I am alergic to Penicillin and cephalosporins both. My doctor said the reason is that penicillin is a natural drug and cephalosporins are a synthetic version of penicillin. When you are alergic to penicillin, there is a 25% cross over rate for people to be alergic to both of them. I am also alergic to sulfa and tetracycline........there goes the cheap antibiotics. I wear a bracelet at all times. Penicillin is life threatening for me.