A cross allergy is two drugs that interact with one another, causing an allergic reaction. One example of a cross allergy is penicillin and cephalosporins. About 2% of the population is allergic to penicillin. Of these patients who are allergic to penicillin, 2-7% of them are also allergic to cephalosporins. For patients that have had an anaphylactic reaction to penicillin, which would be life-threatening respiratory distress, the cross allergy percent increases to 50%. So people who have had an anaphylactic reaction to penicillin should not use cephalosporins due to what could happen. If I was presented with a prescription that could have a cross allergy like I just described I would speak with the doctor that prescribed the prescription.
