Does e-cigarette use in non-smoking young adults act as a gateway to smoking? A systematic review and meta-analysis

Objective: There is evidence that e-cigarette use is less harmful than smoking, but there are also concerns that e-cigarettes may act as a gateway to smoking among young people. The aim of this review was to investigate whether e-cigarette use compared to non-use in young non-smokers is associated with subsequent cigarette smoking. Design: Systematic review and meta-analysis: Data sources: PubMed, Embase, Web of Science, Wiley Cochrane Library databases, and the 2018 Society for Research on Nicotine and Tobacco and Society for Behavioural Medicine conference abstracts. Eligibility criteria for selecting studies: All studies of young people (up to age 30 years) with a measure of e-cigarette use prior to smoking and an outcome measure of smoking where an odds ratio could be calculated were included (excluding reviews and animal studies). Results: Of 9,199 results, 17 studies were included in the meta-analysis. There was strong evidence for an association between e-cigarette use among non-smokers and later smoking (OR 4.59, 95% CI 3.60 to 5.85) when the results were meta-analysed in a random effects model. However, there was high heterogeneity (I2 = 88%). Conclusions: Whilst the association between e-cigarette use among non-smokers and subsequent smoking appears strong, the available evidence is limited by the reliance on self-report measures of smoking history without biochemical verification. None of the studies included negative controls which would provide stronger evidence for whether the association may be causal. Much of the evidence also failed to consider the nicotine content of e-liquids used by non-smokers meaning it is difficult to make conclusions about whether nicotine is the mechanism driving this association.