What is the cross-reactivity concern between penicillins and cephalosporins and how should it guide antibiotic choice?

• A. Cross-reactivity is lower than previously thought; caution with history of severe penicillin allergy; may use cephalosporins with caution or choose alternatives in severe allergies.
• B. Cross-reactivity is 50%; avoid all cephalosporins in any penicillin allergy.
• C. There is no cross-reactivity; all cephalosporins are safe in penicillin allergy.
• D. Cephalosporins are always preferred in penicillin allergy due to broad spectrum.

Answer: (A)

Cross-reactivity between penicillins and cephalosporins is lower than older teachings suggested, so the presence of a penicillin allergy does not automatically exclude cephalosporins. The safest guide is the nature of the allergy. If the reaction was not a severe immediate IgE-mediated event, a cephalosporin can often be used with careful clinical monitoring, especially if the chosen cephalosporin has a different side chain from the penicillin that caused the reaction. In patients with a history of severe anaphylaxis or other life‑threatening penicillin reactions, cephalosporins should be used with caution or alternatives considered, because the risk—although still relatively small—remains higher in those cases. If a cephalosporin is used in a penicillin-allergic patient, selecting one with a dissimilar side chain reduces risk, and some clinicians may perform a graded challenge in appropriate settings when benefits outweigh risks. This approach mirrors the idea that cross-reactivity is not as high as once thought, but caution is warranted in severe penicillin allergies.