Background: Medication nonadherence may reduce the effectiveness of therapies. To our knowledge, the association between medication nonadherence and mortality remains unexplored outside the context of clinical trials.

Methods: A retrospective cohort study of 11 532 patients with diabetes mellitus in a managed care organization. Medication adherence was calculated as the proportion of days covered for filled prescriptions of oral hypoglycemics, antihypertensives, and statin medications. The primary outcomes of interest were all-cause hospitalization and all-cause mortality. Multivariable regression analyses were performed to assess the independent association between medication adherence and outcomes.

Results: Nonadherent patients (proportion of days covered, <80%; prevalence, 21.3%) were younger and had fewer comorbidities compared with adherent patients. During follow-up, nonadherent patients had higher glycosylated hemoglobin, systolic and diastolic blood pressure, and low-density lipoprotein cholesterol levels. In unadjusted analyses, nonadherent patients had higher all-cause hospitalization (23.2% vs 19.2%, P<.001) and higher all-cause mortality (5.9% vs 4.0%, P<.001). In multivariable analyses, medication nonadherence remained significantly associated with increased risks for all-cause hospitalization (odds ratio, 1.58; 95% confidence interval, 1.38-1.81; P<.001) and for all-cause mortality (odds ratio, 1.81; 95% confidence interval, 1.46-2.23; P<.001). The findings were consistent across patient subgroups and using different cutoffs for the proportion of days covered.

Conclusions: Medication nonadherence is prevalent among patients with diabetes mellitus and is associated with adverse outcomes. Interventions are needed to increase medication adherence so that patients can realize the full benefit of prescribed therapies.