E-cigarettes (also known as electronic cigarettes, electronic nicotine delivery systems, vapour pens, and many other terms) are battery-powered devices that heat a solution of humectants (usually propylene glycol or glycerol), nicotine (in most cases), and flavourings (in many cases), to deliver an aerosol that is inhaled by the user. E-cigarette use is increasing in many countries.1, 2 Adults report various motivations for e-cigarette use, including to help them quit cigarettes and allowing them to continue to use nicotine in areas where smoking is prohibited,3, 4, 5, 6, 7 which are common themes in e-cigarette marketing and promotion.8, 9, 10
In 2015, the US Preventive Services Task Force concluded that evidence was insufficient to recommend e-cigarettes for tobacco cessation in adults because of conflicting and limited evidence available at the time the recommendation was prepared.11 Two meta-analyses of combined results from clinical trials have assessed whether e-cigarette use is associated with smoking cessation.12, 13 The first,12 based on two randomised trials,14, 15 concluded that participants using nicotine e-cigarettes were more likely to have abstained from smoking cigarettes for 6 months (relative risk 2·29, 95% CI 1·05–4·96) than were participants using no-nicotine e-cigarettes, although the authors had little confidence in the results because of the small number of trials and small sample sizes. The second,13 based on six studies (the same two randomised trials,14, 15 two cohort studies,16, 17 and two cross-sectional studies18, 19) found the proportion of individuals using nicotine-containing e-cigarettes who quit cigarettes to be 20% (95% CI 11–28). These meta-analyses did not compare e-cigarette users to a control group not using e-cigarettes. A third meta-analysis20 of five population-level studies (four longitudinal21, 22, 23, 24 and one cross-sectional25) found that e-cigarette use was associated with a significant depression in smoking cessation (odds ratio [OR] 0·61, 95% CI 0·50–0·75).
The different results of the meta-analyses of clinical trials and observational studies may relate to discrepancies in how e-cigarettes are used in a controlled study setting versus in the real world. Clinical trials evaluating a treatment or intervention under ideal conditions may differ from observational studies evaluating how a product is actually used in a real-world setting in study design, study population, study environment,26 and ability to control for potential confounders, which can compromise the generalisability of results of observational studies.27 These differences are potentially important for e-cigarettes, which, unlike prescription-only nicotine inhalers, are mass-marketed consumer products. We conducted a systematic review and meta-analysis of clinical trials and observational real-world studies to assess the association between e-cigarettes (as available and used) and cigarette smoking cessation among adults, including all smokers as well as only those interested in quitting smoking.
Research in context
Evidence before this study
We searched PubMed and Web of Science between April 27, 2015, and June 17, 2015, for articles that evaluated the association between e-cigarette use and cigarette smoking cessation among adult cigarette smokers. All relevant papers were included, irrespective of where the research was conducted or quality of the studies. There were no language restrictions, although all the papers were written in English. We included two additional studies published while this report was under peer review. Two earlier meta-analyses, one based on two randomised trials, and another based on six studies (the same two randomised trials plus two cohort studies and two cross-sectional studies), suggested that e-cigarettes might assist smokers in quitting cigarettes. These meta-analyses did not compare e-cigarette users to a control group not using e-cigarettes. The first meta-analysis (of two randomised trials) concluded that participants using nicotine e-cigarettes were more likely to have abstained from smoking cigarettes for at least 6 months (relative risk 2·29, 95% CI 1·05–4·96) than were participants using no-nicotine e-cigarettes. The second meta-analysis concluded that the proportion of individuals using nicotine-containing e-cigarettes who quit cigarettes to be 20% (95% CI 11–28). A third meta-analysis of five population-level observational studies found that e-cigarette use was associated with a significant depression in smoking cessation.
Added value of this study
We include all available (38) studies in our systematic review and all 20 studies with control groups (15 cohort studies, three cross-sectional studies, and two clinical trials) in our meta-analysis. Odds of quitting cigarettes were 28% lower in those who used e-cigarettes compared with those who did not use e-cigarettes (odds ratio 0·72, 95% CI 0·57–0·91). Sensitivity analysis showed that the results were not affected by a wide range of study design factors.
Implications of all the available evidence
As currently being used, e-cigarettes are associated with significantly less quitting among smokers. According to the results of our systematic review and meta-analysis, e-cigarettes should not be recommended as effective smoking cessation aids until there is evidence that, as promoted and used, they assist smoking cessation.