Final answer:
Cephalosporins should not be used if a patient is allergic to penicillin.
Step-by-step explanation:
The drug class of antibiotic that should not be used if a patient is allergic to penicillin is cephalosporins. Cephalosporins are structurally similar to penicillin and may cause an allergic reaction in individuals who are allergic to penicillin. It is important for healthcare providers to be aware of a patient's penicillin allergy and choose an alternative antibiotic class, such as macrolides, tetracyclines, or aminoglycosides.