Phenytoin Therapeutic Drug Monitoring Guidelines

Timing of Level Measurement

Indications for Level Monitoring

Target Therapeutic Range

Expected Outcomes Without Dose Adjustment

Management of Subtherapeutic Phenytoin Levels and Toxicity Monitoring

Causes of Subtherapeutic Phenytoin Concentrations

• Potential contributors to a critically low phenytoin serum concentration (e.g., ≈ 0.6 µg/mL) include severe non‑adherence, malabsorption, drug‑drug interactions that increase metabolism, and dosing that is insufficient for the patient’s metabolic capacity. 8

Monitoring for Phenytoin‑Related Toxicity

• Dose‑related neurologic adverse effects such as ataxia, nystagmus, tremor, and somnolence should be monitored; these effects commonly appear when serum levels exceed 20 µg/mL but may occur at lower concentrations in susceptible individuals. [9][8]