Strengths and limitations
This study was powered for assessing the extent of self-reported adherence. Nevertheless the found associations between adherence and both side effects and side effect burden were significant regardless of the chosen MARS-5 cut-off value. A limitation of this study is the use of only one subjective instrument (MARS-5) to measure implementation adherence. The combination of a self-reported method to assess medication adherence and an objective method is often recommended, however due to automatic repeat prescription services in the participating pharmacies, medication adherence could have been overestimated when refill adherence measures were used. Another limitation of this study is the risk of patients giving socially desirable answers while being interviewed, but one could assume this would rather lead to underestimation of adherence problems than the other way around. That’s why the primary MARS-5 cut-off value of <24 seems justified, all the more because even with the higher cut-off value of <25 differences between non-adherent and adherent patients regarding side effects and burden remain statistically significant. One more limitation is that this study was conducted in only two Dutch community pharmacies, which might hamper the generalization of the results. However, the fact that the results of our study is in line with previous studies confirms the robustness of our results