What is the recommended tapering schedule to discontinue phenytoin 100 mg orally twice daily in an adult while monitoring seizure control and serum drug levels?

Phenytoin Discontinuation Strategy

For a patient on phenytoin 100 mg PO BID (200 mg/day), discontinue by reducing the dose by 100 mg every 7-10 days while monitoring for breakthrough seizures, as phenytoin requires 7-10 days to reach steady-state after any dosage change. 1

Tapering Schedule

The FDA-approved prescribing information explicitly states that "changes in dosage (increase or decrease) should not be carried out at intervals shorter than seven to ten days" because phenytoin requires this timeframe to achieve steady-state blood levels. 1 This pharmacokinetic principle applies equally to dose reductions as to dose increases.

Recommended Taper Protocol

• Week 1-2: Reduce to 100 mg once daily (50% reduction)

  • Monitor for seizure activity daily 1
  • Consider checking phenytoin level at day 7-10 if clinically indicated 2

• Week 2-3: Discontinue completely

  • The final dose reduction should also respect the 7-10 day interval 1
  • Continue seizure monitoring for at least 2-4 weeks after complete discontinuation

Critical Monitoring Parameters

Seizure surveillance is the primary outcome measure during any antiepileptic drug withdrawal. Patients must be counseled to:

• Report any seizure activity immediately, including auras or focal symptoms 1
• Avoid driving and other high-risk activities during the taper period
• Have a plan for emergency medical care if seizures recur

Serum level monitoring may be considered but is not mandatory for discontinuation. If levels are checked, they should be obtained at least 5-7 half-lives after dosage changes (approximately 7-10 days for phenytoin). 2

Special Considerations for Slower Taper

While the standard 7-10 day interval between dose reductions is appropriate for most patients, certain high-risk populations may warrant a more conservative approach:

• Patients with history of status epilepticus or frequent seizures may benefit from 100 mg reductions every 2-3 weeks rather than weekly 1
• Patients with hepatic or renal impairment require closer monitoring during withdrawal 3
• Elderly patients (≥55 years) may experience more adverse effects with rapid medication changes 4

Alternative Antiepileptic Drug Considerations

If phenytoin is being discontinued to switch to another antiepileptic drug rather than complete withdrawal, cross-tapering protocols differ significantly. The evidence shows