Background Low levels of adherence to medication prescribed to treat and manage chronic disease may lead to maladaptive health outcomes. Theory‐based, easy‐to‐administer interventions that promote patients’ effective self‐regulation of their medication‐taking behaviour are needed if adherence is to be maximised. We tested the effectiveness of an intervention adopting planning techniques to promote medication adherence. Methods Outpatients with cardiovascular disease (N = 71) were allocated to either an experimental condition, in which participants were asked to form implementation intentions and coping plans related to their treatment, or to a no‐planning control condition, in which participants received no treatment. Patients also completed self‐report measures of medication adherence, self‐efficacy, and beliefs in medication necessity and concerns. Measures were administered at baseline and at 6‐week follow‐up. Results Results revealed no overall main effect for the intervention on medication adherence. Post‐hoc moderator analyses revealed that the intervention was effective in patients with lower necessity beliefs compared to those with higher necessity beliefs. Conclusion While current findings have promise in demonstrating the conditional effects of planning interventions, there is a need to replicate these findings by manipulating planning and beliefs independently and testing their direct and interactive effects on medication adherence.