A 62-year-old woman with well-controlled generalized epilepsy presents for routine follow-up. She is compliant with phenytoin (300 mg once daily at night) and reports no sides effects and no seizure activity in 4 years. Multiple efforts to wean her off phenytoin have resulted in seizure recurrence. Her current phenytoin level is 24.3 µg/mL (therapeutic range = 10-20 µg/mL). Which of the following is the most appropriate course of action?
A. Increase to dose to 400 mg once daily at night.
B. Maintain the current dose once daily at night. **
C. Change the dose to 200 mg twice daily.
D. Decrease the dose to 200 mg once daily at night.
E. Decrease the dose by 100 mg every 2 weeks until off the medication.
** = Your answer
Several attempts to wean the patient off phenytoin have failed; therefore, the current dose should be maintained. Despite the supratherapeutic level, she remains free of seizures and side effects. Increasing the dose may result in side effects given the zero-order kinetics of phenytoin. Decreasing the dose may result in recurrent seizure activity.
* Richens A. Clinical pharmacokinetics of phenytoin. Clin Pharmacokinet. 1979 May-Jun;4(3):153-169.