Local anesthetic agents consist of three major chemical moieties (Figure 116-1): a lipophilic aromatic ring, a hydrophilic tertiary amine, and an ester or amide linkage. Changes in the amine or ring chemical structure result in marked alterations in lipid/aqueous solubility, potency, and protein binding. Local anesthetics are classified into two major groups based on the linkage between the lipophilic and hydrophilic components: amino esters and amino amides. Though they exert their effect by the same mechanism, they are metabolized differently (esters in the blood by pseudocholinesterase; amides by normal hepatic pathways) and have different allergic potential (ester greater than amide). The most commonly used local anesthetic agents and their physiochemical properties are described in Table 116-1.