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What Is Already Known About This Subject: * Non-adherence is probably an important source of preventable cardiovascular morbidity and mortality. * However, until now there have been very few large effectiveness studies assessing the relationship between adherence levels to antihypertensive medication and major cardiovascular outcomes for primary prevention of cardiovascular disease., What This Study Adds: * The study results suggest that there is an association between better adherence to antihypertensive agents and a relative risk reduction of coronary artery disease. * Adherence to antihypertensive agents needs to be improved so that patients can benefit from the full protective effects of antihypertensive therapies., Aims: Antihypertensive (AH) agents have been shown to reduce the risk of cardiovascular events, including coronary artery disease (CAD). Previous surveys have shown that a substantial number of patients with diagnosed hypertension remain uncontrolled. Non-adherence to AH agents may reduce the effectiveness. The aim was to evaluate the impact of better adherence to AH agents on the occurrence of CAD in a real clinical setting., Methods: A cohort of 83 267 patients was reconstructed using the Régie de l'assurance maladie du Québec databases. Patients were eligible if they were between 45 and 85 years of age without indication of cardiovascular disease, and had been newly treated with AH agents between 1999 and 2004. A nested case-control design was used to study the incidence of CAD. Every case of CAD was matched for age and duration of follow-up to up to 15 randomly selected controls. The adherence level was measured by calculating the medication possession ratio. Cases' adherence was calculated from the start of follow-up to the time of the CAD (index date). For controls, adherence was calculated from the start of follow-up to the time of selection (index date). Rate ratios of CAD were estimated by conditional logistic regression adjusting for covariables., Results: The mean patient age was 65 years, 37% were male, 8% had diabetes and 18% had dyslipidaemia. High adherence level (96%) to AH therapy compared with lower adherence level (59%) was associated with a relative risk reduction of CAD events (rate ratios 0.90; 0.84, 0.95). Risk factors for CAD were male gender, diabetes, dyslipidaemia and developing a cardiovascular condition disease during follow-up., Conclusion: Our study suggests that better adherence to AH agents is associated with a risk reduction of CAD. Adherence to AH agents needs to be improved so that patients can benefit from the full protective effects of AH therapies.