A Systematic Review Investigating Associations Between E-Cigarette Use Among Former Cigarette Smokers and Relapse to Smoking Cigarettes.

As e-cigarette use has steadily increased over the recent years, the public health interest in the potential implications of e-cigarette use on cigarette smoking has grown in parallel. With strict adherence to PRISMA guidelines, this systematic review examined the potential associations between e-cigarette use and relapse to cigarette smoking among former cigarette smokers. The protocol was registered on November 06, 2018 (PROSPERO 2018 CRD42018115674). Literature searches were executed from January 01, 2007 to August 20, 2022 and search results were screened according to the PICOS review method. One RCT and 10 adjusted studies examined relapse to cigarette smoking (evidence grade "moderate") among regular e-cigarette users, reporting mixed and inconsistent findings according to varying definitions of e-cigarette use and relapse. Findings were similarly inconsistent among the 8 adjusted studies examining relapse to cigarette smoking among non-regular e-cigarette users. The inconsistency in findings among studies evaluating regular measures of e-cigarette use, combined with the numerous methodological flaws in the overall body of literature, limit the generalizability of results associated with a causal association between e-cigarette use and relapse to cigarette smoking. Based on findings from this review, more robust studies are required to determine whether a causal association exists between e-cigarette use and relapse to cigarette smoking. Future studies should apply consistent measures of regular e-cigarette use to examine causality with future use patterns, and sufficiently account for known or suspected confounding variables to support inform determinations related to e-cigarette use and cigarette smoking behaviors.