Local anesthetics consist of a lipophilic and a hydrophilic portion separated by a connecting hydrocarbon chain (Fig. 10-1). In almost all instances, an ester (-CO-) or an amide (-NHC-) bond links the hydrocarbon chain to the lipophilic aromatic ring.

The nature of the connecting hydrocarbon chain is the basis for classifying drugs that produce conduction blockade of nerve impulses as ester local anesthetics or amide local anesthetics (Fig. 10-2). The important differences between ester and amide local anesthetics relate to the site of metabolism and the potential to produce allergic reactions.