Investigating the mechanisms of behavior change interventions provides a more fulsome understanding of how and why interventions work (or don’t work). We assessed mechanisms of two interventions (mailouts alone, and mailouts plus telephone support, informed by the Health Action Process Approach (HAPA) and Habit Theory), designed to increase medication adherence after myocardial infarction. We conducted a process evaluation alongside a pragmatic trial. Medication adherence was assessed via self-report at 12-months in the trial, and participants in all trial groups were invited to contemporaneously complete an additional questionnaire assessing targeted mechanisms (HAPA constructs and automaticity). We used multiple regression-based mediation models to investigate indirect effects. Of 589 respondents, 497 were analyzed (92 excluded due to missing data). Mailouts plus telephone support had statistically significant but small effects on intention, social support, action planning, coping planning, and automaticity. There were no indirect effects of interventions on medication adherence via these constructs. Therefore, while this intervention led to changes in proposed mechanisms, these changes were not great enough to lead to behavior change. Refinements (and subsequent evaluation) of the interventions are warranted, and our findings indicate that this could involve offering more intensive support to form plans and identify cues for taking medications, in addition to providing physical supports to encourage self-monitoring, feedback, and habit formation.
Trial registration: ClinicalTrials.gov: NCT02382731.