My dad was allergic to practically every antibiotic. He only developed the allergy in his senior years. It was a big problem for him. Even if the antibiotic seemed to be working okay, he had to take a lot of Benadryl just in case and keep an epi pen around.
Thankfully, my mother doesn't seem to have this problem. She's 81, so if she hasn't developed it yet, I don't think she's going to. Really sucked for my dad though.