The local anesthetics constitute a group of chemically similar agents (esters and amides). Most local anesthetic drugs are esters or amides of simple benzene derivatives. Subgroups within the local anesthetics are based on this chemical characteristic and on duration of action. The commonly used local anesthetics are weak bases with at least 1 ionizable amine function that can become charged through t he gain of a proton (H+). Because the pH of tissue may differ from the physiologic 7.4 (eg, it may be as low as 6.4 in infected tissue), the degree of ionization of the drug will vary. Because the pKa of most local anesthetics is between 8.0 and 9.0 (benzocaine is an exception), variations in pH associated with infection can have significant effects on the proportion of ionized to nonionized drug.