Review Question - QID 214382

A 25-year-old man presents to the emergency department after a witnessed seizure. He is accompanied by a friend, who reports that he has a history of generalized tonic-clonic seizure that was adequately controlled with a seizure medication he cannot recall. His last seizure was 5 years ago, and he self-stopped taking his medication approximately 2 months ago. In the emergency department, the patient developed a generalized tonic-clonic seizure that required a loading dose of fosphenytoin and a maintenance dose of phenytoin. He was admitted to the hospital for further monitoring. A few days later, he developed horizontal nystagmus; however, he has not had seizures since admission. His phenytoin level is 20 mcg/mL (normal range is 10-20 mcg/mL). Which of the following is the most appropriate next step in management?