Risk of smoking relapse with the use of electronic cigarettes: A systematic review with meta-analysis of longitudinal studies.

INTRODUCTION: The use of e-cigarettes has been the subject of a public health debate on their possibility of undermining efforts for tobacco control. The aim of this study was to synthesize the risk of smoking relapse with the use of e-cigarettes by former smokers. METHODS: MEDLINE, EMBASE, PsycInfo and LILACS were searched without restriction to language or date of publication. Longitudinal observational studies evaluating the association between e-cigarette use and smoking relapse were selected by two independent reviewers, and disagreements solved by discussion with a third researcher. Data extraction and risk of bias assessment were also carried out by two independent reviewers. The meta-analysis was performed using the random effect Mantel-Haenszel model. RESULTS: From 632 retrieved records, six studies were eligible and described, while three were included in the quantitative synthesis. The studies were conducted in the USA, UK and France, with final sample size varying from 374 to 4094 former smokers. Risk of relapse was 2.03 (95% CI: 1.39-2.96) among former smoker users than non-users of e-cigarettes, and 1.38 (95% CI: 1.11-1.65) when pooling the adjusted association measures. Long-term former smokers were the main contributors for the higher relapse risk, while the impact of frequency of exposure to e-cigarettes (past, non-daily, daily) was uncertain. CONCLUSIONS: Considering the growing popularity of e-cigarettes among former smokers, our results point to the great potential for an increase in the frequency of relapse to conventional smoking and vaping for those who move to regular use of e-cigarettes.