After giving a patient 600 mg IV Eptoin (phenytoin) stat, what should be the maintenance dose given twice daily?

Phenytoin Maintenance Dosing After 600 mg IV Loading Dose

After a 600 mg IV phenytoin loading dose, initiate maintenance therapy with 300 mg daily, which can be given as a single daily dose or divided into 100 mg three times daily. 1

Standard Maintenance Dosing Strategy

• Start with 300 mg daily of phenytoin sodium as the initial maintenance dose following your loading dose 1
• This can be administered either as:
  • A single 300 mg daily dose, OR
  • 100 mg three times daily (divided dosing) 1
• The typical maintenance range spans 200-700 mg daily depending on individual patient factors, but 300 mg is the recommended starting point 1

Critical Timing Considerations

• Begin maintenance dosing approximately 12-24 hours after the loading dose 1
• At 12 hours post-loading, approximately 50% of patients may already have subtherapeutic levels, making timely initiation of maintenance therapy essential 1
• Most patients (approximately 83%) maintain therapeutic levels at 24 hours after appropriate loading, but this requires proper maintenance dosing 1

Monitoring Requirements

• Check phenytoin levels 2-4 hours after loading dose completion to confirm therapeutic range achievement (10-20 mcg/mL total phenytoin) 1
• The 12-hour timepoint is critical for monitoring, as this is when many patients drop below therapeutic levels 1
• Therapeutic levels are reached within 10 minutes of IV loading completion, but the 2-4 hour timepoint confirms sustained therapeutic concentrations 1

Dose Adjustment Algorithm

Important caveat: Phenytoin exhibits non-linear (Michaelis-Menten) pharmacokinetics, meaning small dose changes can produce disproportionately large changes in serum levels once you approach the therapeutic range 2, 3

• When levels reach 5-10 mcg/mL, adjust doses by small increments of 25-50 mg rather than the typical 100 mg increments 2
• If levels are subtherapeutic, increase phenytoin dose incrementally by 100-200 mg/day at weekly intervals, monitoring for efficacy and toxicity 1
• Maximum typical adult dose is 1200 mg/day 1
• An increment of only 50-100 mg will carry the serum level from the lower to the upper limit of the therapeutic range in most patients 3

Special Population Considerations

• Patients with hepatic or renal impairment require more frequent monitoring and may need lower maintenance doses 1
• Elderly patients and those with cardiovascular comorbidities warrant closer monitoring during the transition from loading to maintenance therapy 4

Monitoring for Toxicity

Watch for dose-related adverse effects including:

• Nystagmus (earliest sign) 1
• Ataxia 1
• Tremor 1
• Somnolence 1
• Cognitive impairment 1

Clinical pearl: Some patients achieve complete seizure control with levels below 10 mcg/mL, while others require concentrations at the upper end or above 15 mcg/mL—clinical judgment trumps rigid adherence to reference ranges 1