Purpose: Narrow therapeutic index (NTI) drugs must be routinely monitored for their severe effects. These drugs share some common features and criteria that focus on the separation of safety and efficacy in relation to toxicity and efficacy. Methods: There are many factors that contribute to drugs’ high toxicity and lack of efficacy, including the following 5: right drug, right dose, right time, right duration, right age. Results: It’s reviewed over 200 drugs, focusing on NTI drugs’ clinical pharmacology, dose-response relationship, physiochemical properties, and potential drug-drug interactions. It is found that any minor change in therapeutic drug concentrations or doses may have significant consequences for patients, such as bleeding due to warfarin overdose or thrombotic events due to underdose. Conclusions: For NTI drugs, therapeutic drug monitoring (TDM) must be accessible in all hospitals and outpatient clinics to ensure patient safety. These approaches and methods must be done in each patient.