Penicillins and Cephalosporins

Cross-reactivity between penicillins and cephalosporins may occur due to the similarities in the C3 and C7 side chains, as noted below.

Risk of cross-reactivity is about 0.5% between first-generation cephalosporins and penicillins, but this lowers with each subsequent generation of cephalosporins.

Cross-reactivity amongst cephalosporins is low, due the heterogeneity of side chains. Therefore a patient with a cephalosporin allergy may be prescribed another cephalosporin if it contains both dissimilar C3 and C7 side chains.

Cefazolin does not share a side chain with any beta-lactam and is not expected to cross react with other agents

Cross reactions are possible within the same group.

Cross reactions are possible within the same group.

For patients who do NOT have evidence of an immediate (type 1) allergic reaction to a penicillin (see below for definition), any cephalosporin can be used.

For patients who have a history of an immediate (type 1) hypersensitivity reaction to a penicillin, some cephalosporins with a different side chain may be used with caution.

If there is no suitable alternative antibiotic and a cephalosporin is deemed necessary, the cephalosporin may be used following successful desensitization.