11th longitudinal study shows that kids who start with ecigs more likely to go on to cigs, this time from Canada

David Hammond and colleagues just published “Electronic cigarette use and smoking initiation among youth: a longitudinal cohort study,” a large well-done longitudinal study of Canadian high school students that found that, like all the other similar studies from the US and UK, kids who started with e-cigarettes we more likely to be smoking conventional cigarettes a year later and also more likely to have become daily smokers. 
 
In addition to controlling for a wide range of demographic and behavioral factors, they controlled susceptibility to cigarette smoking which contributes to the strength of the study.
 
The paper is especially interesting because nicotine-containing e-cigarettes are illegal in Canada, which strongly suggests that the gateway effect of e-cigarette use does not simply depend on the availability of nicotine.  Nicotine containing e-cigs are illegally available at vape shops, but only nicotine-free e-cigs are available in supermarkets and convenience stores.  There are no legal restrictions on youth purchase of these e-cigarettes in Canada.
 
This is the 11th longitudinal study showing that kids who start with e-cigs are more likely to be smoking cigarettes a year later, with 9 from the US and 1 from UK showing the same thing.
 
Perhaps responding to the aggressive denialism of the link between e-cigarette use and subsequent smoking coming from some quarters, Hammond and colleagues went out of their way to say that they did not find “causality:”
 

It is unclear the extent to which this association is causal. At the individual level, e-cigarettes may be causally related to cigarette smoking because they provide early exposure to nicotine or greater exposure to environmental risk factors, including greater exposure to smokers or certain social settings. E-cigarette use may also help to “re-normalize” smoking by promoting more positive normative beliefs about nicotine use and smoking, which are important predictors of uptake. Alternatively, both e-cigarettes and cigarette smoking could be the result of unmeasured “common factors.” Several studies to date have adjusted for factors such as sensation seeking, parental support and rebelliousness, and have found that the association with e-cigarette use persists; however, it is doubtful that any analysis can adequately control for the range of “common factors” that may account for the use of multiple nicotine products. Put more simply, youth who try e-cigarettes may be different from those who do not. The temporal order, whereby e-cigarette use precedes cigarette-smoking initiation, may be explained by the fact that e-cigarettes are more accessible than cigarettes to Canadian youth, and are therefore likely to be encountered and used first. [citations deleted]

 
While all these things are possible, the authors are really stretching to try and avoid the obvious conclusion from their data, namely that e-cigs promote cigarette smoking.  An important point that they make is that some of the kids who start with e-cigs are different from kids who start with cigarettes; several studies show that they are kids at low risk of starting with cigarettes, which means that e-cigarettes are expanding the nicotine addiction market.  To the extent that this situation exists (about 1/3 of new e-cig users, they do not share “common factors” underlying the use of both e-cigarettes and conventional cigarettes.  These kids would are unlikely to ever start nicotine use with conventional cigarettes.
 
More important, the fact that, despite being done in different places with different populations and using different measures, all 11 longitudinal studies show increased cigarette smoking among kids who start with e-cigarettes is about a strong as the evidence gets.
 
And there are also the earlier cross-sectional studies showing the same association.
 
At this point the evidence that e-cigarette use causes cigarette smoking in youth is stronger and more consistent than the evidence was that cigarette smoking caused lung cancer caused lung cancer back in 1964, when the Surgeon General first drew that conclusion.
 
Finally, the real challenge to the denialists should be to come up with a plausible alternative explanation for all the studies together other than that e-cigarette use increases the likelihood that an adolescent will go on to cigarettes that simultaneously explains the results of all the studies.
 
Here is the abstract for the Hammond paper:
 

BACKGROUND: The influence of ecigarette use on smoking initiation is a highly controversial issue, with limited longitudinal data available for examining temporal associations. We examined e-cigarette use and its association with cigarette-smoking initiation at 1-year follow-up within a large cohort of Canadian secondary school students.
METHODS: We analyzed data from students in grades 9–12 who participated in 2 waves of COMPASS, a cohort study of purposefully sampled secondary schools in Ontario and Alberta, Canada, at baseline (2013/14) and 1-year followup (2014/15). We assessed cigarette smoking and e-cigarette use at baseline and follow-up using self-completed surveys. We used generalized linear mixed effects models to examine correlates of past 30-day e-cigarette use at baseline and smoking initiation between waves within the longitudinal sample.
RESULTS: Past 30-day e-cigarette use increased from 2013/14 to 2014/15 (7.2% v. 9.7%, p < 0.001), whereas past 30-day cigarette smoking decreased slightly (11.4% v. 10.8%, p = 0.02). Among the 44 163 students evaluated at baseline, past 30-day e-cigarette use was strongly associated with smoking status and smoking susceptibility. In the longitudinal sample (n = 19 130), past 30-day use of e-cigarettes at baseline was associated with initiation of smoking a whole cigarette (adjusted odds ratio [OR] 2.12, 95%
confidence interval [CI] 1.68–2.66) and with initiation of daily smoking (adjusted OR 1.79, 95% CI 1.41–2.28) at follow-up.
INTERPRETATION: E-cigarette use was strongly associated with cigarette smoking behaviour, including smoking initiation at follow-up. The causal nature of this association remains unclear, because common factors underlying the use of both e-cigarettes and conventional cigarettes may also account for the temporal order of initiation.

 
The full citation is David Hammond, Jessica L. Reid, Adam G. Cole, Scott T. Leatherdale.  Electronic cigarette use and smoking initiation among youth: a longitudinal cohort study. CMAJ 2017 October 30;189:E1328-36. doi: 10.1503/cmaj.161002.  It is available here.

Comments

Still waiting

With all these longitudinal studies showing an association between prior e-cigarette use and initiating smoking, why are we still waiting for proof of a trajectory from never smoker to regular smoker that passed through e-cigarette use?

Could it be because it doesn't happen or it happens so infrequently that it's insignificant?

When will these studies stop hinting at people becoming smokers by pointing at people that try cigarettes as proof and actually show e-cigarette use leading to regular smoking?

That's already been demonstrated

As my blog post noted, there are now 11 longitudinal studies showing that nonsmoking kids who usee-cigarettes are more likely be smoking cigarettes a year later.  The definition of "smoking" cigarettes varies in the studies, with some measuring initiation (smoked a puff or more), some measuring current smoking (smoking in the last 30 days, the standard definition for adolescent smoking), and some using having smoked 100 cigarettes (the definition of established smoker). 
 
We also already published a cross-sectional study showing that among kids who have smoked a puff (experimenting with cigarettes) using e-cigarettes is strongly associated with having become an established smoker (100 cigarettes).
 
Also important:  There are no studies contradicting these results.  They are completely consistent.