Articles
E-cigarettes and smoking cessation in real-world and clinical settings: a systematic review and meta-analysis

https://doi.org/10.1016/S2213-2600(15)00521-4Get rights and content

Summary

Background

Smokers increasingly use e-cigarettes for many reasons, including attempts to quit combustible cigarettes and to use nicotine where smoking is prohibited. We aimed to assess the association between e-cigarette use and cigarette smoking cessation among adult cigarette smokers, irrespective of their motivation for using e-cigarettes.

Methods

PubMed and Web of Science were searched between April 27, 2015, and June 17, 2015. Data extracted included study location, design, population, definition and prevalence of e-cigarette use, comparison group (if applicable), cigarette consumption, level of nicotine dependence, other confounders, definition of quitting smoking, and odds of quitting smoking. The primary endpoint was cigarette smoking cessation. Odds of smoking cessation among smokers using e-cigarettes compared with smokers not using e-cigarettes were assessed using a random effects meta-analysis. A modification of the ACROBAT-NRSI tool and the Cochrane Risk of Bias Tool were used to assess bias. This meta-analysis is registered with PROSPERO (number CRD42015020382).

Findings

38 studies (of 577 studies identified) were included in the systematic review; all 20 studies with control groups (15 cohort studies, three cross-sectional studies, and two clinical trials) were included in random effects meta-analysis and sensitivity analyses. Odds of quitting cigarettes were 28% lower in those who used e-cigarettes compared with those who did not use e-cigarettes (odds ratio [OR] 0·72, 95% CI 0·57–0·91). Association of e-cigarette use with quitting did not significantly differ among studies of all smokers using e-cigarettes (irrespective of interest in quitting cigarettes) compared with studies of only smokers interested in cigarette cessation (OR 0·63, 95% CI 0·45–0·86 vs 0·86, 0·60–1·23; p=0·94). Other study characteristics (design, population, comparison group, control variables, time of exposure assessment, biochemical verification of abstinence, and definition of e-cigarette use) were also not associated with the overall effect size (p≥0·77 in all cases).

Interpretation

As currently being used, e-cigarettes are associated with significantly less quitting among smokers.

Funding

National Institutes of Health, National Cancer Institute, FDA Center for Tobacco Products.

Introduction

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.

Section snippets

Data sources and searches

To identify studies, we began by manually searching the references from the three earlier meta-analyses12, 13, 20 and completed a comprehensive literature search of PubMed and the Web of Science Core Collection between April 27, 2015, and June 17, 2015. As detailed in the appendix, search terms included “electronic cigarette”, “e-cigarette”, “electronic nicotine delivery”, “stop”, “quit”, “cessation”, “abstain”, and “abstinence”. Search results were not limited by language, but all identified

Results

Of 577 studies identified, 38 were included in the systematic review (appendix) and 20 (table 1) in the meta-analysis. Studies excluded from the systematic review did not include smoking cessation as an outcome, did not include adults as the study population, were opinion pieces or commentaries, or were review articles (figure 1). Of the 38 studies included in the systematic review, 16 were excluded from the meta-analysis because they lacked a control group that did not use e-cigarettes,15, 16,

Discussion

In this systematic review and meta-analysis, pooled results from 18 real-world observational studies and two clinical trials showed 28% (OR 0·72, 95% CI 0·57–0·91) lower odds of cigarette smoking cessation among those who used or had used e-cigarettes compared with those who had not used e-cigarettes. This conclusion was insensitive to a wide range of study design factors, including whether or not the study population consisted only of smokers interested in smoking cessation, or all smokers

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