Penicillin Allergy


Over 90% of patients who report they are “allergic” to penicillin (PCN) actually have negative skin tests and are not at increased risk of an IgE-mediated allergic reaction.

Adverse reactions to penicillin occur in 0.7 to 10% of all patients who get the drug BUT the incidence of anaphylactic reactions is 0.0024% to 0.015%.

Cross Reactions

  • Rates of cross-reactive IgE-mediated allergic reactions to cephalosporins in PCN allergic patients appear to be uncommon (<5%), but do occur.

  • First generation cephalosporins do have cross-reactivity with penicillins, but cross-reactivity is negligible with second and third-generation cephalosporins.

  • There is a very low cross-reactivity (0.8-1%) between penicillin and carbapenems

  •  Aztreonam does not cross-react with other beta-lactams except for ceftazidime, with which it shares an identical R-group side chain.