Therapeutic drug monitoring of phenytoin was carried out to make sure 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. A situation where monitoring levels may be clinically useful in establishing an individual therapeutic concentration, aiding in the diagnosis of clinical toxicity, assessing patient compliance, and 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.