According to WHO, in developed nations, the average adherence rate for individuals with chronic illnesses is barely 50%. Given that pharmaceutical no adherence results in negative health outcomes and higher healthcare costs, this is acknowledged as a serious public health concern. Therefore, increasing medication adherence is essential, and numerous studies have shown that treatments can increase drug adherence. Understanding the extent of medication adherence is an important part of the measures to improve it. In order to plan future interventions, researchers and medical practitioners lack general guidelines on how to select the right instruments to investigate the level of drug adherence and the causes of this issue. Both subjective and objective medication adherence measures are reviewed in this work, including direct measurements, those that use electronic medication packaging (EMP) devices, secondary database analysis, pill counts, clinician assessments, and self-report. While objective assessments help to create a more accurate record of a patient's medication-taking behavior, subjective measures typically offer explanations for the patient's nonadherence. Researchers and medical practitioners should weigh practicality, particularly cost effectiveness, against reliability when selecting an appropriate approach. In the meantime, a multimeasure method appears to be the best option at the moment because there is no perfect measure.