. In my opinion, if you take into consideration every side effect of every medication, you would be afraid to take any medication. The best advice I can give you is to make sure all doctors who are treating you are aware of any and all medications you are taking. They probably put you on a low dose of Advair because there may be a slight possibility that the higher dose of Advair 500, might trigger your reflux. Please remember I am not a medical professional. I know before I had my esophagus repaired, my allergist had me on a lower dose of Advair for that very reason.
Good luck to you!