OBJECTIVES: Measuring the level of adherence to drugs is relevant to assess the clinical benefits of prescribed treatments. Adherence can be defined as the extent to which a patient intake of medicines coincides with the medical prescription. The purpose of this study is to review the degree of adherence to drugs among Mexican patients as part of the overall medical therapy. METHODS: A systematic review was performed to retrieve information on quantity measures of drug adherence to medical treatments in Mexico. Key words such as “treatment and patient adherence,” “drug compliance” and “drug utilization” were searched in Pub Med, Medline, Embase, Medic Latina, and the Cochrane Library of Systematic Reviews from 1998 to 2010. RESULTS: Few published studies in Mexico quantify the extent of adherence to drugs among Mexican patients. Most of these studies measured drug adherence in three chronic diseases: diabetes, HIV/AIDS, and rheumatoid arthritis. These were carried out at the regional level with patients from public health institutions. The main methods used were pill-count, questionnaires, and interviews with patients. Adherence to diabetes medication reported frequencies in the range of 17.2%- 54.2%, while in antiretroviral treatment for HIV/AIDS, the range was from 42% to 85.3%. One study reported adherence to disease-modifying antirheumatic drugs in stable patients with early rheumatoid arthritis of 50.5%, with an increased risk of non-adherence as the drug treatment scales-up. CONCLUSIONS: The range of frequencies for drug adherence among diabetic and HIV/AIDS patients varies widely. This can be attributed to the different methods used to measure adherence and the lack of a standardized measuring technique. Adherence results are derived from regional studies; therefore, further research is needed in order to obtain estimates with national representation. This is important for the design of drug policies aimed at enhancing drug adherence to maximize the health benefits from treatments.