When given in equal doses, phenytoin chewable tablets yield higher plasma levels than extended phenytoin sodium capsules. For this reason serum concentrations should be monitored and care should be taken when switching a patient from the sodium salt to the free acid form.
Extended phenytoin sodium capsules are formulated with the sodium salt of phenytoin. The free acid form of phenytoin is used in phenytoin oral suspensions and phenytoin chewable tablets. Because there is approximately an 8% increase in drug content with the free acid form over that of the sodium salt, dosage adjustments and serum level monitoring may be necessary when switching from a product formulated with the free acid to a product formulated with the sodium salt and vice versa.
General: Not for once-a-day dosing.
Dosage should be individualized to provide maximum benefit. In some cases, serum blood level determinations may be necessary for optimal dosage adjustments—the clinically effective serum level is usually 10 to 20 mcg/mL. With recommended dosage, a period of seven to ten days may be required to achieve steady-state blood levels with phenytoin and changes in dosage (increase or decrease) should not be carried out at intervals shorter than seven to ten days.
Phenytoin chewable tablets can be either chewed thoroughly before being swallowed or swallowed whole.
Adult Dosage: Patients who have received no previous treatment may be started on two phenytoin chewable tablets three times daily, and the dose is then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be six to eight phenytoin chewable tablets daily; an increase to twelve phenytoin chewable tablets daily may be made, if necessary.
Dosing in Special Populations
Patients with Renal or Hepatic Disease: Due to an increased fraction of unbound phenytoin in patients with renal or hepatic disease, or in those with hypoalbuminemia, the interpretation of total phenytoin plasma concentrations should be made with caution. Unbound phenytoin concentrations may be more useful in these patient populations.
Elderly Patients: Phenytoin clearance is decreased slightly in elderly patients and lower or less frequent dosing may be required.
Pediatric: Initially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum 300 mg daily. A recommended daily maintenance dosage is usually 4 to 8 mg/kg. Children over 6 years old and adolescents may require the minimum adult dose (300 mg/day). If the daily dosage cannot be divided equally, the larger dose should be given before retiring.
