Phenytoin: A Guide to Therapeutic Drug Monitoring

Therapeutic drug monitoring of phenytoin is carried out to ensure effective and safe levels. Some of the factors complicating phenytoin dosing include a narrow therapeutic window, high degree of protein binding, and nonlinear pharmacokinetics. However, serum drug levels should only be taken when there is a clear indication to guide patient management. Scenarios where monitoring levels may be clinically useful include: (1) establishing an individual therapeutic concentration, (2) aiding in diagnosis of clinical toxicity, (3) assessing patient compliance, and (4) guiding dosage adjustments in patients likely to have greater pharmacokinetic variability. This paper aims to provide an overview on of the correct timing and interpretation of phenytoin levels. It will also address phenytoin dosing, other monitoring parameters, and management of phenytoin adverse effects