"Vaping is a gateway to smoking," the Mail Online reports, seriously overstating the evidence of a new US study.

While the study did find teens who experimented with e-cigs were more likely to smoke "traditional" tobacco products – mainly cigarettes – no direct link between the two was proven.

Following a survey, around 300 adolescents aged 17 from California who had never smoked tobacco were included in the study. Half of them had used e-cigarettes.

Sixteen months later, those that had smoked e-cigarettes were six times more likely to have started smoking tobacco.

An obvious response to this finding is that certain teenagers are less risk-averse than others – those who experimented with e-cigarettes would have probably ended up trying cigarettes anyway, regardless of whether or not e-cigarettes existed.

The researchers did try to account for this effect by asking teenagers at the beginning of the study if they had a "firm commitment" to never start smoking. But you could make the case that nothing changes faster than a teenager's mind.

So, in spite of the Mail's headline, this study is not able to prove the use of e-cigarettes is responsible for a progression to smoking.

Rates of teen smoking are low in England, at around 4% for under-16s. The increasing popularity of e-cigarettes has not had any impact on this welcome trend – at least at the moment.

Under recent legislation, it is now illegal to supply e-cigarettes to under-18s, unless they have a prescription.

Where did the story come from?

The study was carried out by researchers from the University of Southern California.

It was funded by the US National Cancer Institute at the National Institutes of Health and the Food and Drug Administration Center for Tobacco Products.

The study was published on an open access basis in the peer-reviewed journal Pediatrics, so it is free to view online.

While the Mail reports the study accurately in the body of its report, its headline "Vaping is a gateway to smoking" is unsupported by the evidence from the study.

What kind of research was this?

This prospective cohort study aimed to assess whether the use of e-cigarettes in adolescents leads to the use of combustible tobacco products such as cigarettes, cigars and pipes.

This type of study is able to make a link between vaping and smoking, but it is unable to prove e-cigarettes are the cause of adolescents progressing to smoking.

What did the research involve?

The researchers included adolescents in the eleventh and twelfth grade from high schools in south California, with an average age of 17.

At the start of the study, teenagers filled out a questionnaire under the supervision of research staff, designed to evaluate whether they had smoked tobacco or used e-cigarettes.

They were asked whether they had tried e-cigarettes, cigarettes, cigars, pipes or hookah in the past 30 days, classified as "never tried" and "ever users".

The researchers restricted their analyses to those who had never smoked cigarettes at the first evaluation.

Susceptibility to cigarette use was defined as the "absence of a firm commitment not to smoke".

When questioned about intention to smoke in the future, students who answered "definitely not" were considered "not susceptible".

The researchers also asked questions about the social acceptability of smoking within the teenagers' social environment.

Questions and possible responses were as follows:

How many of your four closest friends use cigarettes? (0 to 4 friends)

How would your best friends act toward you if you used cigarettes? (very unfriendly, unfriendly, friendly, or very friendly)

Does anyone who lives with you now use cigarettes? (yes or no)

Information on sociodemographic factors was also collected.

The researchers then invited the never-smoking e-cigarette users and a sample of never-smoking and never e-cigarette users to complete a follow-up questionnaire 16 months later.

Never-users were matched to e-cigarette users by gender, ethnicity and grade.

What were the basic results?

The study presented findings from 152 never users and 146 e-cigarette users.

Cigarette smoking during the follow-up period was reported by 40.4% of e-cigarette users and 10.5% of those who had never vaped.

After adjusting for possible confounders, e-cigarette users were 6.17 times more likely to start smoking cigarettes than never e-cigarette users (95% confidence interval 3.30 to 11.6).

E-cigarette users were more likely to start using any combustible product, including hookah, cigars and pipes.

How did the researchers interpret the results?

The researchers concluded: "E-cigarette use in never-smoking youth may increase risk of subsequent initiation of cigarettes and other combustible products during the transition to adulthood, when the purchase of tobacco products becomes legal.

"Stronger associations in participants with no intention of smoking suggests that e-cigarette use was not simply a marker for individuals who would have gone on to smoke, regardless of e-cigarette use."

Conclusion

This prospective cohort study found adolescents who used e-cigarettes were more likely to go on to use combustible tobacco products such as cigarettes, cigars and pipes than those who never used e-cigarettes.

But the strength of these findings is limited by a number of factors:

The study design is not able to prove the use of e-cigarettes is responsible for progression to smoking. However, the researchers did attempt to investigate susceptibility in both e-cigarette users and non-users.

The sample of participants was small, from a particular age group and from one location, reducing the reliability and generalisability of the findings.

No information was collected on the type of e-cigarette used or nicotine content.

A large number of participants were lost to follow-up – whether these teens went on to smoke or not would have altered the results.

The initial grouping of participants into users and non-users was based on their responses to a questionnaire completed with a researcher. Students may have felt the need to give responses they felt were more appropriate, rather than accurate.

Under new legislation, it is now illegal to sell e-cigarettes – and e-liquids, or nicotine-containing liquid – to under-18s, unless they have a prescription for a brand of e-cigarette recognised as a stop-smoking aid.

Read more advice about helping teens quit smoking.

Analysis by Bazian

Edited by NHS Website