I also seem to be allergic to Byetta - having been on it for 2 years and then having a bad reaction when they added insulin to my treatment. At the last appointment with my consultant, he explained that I had had an allergic reaction to it, which rather stunned me. I had been on the drug for 2 years and thought I'd have had a reaction long before then. Obviously not, as your body can take time to develop an allergy to meds.
I frequently get asked - by the health professionals - how I know I am allergic to these meds and once I reel of the list of the things that happened when I take them, I seem to be believed. I find this very annoying as I know exactly what can happen when I take certain drugs, I certainly wouldn't tell anyone I was allergic to something if I wasn't.
It's going to be fun when I have my lumbar puncture, as I am highly allergic to codeine - swollen lips, eyes and an itchy rash developing within seconds of me taking it - so sorting out a pain killer will be a challenge. This allergy means I can't take cough and cold preparations either.