I'm sorry I just found your post to me. I've been off of here a lot lately due to things dealing with our never ending move. But that is a different story. I know I am allergic to sulfa drugs because they gave me one of the new antibiotics, Cipro. My whole upper body broke out in huge hives and also the inside of my mouth and in my throat. I could just barely swallow it was so swollen. My mother had similar reaction to Penicillin. The doctor told me that my reaction was completely typical of sulfa allergy and stopped the medication at once. So no, it wasn't a "herx" reaction. Thanks for asking.