PENICILLIN ALLERGY: WHEN IS IT OK TO USE A CEPHALOSPORIN?

Penicillin and penicilin-based antibiotics are the most widely used antibiotics for common infections and are also the antibiotics which most often cause allergic reactions. Penicillins and cephalosporins share a common beta-lactam ring structure as well as similar side chains which could potentially lead to clinical cross-reactivity. IgE-mediated allergy to penicillins and cephalosporins may be due to the beta-lactam ring structure that is common to this group of drugs, or due to the R-group side chain that distinguishes the different penicillins from each other. After administration penicillin undergoes degradation resulting mainly in the formation of benzyl penicilloyl which is the major antigenic determinant to which the majority of allergic patients react. The beta-lactam ring may act as a hapten by covalently binding to tissue or serum proteins. When cephalosporins degrade, the beta-lactam ring as well as the R-group side chain are exposed. It is the R-group that is believed to play a more important role in the cross-reactivity between penicillins and cephalosporins. Most patients with proven penicillin allergy can safely receive cephalosporin antibiotics after a thorough history is taken and skin prick testing and a drug provocation test performed.