What can I do to my doctor who prescribed me something I was allergic to?

Few people are actually allergic to codeine but lots of people say they are because it upsets their stomach, so doctors tend not to put a lot of credence in it unless they have actually had a rash or anaphylactic reaction. Many of the drugs with cousins to codeine can cause a reaction if you're truly allergic, but not necessarily upset your stomach the same way. If you are truly allergic, make sure you make that clear with codeine.