Therapeutic drug monitoring (TDM) has been used to individualize drug therapy since the early 1970s. In addition, application of pharmacokinetic principles as part of TDM has been used to assess drug behavior. The aim of TDM is to optimize pharmacotherapy by maximizing therapeutic efficacy, while minimizing adverse events, in those instances where the blood concentration of the drug is a better predictor of the desired effect(s) than the dose. The Cochrane database and Medline were searched. References identified by this approach were then searched manually for relevant articles. Very few studies have been performed that document the cost-effectiveness of TDM.