In a 68-year-old male nursing-home resident on phenytoin (Dilantin) 200 mg twice daily with a serum phenytoin concentration of 6.3 µg/mL, how should the dose be adjusted?

Phenytoin Dose Adjustment for Subtherapeutic Level

This patient's phenytoin level of 6.3 µg/mL is significantly subtherapeutic (normal range 10-20 µg/mL), and the dose should be increased by 100-200 mg/day, but given phenytoin's nonlinear pharmacokinetics at higher doses, start with a 100 mg/day increment to avoid toxicity. 1

Understanding the Current Situation

Your patient is receiving 400 mg/day total (200 mg twice daily) with a level of 6.3 µg/mL, which is well below the therapeutic range of 10-20 µg/mL needed for seizure control. 2 The typical maintenance range is 200-700 mg daily depending on individual factors, so there is room to increase. 1

Critical Pharmacokinetic Consideration

Phenytoin exhibits zero-order (saturable) pharmacokinetics at therapeutic doses, meaning small dose increases can produce disproportionately large increases in serum levels. 2 The FDA label specifically warns that "small incremental doses may increase the half-life and produce very substantial increases in serum levels when these are in the upper range," and that "the steady-state level may be disproportionately increased, with resultant intoxication, from an increase in dosage of 10% or more." 2

Recommended Dosing Strategy

• Increase the daily dose by 100 mg (to 500 mg/day total), which can be given as 200 mg in the morning and 300 mg at night, or 250 mg twice daily. 1
• When phenytoin concentrations reach 5-10 µg/mL, subsequent adjustments should be made in smaller 25-50 mg increments due to the steep dose-concentration relationship. 3
• Monitor for dose-related adverse effects including ataxia, nystagmus, tremor, somnolence, and cognitive impairment. 1

Timing of Follow-up Level

• Check the next phenytoin level in 7-10 days (5-7 half-lives) after the dose change to allow achievement of steady state. 2
• Obtain a trough level just prior to the morning dose to assess therapeutic adequacy and guide further adjustments. 2

Nursing Home-Specific Concern

Verify that the patient is not receiving continuous tube feedings, as enteral nutrition severely impairs phenytoin absorption and can result in almost undetectable levels despite standard dosing. 4 If tube feedings are present, phenytoin may need to be held 1-2 hours before and after administration, or doses may need to be dramatically increased (one case required 1800 mg/day to achieve a level of only 9 µg/mL with continuous feeds). 4

Common Pitfalls to Avoid

• Do not increase by 200 mg/day initially unless the patient has documented rapid metabolism or urgent seizure control is needed, as this 50% dose increase risks overshooting into the toxic range given saturable kinetics. 2
• Do not assume non-compliance without investigation—in nursing home residents, drug-drug interactions, tube feeding interference, or altered protein binding (common in elderly with low albumin) are more likely explanations. 2, 4
• Watch for drug interactions that could affect phenytoin metabolism through CYP2C9 or CYP2C19 pathways. 5

Monitoring Plan

• Recheck phenytoin level in 7-10 days after dose adjustment. 2
• Perform neurologic examination at follow-up, specifically assessing for nystagmus, ataxia, and tremor as early signs of toxicity. 1
• If levels remain subtherapeutic after the first adjustment and are now 8-12 µg/mL, make smaller 50 mg increments; if still <8 µg/mL, another 100 mg increase is reasonable. 1, 3