Pharmacology. The neuronal membrane–stabilizing actions of phenytoin make this drug popular for sustained control of acute and chronic seizure disorders and useful for certain cardiac arrhythmias. Because of the relatively slow onset of anticonvulsant action, phenytoin usually is administered after diazepam. At serum concentrations considered therapeutic for seizure control, phenytoin acts similarly to lidocaine to reduce ventricular premature depolarization and suppress ventricular tachycardia. After intravenous administration, peak therapeutic effects are attained within 1 hour. The therapeutic serum concentration for seizure control is 10–20 mg/L. Elimination is nonlinear, with an apparent half-life averaging 22 hours. Fosphenytoin, a prodrug of phenytoin for intravenous use, is converted to phenytoin after injection, with a conversion half-life of 8–32 minutes.

Indications

1. 
   

   Control of generalized tonic-clonic seizures or status epilepticus. However, benzodiazepines (See Benzodiazepines (Diazepam, Lorazepam, and Midazolam) and phenobarbital (See Phenobarbital) are generally more effective for treating drug-induced seizures.

   
   
2. 
   

   Control of cardiac arrhythmias, particularly those associated with digitalis intoxication.

Contraindications. Known hypersensitivity to phenytoin or other hydantoins.

Adverse effects

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