Norlidocaine (monoethylglycinexylidide, MEGX) is the major metabolite of lidocaine. The lidocaine metabolites formed on the first pass, which include MEGX and GX, have additive central nervous system (CNS) toxicity to lidocaine itself, making oral administration unacceptable. Monoethylglycinexylidide (MEGX) also has local anesthetic activity. It contributes to central nervous system (CNS) toxicity in some patients treated with lidocaine. Lidocaine is eliminated mainly by biotransfomation to monoethylglycinexylidide. MEGX has pharmacologic activity. MEGX has an antiarrhythmic potency of the same order as lidocaine in vitro in a guinea pig atrium model and is about half as potent in vivo in dogs.