phenytoin /fen′ətō″in/ , a drug that alters cell membrane conduction of Na+, thereby blocking sodium-dependent action potentials in nerve, heart, and muscle tissues. ▪ INDICATIONS: It is prescribed as an anticonvulsant for the treatment and prevention of tonic-clonic seizures, complex partial seizures, and seizures resulting from head trauma or surgery. It has an unlabeled use as an antiarrhythmic agent, particularly in digitalis-induced ventricular arrhythmias. Establishing and maintaining the desired plasma concentration can be difficult because phenytoin has a high but variable amount of protein binding (typically 80%) and undergoes capacity-limited metabolism (Michaelis-Menten kinetics), with a half-life ranging from approximately 7 to 42 hrs. ▪ CONTRAINDICATIONS: Known hypersensitivity to this drug or to other hydantoins prohibits its use. It is used with caution in patients with a history of hepatic or hematologic abnormalities and in the presence of certain arrhythmias. Phenytoin has been shown to cause fetal malformations, but the benefit to risk ratio can often justify continued use during pregnancy. ▪ ADVERSE EFFECTS: Among the more serious adverse effects are ataxia, nystagmus, hypersensitivity reactions, and gingival hyperplasia. Rarely, a variety of severe reactions occurs. This drug interacts with many other drugs.
