Why aren’t I allergic to Demerol?

Patient: Why is it that i experience many side effects to most opioids, such as codeine and several synthetic forms, but not from Demerol. And I ask because I haven’t had to take prescription pain killers many times, however, I recently fell from a 14ft ladder at work and have several broken bones, and will have surgery soon on a torn meniscus. I told my doctor of my allergies and he gave me Pentazocine Naloxone, having never taken it before, I took one. Needless to say, I had the same aweful reactions as the others!

Doctor: