You are not taking anything too personally at all. I don't what that particular dentist was thinking. It certainly has nothing to do with your medications. In fact we will often put people on 40 mg of prednisone when they have thier teeth extracted to lessen the swelling following the procedure. Canasa has nothing to do with it. In the end I would definitely find another dentist. No hesitation on calling this one.
My only other input is that it's not true that you will 100% need an antibiotic. It's true that 70-80% of the time one will get put on one but it's rare that one will HAVE to have one at all. The best treatment is to have the root canal. I often do not put patients on abs if I can avoid it because the best treatment in the end is to directly drain and clean the source of the infection, hence the root canal procedure itself. If you do need to go on one Z pack works nicely on the gut. You can also shorten the duration, instead of 10 or 7 days one can usually get away with 5 days. I also highly recommend a probiotic while you on the antibioitic anyways.