Over time young adults who use e-cigarettes smoke more not less; ecigs enhancing harm by making tobacco epidemic worse

In March 2014 Lauren Dutra and I published Electronic Cigarettes and Conventional Cigarette Use Among US Adolescents: A Cross-sectional Study in JAMA Pediatrics.  This paper showed that those youth who experimented with cigarettes (i.e., answered “yes” to the question “Have you ever smoked a cigarette, even a puff?”) were much more likely to be established smokers (smoked 100 cigarettes) than youth who did not use e-cigarettes.  We concluded “Use of e-cigarettes does not discourage, and may encourage, conventional cigarette use among U.S. adolescents.”
 
In writing the paper, Lauren Dutra and I took care not to use the word “cause” and noted that cross-sectional studies do not allow causal conclusions because they are a snapshot in time.  (Of course, you cannot smoke your 100th cigarette before smoking your 1st cigarette, so reverse causation is not an issue in this case.)
 
Despite the care we took on this point, Farsalinos and Polosa criticized this paper on the grounds that one cannot conclude causation from a cross-sectional study. 
 
Now we have the first longitudinal study on this issue, which does allow drawing causal conclusions.  The new paper, Does e-cigarette use predict cigarette escalation? A longitudinal study of young adult non-daily smokers?, by Neal Doran and colleagues, which appears in the July, 2017 issue of Preventive Medicine. As one would predict from our earlier cross-sectional study, they found that “among non-daily smokers, young adults who use e-cigarettes tend to smoke more cigarettes and to do so more frequently. Such individuals may be at greater risk for chronic tobacco use and dependence.”
 
This paper is very carefully done, including the fact that it controls for propensity to use e-cigarettes at baseline.
 
Here is the abstract:
 

Recent studies suggest that e-cigarette use among youth may be associated with increased risk of cigarette initiation. The goal of this study was to test the hypothesis that use of e-cigarettes among young adult non-daily cigarette smokers would be associated with increased cigarette consumption. Participants (n=391; 52% male) were 18-24year-old non-daily cigarette smokers recruited from across California. Cigarette and e-cigarette use were assessed online or via mobile phone every three months for one year between March 2015 and December 2016. Longitudinal negative binomial regression models showed that, adjusted for propensity for baseline e-cigarette use, non-daily smokers who reported more frequent use of e-cigarettes upon study entry reported greater quantity and frequency of cigarette smoking at baseline and greater increases in cigarette quantity over 12 months than non-daily cigarette only smokers (ps

 
The full citation is Doran N, Brikmanis K, Petersen A, Delucchi K, Al-Delaimy WK, Luczak S, Myers M, Strong D.  Does e-cigarette use predict cigarette escalation? A longitudinal study of young adult non-daily smokers.  Prev Med. 2017 Jul;100:279-284. doi: 10.1016/j.ypmed.2017.03.023.  The accompanying editorial by Thomas Wills and James Sargent is available here.