Epilepsy treatment. Bottle with pills containing the anticonvulsant drug known as phenytoin. It is commonly prescribed for long-term treatment of epilepsy. Phenytoin is also given to treat trigeminal neuralgia and infrequently to control certain types of arrhythmia (irregularity of the heartbeat). Prolonged use of this drug may cause slurred speech, dizziness, confusion and overgrowth of the gums.