Wednesday, September 9, 2015

Teen E-Cigarette Use & Smoking: Correlation, Not Causation

Here we go again.  Researchers funded by the National Institutes of Health issued a release saying that “teens who use e-cigarettes were more likely to transition to smokeable tobacco products. However, researchers cautioned that additional studies are needed to determine whether the association is causal.”  Their study appeared in JAMA (here).  

It may sound like the authors are denying a gateway claim, but lead author Adam Leventhal pushed the envelope in the press release: “E-cigarettes may be drawing a new generation of teens into recreational nicotine use because they are high-tech, can be purchased somewhat easily, come in enticing flavors and have a perception that they’re not harmful…If you enjoy the experience of inhaling nicotine in e-cigarettes, it makes sense that you would be open to trying other nicotine products, like cigarettes, hookah, and cigars.”

The media, naturally, saw the study as evidence that e-cigarettes are a “gateway” to smoking among teenagers.  This is a misrepresentation of the data. 

The researchers collected baseline information on “ever” lifetime use of e-cigarettes.  The 6- and 12-month follow-up outcomes were “any” use (even a few puffs) of several combustible tobacco products in the past 6 months.  It is unsurprising that the results revealed that “ever” use of e-cigarettes at baseline was associated with combustible use at follow-up.  It is well known that use of one tobacco product is associated with use of other products among teenagers, especially when the comparison group is composed of kids who have never used any tobacco product.

The bottom line: If you have “ever” used one tobacco product, you are likely to have “ever” used another.  That is correlation, not causation.  This phenomenon was also confirmed by information in Table 2 of the publication, which crosstabs students’ combustible tobacco use with their use of e-cigs at baseline.  Here are the results:

Number of Combustible Products Ever UsedNumber of StudentsNumber (%) Ever Using E-Cigs
02,558225 (9%)
1401156 (39%)
2233136 (58%)
3134100 (75%)

Among the 2,558 students who never smoked, e-cigs were only used by 8.8%.  Among students who had smoked one product, e-cig use was 39%.  Students using 2 and 3 products had e-cig usage rates of 58% and 75%, respectively.

This study shows that students who have “ever” used one tobacco product are likely to have “ever” used another, and that includes e-cigarettes.

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