What is the typical dose of phenytoin (antiepileptic medication) for a patient at discharge?

Phenytoin Dosing at Discharge

For patients being discharged on phenytoin, prescribe a maintenance dose of 300 mg daily (typically divided as 100 mg three times daily or 300 mg once daily at bedtime), with dose adjustments of 100 mg increments based on serum levels and clinical response, recognizing that most patients (85.6%) will not achieve therapeutic levels on 300 mg daily alone. 1

Initial Maintenance Dosing Strategy

Standard starting dose:

• Begin with 300 mg daily of phenytoin sodium, which serves as the conventional starting point 1
• This can be administered as a single daily dose or divided (100 mg three times daily) 2
• However, recognize upfront that the majority of patients will require dose adjustment, as 85.6% fail to achieve therapeutic concentrations (10-20 mcg/mL) on this standard dose 1

Weight-based considerations:

• Initial dosing based on patient weight or body surface area may be useful for determining starting requirements 1
• Typical maintenance range is 200-700 mg daily depending on individual patient factors 2

Dose Adjustment Algorithm

When to adjust:

• Check serum phenytoin levels 3-7 days after initiating regular oral maintenance dosing (without a loading dose) to assess if therapeutic levels have been achieved 3
• If levels are subtherapeutic, increase dose incrementally by 100-200 mg/day at weekly intervals, with maximum typical adult dose of 1200 mg/day 3

Critical dosing principle:

• Phenytoin exhibits Michaelis-Menten (saturable) kinetics, meaning small dose changes can produce disproportionately large changes in serum concentration 4, 1
• When serum concentration reaches 5-10 mcg/mL, adjust doses by smaller steps of approximately 25 mg rather than 100 mg increments to avoid overshooting into toxic range 4
• The steep dose-concentration relationship means patients tend to have levels that are either too low or too high with standard 100 mg adjustments 4

Timing Optimization for Discharge Prescriptions

Chronotherapeutic dosing:

• Consider prescribing most or all of the daily dose to be taken at 20:00 hours (8 PM) rather than divided dosing 5
• This chronotherapeutic approach can improve response in patients, achieve therapeutic drug levels more reliably, and reduce toxic manifestations 5
• This strategy is particularly effective for diurnally active epileptic patients 5

Monitoring Plan at Discharge

Follow-up timing:

• Schedule serum level check within 3-7 days after discharge if starting maintenance dosing without a loading dose 3
• More frequent monitoring required for patients with hepatic or renal impairment 3

Signs requiring urgent reassessment:

• Educate patients to watch for toxicity signs: nystagmus, ataxia, tremor, somnolence, cognitive impairment 3
• These symptoms warrant immediate level check and potential dose reduction 3

Special Considerations

Enteral feeding interactions:

• If patient will be on tube feedings at home, be aware that phenytoin absorption is severely impaired with continuous enteral feedings 6
• Standard doses (300-500 mg/day) may result in almost undetectable levels 6
• May require dramatically higher doses (up to 1800 mg daily in divided doses) to achieve therapeutic levels in this context 6

Pharmacokinetic variability:

• Individual patients have significant variability in Vmax (maximum rate of drug metabolism) and Km (Michaelis-Menten constant) 1
• Estimated pharmacokinetic values provide the best guide for dosage adjustment beyond simple weight-based dosing 1