OBJECTIVE A previous study suggests an association between poor medication adherence and excess mortality in chronic disease. The purpose of this study was to assess the association between medication adherence and risk of cardiovascular disease (CVD), all-cause mortality, and hospitalization in type 2 diabetes.

RESEARCH DESIGN AND METHODS We conducted an electronic search on many electronic databases from inception to 27 April 2016. We selected randomized controlled trials and case-control and cohort studies reporting on CVD, all-cause mortality, or hospitalization outcomes by adherence in adults with type 2 diabetes. Two reviewers independently screened for eligible studies and extracted outcome data. Pooled relative risks (RRs) were calculated using a random-effects meta-analysis; risk of bias in each of the included studies was assessed using the GRADE approach.

RESULTS Eight observational studies were included (n = 318,125). The mean rate of poor adherence was 37.8% (95% CI 37.6–38.0). Adjusted estimates were provided by five studies only. The RRs of good (≥80%) versus poor adherence to medication were 0.72 (95% CI 0.62–0.82, I² = 0%, three studies) for all-cause mortality and 0.90 (0.87–0.94, I² = 63%, seven studies) for hospitalization. No evidence of small study bias was observed. Only one study reported CVD outcomes by adherence.

CONCLUSIONS We identified no trials reporting on outcomes by adherence, suggesting a systematic failure to include this information. Pooled estimates from available observational studies suggest that good medication adherence is associated with reduced risk of all-cause mortality and hospitalization in people with type 2 diabetes, although bias cannot be excluded as an explanation for these findings.