Purpose: The aim of this study was to apply the theory of planned behavior (TPB) with two other factors (action planning and coping planning) to the medication adherence of adults with epilepsy.

Methods: We measured the elements of the theory of planned behavior (attitude, subjective norm, perceived behavioral control, and behavioral intention), action planning, and coping planning at baseline among adults with epilepsy (n = 567, mean +/- SD age = 38.37 +/- 6.71 years, male = 48.5%). Medication adherence was measured using the Medication Adherence Report Scale (MARS) and antiepileptic serum level at the 24-month follow-up. Structural equation modeling (SEM) examined three models relating TPB elements to medication adherence.

Results: Three SEM models all had satisfactory fit indices. Moreover, attitude, subjective norms, perceived behavioral control, and intention together explained more than 50% of the variance for medication adherence measured using MARS. The explained variance increased to 61.8% when coping planning and action planning were included in the model, with coping planning having greater association than action planning. In addition, MARS explained 3 to 5% of the objective serum level.

Conclusion: The theory of planned behavior is useful in understanding medication adherence in adults with epilepsy, and future interventions may benefit by improving such beliefs as well as beliefs about coping planning.