Phenytoin Dosing Assessment
No, this prescription is incorrect and potentially dangerous—phenytoin sodium extended 100 mg, 2 capsules twice daily (400 mg/day total) exceeds the typical maintenance dose and uses an inappropriate dosing schedule for extended-release formulations. 1
Critical Issues with This Prescription
Dosage Problems
- The FDA-approved maintenance dosage for most adults is 300 mg daily (one 100 mg capsule three to four times daily), not 400 mg daily 1
- Extended phenytoin sodium capsules are specifically designed for once-daily dosing at 300 mg when seizure control is established, not twice-daily administration 1
- The twice-daily schedule of 200 mg doses contradicts the pharmacokinetic properties of extended-release formulations 1
Correct Dosing Approach
For standard maintenance therapy:
- Start with 100 mg three times daily (300 mg/day total) 1
- Adjust only after 7-10 days, as this is the time required to reach steady-state blood levels 1
- Maximum typical dose is 100 mg four times daily (400 mg/day), but this should only be used if necessary and with close monitoring 1
For once-daily dosing (if appropriate):
- Only use 300 mg once daily after seizure control is established with divided doses 1
- This convenience dosing is reserved for compliant patients on stable regimens 1
Safety Monitoring Requirements
Phenytoin requires careful monitoring due to its narrow therapeutic index and saturation kinetics:
- Therapeutic serum levels are 10-20 mcg/mL 1
- Check levels every 3-6 months in stable patients 2
- Monitor more frequently when adding/removing interacting medications, signs of toxicity appear (nystagmus, ataxia, tremor, somnolence, cognitive impairment), or hepatic disease develops 2, 3
Common Pitfalls
The steep dose-response relationship of phenytoin means small dose changes can cause large serum level changes 4. When concentrations reach 5-10 mcg/mL, adjust doses by small increments of approximately 25 mg, not 100 mg jumps 4. The prescribed 400 mg daily dose risks toxicity, particularly given phenytoin's saturation kinetics where elimination capacity becomes overwhelmed at higher doses 5.
Cardiovascular risks exist primarily with rapid IV administration, but oral dosing at recommended levels is generally safe 6. However, the excessive dose in this prescription increases risk of phenytoin encephalopathy, manifesting as cognitive impairment and cerebellar syndrome 5.
Recommended Correction
Rewrite the prescription as:
- Phenytoin sodium extended 100 mg capsule
- Dispense: 90 capsules
- Sig: Take 1 capsule by mouth three times daily
- Duration: 30 days
Or if once-daily dosing is appropriate for an established patient:
- Dispense: 30 capsules
- Sig: Take 3 capsules by mouth once daily