Objectives: To estimate the pooled effect size of oral antidiabetic drug (OAD) adherence-enhancing interventions and to explore which of the behavior change techniques (BCTs) applied in the intervention groups modified this pooled intervention effect size. Methods: We searched relevant studies published until September 3, 2013, on MEDLINE, Embase, PsycInfo, the Cochrane Library, CINAHL, Current Contents Connect, and Web of Science. Selected studies were qualitatively synthesized, and those of at least medium quality were included in the meta-analysis. A random-effects model was used to pool effectiveness (Hedges's g) and to examine heterogeneity (Higgins I <sup>2</sup>). We also explored the influence on the pooled effectiveness of unique intervention BCTs (those delivered to the intervention groups but not control groups in a trial) by estimating their modifying effects. Results: Fourteen studies were selected for the qualitative synthesis and 10 were included in the meta-analysis. The pooled effectiveness of the interventions was 0.21 (95% confidence interval -0.05 to 0.47; I <sup>2</sup> = 82%). Eight unique BCTs were analyzed. "Cope with side effects" (P = 0.003) and "general intention formation" (P = 0.006) had a modifying effect on the pooled effectiveness. The pooled effectiveness of the interventions in which "cope with side effects" was applied was moderate (0.64; 95% confidence interval 0.31-0.96; I <sup>2</sup> = 56%). Conclusions: The overall effectiveness of OAD adherence-enhancing interventions that have been tested is small. Helping patients cope with side effects or formulate desired treatment outcomes could have an impact on the effectiveness of OAD adherence-enhancing interventions. Only those interventions that include helping patients to cope with side effects appear to be particularly effective in improving OAD adherence.
- Oral antidiabetic drug