From the FDA Drug Label
Adult Dosage: Divided daily dosage: Patients who have received no previous treatment may be started on one 100 mg extended phenytoin sodium capsule, USP three times daily and the dosage then adjusted to suit individual requirements. For most adults, the satisfactory maintenance dosage will be one capsule three to four times a day An increase up to two capsules three times a day may be made, if necessary. Pediatric: Initially, 5 mg/kg/day in two or three equally divided doses, with subsequent dosage individualized to a maximum of 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) 1
The usual dosing of phenytoin for adults is 300-400 mg/day, divided into 3-4 doses, with a maximum dose of 600 mg/day. For children, the initial dose is 5 mg/kg/day, divided into 2-3 doses, with a maximum daily dose of 300 mg and a recommended maintenance dose of 4-8 mg/kg/day. Children over 6 years old and adolescents may require the minimum adult dose of 300 mg/day.
From the Research
Phenytoin dosing for adults typically starts with a loading dose of 15-20 mg/kg IV or orally, followed by a maintenance dose of 300-400 mg/day (4-6 mg/kg/day) divided into 1-3 doses, while for children, the loading dose is similar at 15-20 mg/kg, with maintenance dosing of 5-8 mg/kg/day divided into 2-3 doses, as supported by the study 2 which found that a phenytoin loading dose of 18 mg/kg gave serum concentrations within the recommended therapeutic range in most children. The dosing of phenytoin is crucial due to its non-linear pharmacokinetics, requiring therapeutic drug monitoring to achieve a target serum concentration of 10-20 mcg/mL 3. Key considerations include:
- Loading dose: 15-20 mg/kg IV or orally for both adults and children
- Maintenance dose:
- Adults: 300-400 mg/day (4-6 mg/kg/day) divided into 1-3 doses
- Children: 5-8 mg/kg/day divided into 2-3 doses
- Therapeutic drug monitoring is essential due to the drug's narrow therapeutic index
- Patients with hepatic impairment, hypoalbuminemia, or taking interacting medications may require lower doses
- Administration with food can reduce gastrointestinal side effects, though this may affect absorption
- Phenytoin works by blocking voltage-dependent sodium channels in neurons, preventing repetitive firing of action potentials and limiting seizure spread
- Common side effects include gingival hyperplasia, nystagmus, ataxia, and skin rashes, which should be monitored during treatment, as noted in studies such as 4 which examined neuro-ophthalmological signs during rapid intravenous administration of phenytoin. It's also important to consider the age-dose-concentration relationship in children, as shown in the study 3, which found that many patients receiving commonly recommended pediatric phenytoin doses achieved concentrations below the reference range, especially children under 3 years of age.