Cross-Reactivity of Penicillins and Cephalosporins Cross reactivity of penicillin-allergic patients to cephalosporins has more to do with the R1 side chain than the β-lactam ring. Carbapenem cross-reactivity is <1%, and there is no cross-reactivity with monobactams. #CrossReactivity #Penicillins #Cephalosporins #Allergy #Management #Carbapenem #Reaction #Incidence #PCN #Pharmacology