LAUSR

The prevalent new user design includes a broader study population than the traditional new user approach that is frequently used in pharmacoepidemiologic research. In an article appearing in this issue (Am J Epidemiol. XXXX;XXX(XX):XXXX-XXXX), Webster-Clark and colleagues describe the treatment initiator types included in the prevalent new user design and contrast the causal questions assessed using a prevalent new user design versus a new user design. They further applied a series of simulation studies showing the importance of accounting for treatment history in addition to time since initiation of the comparator in the prevalent new user design. In this commentary, we put their findings in the broader context with a discussion of the strengths and limitations of the prevalent new user design and settings where it may be most useful. The prevalent new user design and new user design both address unique questions of clinical and public health importance. Real-world evidence generated by pharmacoepidemiologic research is increasingly being used by regulators and other knowledge users to inform their decision making. Understanding the causal questions addressed by different designs is crucial in this process; the study by Webster-Clark and colleagues represents an important step in addressing this issue.