Use of prescription drug samples in the US and implications for pharmacoepidemiologic research: a systematic search of the literature.

INTRODUCTION

Free drug samples are not captured in the pharmacy claims databases used in many pharmacoepidemiologic studies, which could lead to misclassification of drug exposure status and thus bias study results.

AREAS COVERED

We systematically searched the literature in PubMed/MEDLINE, Embase, and Scopus from database inception to August 2020 for studies assessing the magnitude of exposure misclassification in pharmacy claims data associated with uncaptured drug sample utilization. Our review identified five US-based studies with substantially different characteristics, contexts, methods, and results. Taken together, these studies suggest that the risk of sample-related bias may be higher for (1) studies of newly approved, patented brand-only drugs in specific classes and contexts; (2) studies of populations where sample use is common and the unexposed cohort is small; and (3) studies where the outcomes of interest are expected to be early-onset or acute, with non-constant hazards.

EXPERT OPINION

In light of declining overall trends in sample use, future research on sample-related exposure misclassification should focus on delineating bias across those modern contexts where sample use remains high and optimizing bias quantification methods to create a more standardized approach. Additionally, further assessment is warranted for other sources of misclassified exposure status in claims-based pharmacoepidemiology research.