Objective The study aimed to identify the determinants of medication adherence in a population of hypertensive outpatients. The principle objective of the present study was to determine the utility of self-efficacy and the Theory of Planned Behaviour (TPB) in predicting adherence with antihypertensive medication.

Setting An outpatient hypertension clinic at the Belfast City Hospital, Northern Ireland.

Method Outpatients who had attended the hypertension clinic on at least one previous occasion were invited to participate in the study. Participants completed a questionnaire incorporating measures of adherence with medication, medical and sociodemographic factors, together with measures of self-efficacy and the TPB. There were no specific study exclusion criteria.

Key findings A total of 139 participants took part in the study; 20.9% of the participants reported non-adherence with their prescribed antihypertensive medication. A statistically significant difference in self-efficacy scores between the adherent and non-adherent groups was revealed by a Mann-Whitney test, with adherent patients perceiving higher levels of self-efficacy. Regression analysis with the TPB variables revealed that adherence was predicted by intentions and subjective norms, with 41% of the variance explained. Intention was predicted by attitudes and perceived behavioural control, with 61.9% of the variance explained.

Conclusion The present study provides support for the use of self-efficacy and the TPB in predicting medication adherence; however, further research is required on the application of the TBP in the prediction of health behaviour. These findings have implications for the design of adherence-enhancing interventions.