Background and Objectives: Phenytoin is a narrow therapeutic index antiepileptic drug. In Thailand, phenytoin capsules are available in two formulations including immediate-release (IR phenytoin) and extended-release (ER phenytoin). The objectives were (1) to report the number of patients who had been using IR phenytoin once-daily or multiple daily dosing and (2) to compare the seizure frequencies between IR phenytoin once-daily or multiple daily and ER phenytoin once-daily.
Methods: This was a retrospective study conducted at the Epilepsy Clinic of the King Chulalongkorn Memorial hospital. Medical charts of all patients were reviewed. The seizure frequencies were compared within the same patient between using IR phenytoin and being switched to use ER phenytoin at the same daily dose without changing any other concomitant drugs for 3 months.
Results: Out of a total of 4,475 patients, 696 patients received IR phenytoin. Most of them (77.6%) were found to take IR phenytoin once-daily. Among these patients, 31 patients underwent switching of phenytoin from once daily IR phenytoin to once daily ER formulation at the same daily dose without changing any other concomitant drugs for 3 months. Seizure frequencies were significantly lower when the patients were switched to ER phenytoin (p < 0.05).
Conclusion: The proportion of incorrect using IR phenytoin prescribing once daily was high. Our findings highlighted that care must be taken in IR phenytoin prescribing. Frequency of administration should be proper for the chosen dosage formulation to achieve the best outcomes.