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