"Vaping raises likelihood of teenagers starting to smoke, study suggests," The Guardian reports.

A study of US teens found those who regularly vaped were more likely to progress to tobacco smoking than their non-vaping peers.

The study used questionnaires to assess e-cigarette and cigarette use in 3,000 adolescents aged 15.

The teenagers completed questionnaires twice: at the start of the study and six months later.

Researchers found there was an association between frequent use of e-cigarettes at the start of the study and smoking tobacco at follow-up.

Despite the association, it's difficult to say that the cigarette smoking was caused directly and independently by the use of e-cigarettes.

While the researchers took into account other risk factors for smoking, such as family smoking history, they did not look at all possible contributing factors.

For example, it could be the case that if e-cigarettes didn't exist, some teenagers may have started smoking tobacco anyway.

And these results are based on a small number of people.

The overall prevalence of smoking three or more cigarettes in the past month, or vaping three or more times, was below 5%.

Daily use of either, which may indicate a more serious habit, was also not examined.

E-cigarettes are best used as a quitting aid for people addicted to tobacco. Recreational use may be unwise.

While they are much safer than tobacco, e-cigarettes may still pose both short- and long-term health risks.

Where did the story come from?

The study was carried out by researchers from the University of Southern California Keck School of Medicine, the University of California, and the University of Pennsylvania Perelman School of Medicine, all in the US.

The research was funded by grants from the US National Institutes of Health. The authors report no conflict of interest.

The study was published in the peer-reviewed Journal of the American Medical Association (JAMA).

The UK media generally reported the story accurately, suggesting the link between e-cigarette use and smoking uptake was "tentative", and acknowledging that the number of adolescents who used e-cigarettes or cigarettes at all in the study was very small.

The media also acknowledged that other factors could have contributed to the uptake of smoking, such as the home environment.

The one exception to this measured reporting was on the Mail Online, which ran the headline: "E-cigarettes are a gateway to smoking". This implies that the research proved a direct causal relationship – but this is not the case.

What kind of research was this?

This prospective cohort study followed adolescents over time to see whether e-cigarettes were associated with progression to cigarette smoking. It could be that using e-cigarettes is associated with the beginnings of a smoking habit.

But as some adolescents who smoke cigarettes use e-cigarettes as an aid to help them quit, it could be that those who use e-cigarettes are more likely to cut back on how much they smoke over time.

The researchers therefore wanted to see what the associations were between e-cigarettes and subsequent smoking frequency and heaviness.

A prospective cohort study is the best way of examining whether a particular factor is linked with a particular outcome.

But it can be difficult to account for all other variables that may be involved – for example, previous smoking, the frequency of other risk behaviours, or other environmental influences.

For this reason, cohort studies cannot prove cause and effect.

A randomised controlled trial, which could prove cause and effect, would not be ethical because we know smoking has harmful effects.

What did the research involve?

The researchers included students from 10 public high schools in Los Angeles County, California who were already enrolled in a longitudinal study.

The analysis used data from 3,084 students who completed surveys twice: once at the start of the study and again six months later. Their average age was 15.5 at baseline.

The surveys categorised e-cigarette use at baseline into "never", "prior" (ever used, but not in the previous 30 days), "infrequent" (1 to 2 days during the past 30 days), or "frequent" (3 or more days in the past 30 days).

Smoking use was also recorded at baseline and follow-up. Smoking frequency was categorised into "non-smoker", "infrequent smoker" (1 to 2 days in the previous 30 days) or "frequent smoker" (3 or more days in the past 30 days).

The amount smoked was categorised into none, less than one, one, or two or more cigarettes a day on smoking days.

The researchers assessed the association between e-cigarette use at baseline and how often and how heavily teenagers smoked at the follow-up stage.

The results were adjusted for confounders, including:

age

sex

ethnicity

highest parental education

whether the student lived with both parents

ever used alcohol or drugs

ever used any combustible tobacco product

family history of smoking

depressive symptoms

impulsive behaviour

sensation seeking

peer smoking

smoking susceptibility

smoking expectancies

What were the basic results?

At follow-up, those who had used e-cigarettes more frequently at baseline were more likely to have become smokers.

Frequent e-cigarettes use was associated with a subsequent increased possibility of frequent tobacco smoking (odds ratio [OR] 1.37, 95% confidence interval [CI] 1.16 to 1.61) and heaviness (OR 1.26, 95% CI 1.07 to 1.48).

Of those who had:

never used e-cigarettes – 0.9% were infrequent smokers and 0.7% were frequent smokers

used e-cigarettes at some point prior to the study – 4.1% were infrequent smokers and 3.3% were frequent smokers

used e-cigarettes infrequently at baseline – 9% were infrequent smokers and 5.3% were frequent smokers

used e-cigarettes frequently at baseline – 11.6% were infrequent smokers and 19.9% were frequent smokers

These trends were found to be stronger for those who had not been smokers at baseline (OR 2.51, 95% CI= 2.30 to 2.75).

How did the researchers interpret the results?

The researchers concluded "vaping more frequently was associated with a higher risk of more frequent and heavy smoking six months later".

They added: "Although some youths use e-cigarettes for cessation purposes, vaping was not associated with smoking reductions in baseline smokers.

"However, because the reason for vaping was not assessed, further investigation is required."

Conclusion

This research shows an association between using e-cigarettes at baseline and smoking frequency six months later among adolescents in US high schools.

The study has several strengths, including:

data was collected prospectively, meaning the researchers did not know the outcomes at the start of the study

adolescents were followed up over six months, which is a reasonable time period

However, the use of e-cigarettes and cigarette smoking was measured by self-reporting, and may be inaccurate.

While some factors were accounted for, it is difficult to account for all factors that may make smoking more likely.

These could include engaging in other risky behaviour or living in a home environment where teens are exposed to e-cigarettes or cigarette smoking.

Although it was a reasonably large sample size, with data from more than 3,000 adolescents, the number of teens who actually reported using e-cigarettes or cigarettes was low and therefore a small sample to base any conclusions on.

The categories used were fairly broad – "prior" use included people who had used e-cigarettes just once. "Frequent" users included those who used e-cigarettes three times in the previous month, which could be considered fairly low.

This study looked at students from US high schools, and the findings may not have as much relevance in the UK.

Find out more about quitting smoking if you're a teenager.

Free stop smoking treatments are available for both adults and children aged 12 to 18. Get more advice about quitting smoking.

Analysis by Bazian

Edited by NHS Website