E-cigarettes may be prompting UK teens to start smoking the real thing, and to escalate tobacco consumption, finds the first UK study to report this trend, and published online in the journal Tobacco Control.
But the researchers call for caution in interpreting the survey data on which the findings are based: while vaping prevalence has increased in the UK, smoking prevalence has continued to fall, they point out.
Rates of e-cigarette use among teens are low, but the proportion of those who have tried them at least once are reasonably high, at 13-22%, and the trend is upwards. But the studies suggesting that experimentation with e-cigarettes may act as a gateway to smoking in adolescents have been carried out in the US.
The researchers therefore wanted to see if there were any similarities in patterns in the UK, as well as to explore several potential risk factors and influences that have not been looked at before.
They mined data from the survey responses of 2386 teens from 20 schools across England in 2014 (baseline), when respondents were aged 13 and 14, and again a year later.
At baseline, the teens were asked about their vaping and smoking behaviours--how much and how often. And they were asked whether any of their friends or family smoked; and what their attitude to smoking was--factors associated with smoking uptake among the young.
Levels of carbon monoxide in their breath were assessed: this gas indicates whether someone has been smoking. And information was also collected on whether they had free school meals--a measure of household financial hardship.
A year later they were asked whether they smoked cigarettes, and if so, how many; and their breath carbon monoxide levels were re-assessed.
At baseline, nearly two thirds (61.5%, 1726) of the sample had neither tried vaping nor smoking; 16% said they had only tried e-cigarettes; 4.4% had tried the real thing, but not e-cigarettes; and nearly one in five (18.1%) had tried both.
Starting to smoke over the next 12 months was significantly more common among those who had friends and two or three family members who smoked. And it was significantly less likely among those with negative attitudes towards smoking.
But it was strongly associated with e-cigarette use, particularly among those without friends who smoked--a group usually thought to be less vulnerable to taking up smoking.
Among those who had never smoked cigarettes but had tried e-cigarettes at baseline, a third (34.4%) said that they had tried cigarettes 12 months later compared with only 9% in the group who had not tried e-cigarettes at baseline.
After taking account of other potentially influential factors, those who vaped were four times as likely to start smoking conventional cigarettes as were those who didn't use e-cigarettes.
And occasional smokers at baseline were nearly twice as likely to escalate their habit if they had tried e-cigarettes as were those who hadn't experimented with vaping (just over 24% compared with just under 13%).
After taking account of other potentially influential factors, this was no longer significant, but the numbers involved were small, caution the researchers.
They suggest that there may be plausible explanations for their findings, including that e-cigarette use among teens may simply be an indicator of those who would have started smoking and escalated their habit anyway.
E-cigarette use may also have 'normalised' any kind of nicotine use through developing addiction to it, but they point out that there is no direct evidence as yet to back this up.
Despite attempts to account for a broad range of potential influences, there may be other as yet unexplored factors that are responsible, they add.
This is an observational study, so no firm conclusions can be drawn about cause and effect, something which the researchers are eager to emphasise.
"While acknowledging that a causal relationship may be plausible, we cannot confirm this, based on our findings and the trends observed over the same period in the UK," they write.
"Given the lack of clarity regarding the mechanism linking e-cigarette and cigarette use, we need to be cautious in making policy recommendations based on our findings," they insist.