R1 side chains and side chain groups that are shared among cephalosporins

Several cephalosporins share the same moieties at the R1 side chain position (shown in the boxes). The more similar the R1 side chains, the more likely it is that a patient allergic to one will also react to another. Specifically, ceftriaxone, cefuroxime, cefotaxime, cefepime, and cefodizime each have a methoxyimino group (shown in the circles) within the R1 side chain, and cross-reactivity is observed in patients allergic to these agents. In contrast, ceftazidime has a different structure in the R1 position.