What drug should be avoided in patients allergic to penicillin due to cross-reactivity?

• A. Vancomycin
• B. Cephalosporin
• C. Aztreonam
• D. Linezolid

Answer: (B)

Cross-reactivity among beta-lactam antibiotics occurs because they share the beta-lactam ring and, in some cases, similar chemical side chains. Because penicillin allergy can raise concern for reactions to related drugs, cephalosporins—being another beta-lactam class—are the ones most likely to cause a cross-reaction. That’s why they’re the drug to avoid in this context.

Vancomycin and linezolid are non-beta-lactam antibiotics, so penicillin allergy doesn’t drive their use patterns. Aztreonam is a monobactam and, although related to beta-lactams, has very low cross-reactivity with penicillin allergies and is often considered safe for patients with penicillin sensitivity, especially when a beta-lactam is needed but broader risk is a concern. The emphasis here is that cephalosporins carry the greatest potential for cross-reactivity among the options.