Allergic reactions occur with amino ester local anesthetics due to its metabolite:

p-Aminobenzoic acid is one of the metabolites of ester-type compounds that can induce allergic-type reactions in a small percentage of patients. The aminoamides are not metabolized to p-aminobenzoic acid, and repos of allergic reactions to these agents are extremely rare. Ref: Miller's anesthesia-7th ed , Chapter 30.