What will happen if you take one cefprozil (cefzil) and you are allergic?

It depends: Are you allergic to cepfrozil? If you are, it depends on what kind of a reaction you had in the past. If it was a late small rash, another exposure may be just a somewhat worse rash. If you had a severe allergic reaction, the next may be life-threatening. Without knowing what kind of a reaction you had, I can't be more specific.

Don't do it: Too many possibilities here. Depends on the type of allergies - don't fool around. Blistering, mucous membrane reactions can get started and are very difficult to control, even life threatening. Go see an allergiest who can guide you through, and have testing that can help eliminate possible problems.