What is the goal therapeutic level for Dilantin (phenytoin)?

Therapeutic Level for Dilantin (Phenytoin)

The goal therapeutic level for Dilantin (phenytoin) is 10-20 mcg/mL (or 10-20 μg/mL) in serum. 1

Pharmacological Considerations

Phenytoin is an antiepileptic medication with a narrow therapeutic index, requiring careful monitoring of serum levels to balance efficacy and toxicity. Key points to consider:

• Phenytoin has a long half-life averaging 22 hours (range: 7-42 hours) in adults
• Steady-state levels are typically achieved after 7-10 days (5-7 half-lives) of consistent dosing
• Protein binding characteristics significantly affect free phenytoin levels, which is the pharmacologically active component

Monitoring Recommendations

When to Check Levels

• Trough levels (just before next dose) provide information about clinically effective range
• Peak levels (4-12 hours after administration for extended capsules) indicate threshold for dose-related side effects
• Allow 5-7 half-lives after any dosing change before measuring levels to ensure steady-state has been reached

Special Considerations

• Phenytoin exhibits non-linear pharmacokinetics due to saturable hepatic metabolism
• Small dosage increases (even 10%) may disproportionately increase serum levels when concentrations are already in the upper therapeutic range 1
• Wide interpatient variability exists in phenytoin metabolism, requiring individualized monitoring

Loading and Maintenance Dosing

Adult Dosing

• Initial maintenance: 300 mg/day (typically 100 mg three times daily)
• May increase up to 200 mg three times daily if needed
• Once-daily dosing with 300 mg may be considered if seizure control is established with divided doses

Loading Dose (when rapid therapeutic levels are needed)

• 1 gram divided into three doses (400 mg, 300 mg, 300 mg) administered at two-hour intervals
• Should only be used in hospital settings where levels can be closely monitored
• Not recommended for patients with liver or renal disease

Common Pitfalls to Avoid

1. Formulation differences: Different phenytoin products (extended vs. prompt release, sodium salt vs. free acid) have different bioavailability. Monitor levels when changing formulations. 1

2. Food effects: Taking phenytoin with food can alter absorption by up to 13%, potentially resulting in significant changes in serum levels due to its non-linear pharmacokinetics. 2

3. Drug interactions: Many medications affect phenytoin metabolism, potentially leading to toxic levels or therapeutic failure.

4. Treating the number instead of the patient: While 10-20 mcg/mL is the standard therapeutic range, some patients may require higher levels for seizure control while others may respond at lower concentrations. 3

5. Inadequate monitoring frequency: Due to phenytoin's non-linear pharmacokinetics, more frequent monitoring is needed when making dosage adjustments, especially near the upper therapeutic range.

By maintaining serum phenytoin concentrations within the therapeutic range of 10-20 mcg/mL, clinicians can optimize seizure control while minimizing the risk of adverse effects for most patients.