Cross-reactivity between penicillins and cephalosporins: what is a reasonable approach depending on reaction history?

• A. Always avoid cephalosporins if penicillin allergy is reported.
• B. Consider potential cross-reactivity and use cephalosporins with caution; consider skin testing or beta-lactam challenge in low-risk cases.
• C. Assume complete cross-reactivity and avoid all beta-lactams.
• D. Prescribe a penicillin anyway.

Answer: (B)

Cross-reactivity between penicillins and cephalosporins depends on the reaction history and the chemical similarities between the drugs, not a blanket rule. The safest approach is to assess how the patient reacted to penicillin and use that to guide cephalosporin use. If the reaction was non-severe or remote, cephalosporins can often be used with caution—especially cephalosporins that have side chains different from penicillin—because the true risk of cross-reactivity is lower than once believed. When there’s concern for true penicillin allergy, penicillin skin testing (to major and minor determinants) can help determine tolerance; a negative result supports using penicillin if needed and informs future decisions, while a positive result indicates caution. In low-risk cases where a cephalosporin is clearly indicated, a supervised beta-lactam challenge or graded-dose exposure can confirm safety. Be mindful that cephalosporins with similar side chains to the penicillin in question carry higher risk, so choosing a cephalosporin with a dissimilar side chain when possible is sensible. A blanket rule to avoid cephalosporins in any penicillin allergy isn’t necessary and isn’t supported by current evidence.