OBJECTIVES: To determine the association between medication adherence and adverse drug events (ADEs), health related quality of life (HRQOL) and hospitalisation in older community dwelling patients. METHODS: A retrospective cohort study of 855 patients aged ≥70 years from 15 general practices in Ireland in 2010.  Medication adherence was measured by: (i) the Medication Possession Ratio (MPR) using national pharmacy claims dispensing data; and (ii) self-report using the Morisky Medication Adherence Scale. ADEs and hospitalisation were measured by patient medical record and self-report for the previous 6 months. ADEs were reviewed by two independent clinicians. HRQOL was measured using EQ-5D. Multilevel Poisson and linear regression were used to examine how the number of ADEs, utility and hospitalisation varied by adherence after adjusting for patient and practice level covariates; socioeconomic status, deprivation, co-morbidity, number of drugs, functional disabilities, social support and health insurance. RESULTS: 592 (69%) patients were adherent based on dispensed pharmacy claims data (MPR ≥80%) and 553 (63%) self-reported adherence to their medication. The median MPR for self-reported adherent patients was 0.88 (IQR: 0.78, 0.95) compared to 0.86 (IQR: 0.71, 0.93) for non-adherent patients (p<0.01). Non-adherence (MPR<80%) was not significantly associated with any ADEs but self-reported non-adherent patients had an increased risk of any ADEs (IRR 1.18; 95% CI 1.05, 0, 1.33 p<0.01). Non-adherent patients had a significantly lower mean HRQOL utility (MPR coefficient, -0.11, SE 0.03, p<0.001; self-report coefficient, -0.06, SE 0.01, p<0.001) and an almost two-fold increased risk in the expected rate of any hospitalisation (MPR IRR, 1.75; 95% CI, 1.42, 2.15, p<0.001; self-report IRR, 1.53; 95% CI, 1.16, 2.01, p<0.01) compared to adherent patients. CONCLUSIONS: Non-adherence was significantly associated with adverse health outcomes. Developing methods to assist older adults in accurate and safe management of their medications may increase their quality of life.