Teenagers who use electronic cigarettes are six times more likely to try regular cigarettes within two years than those who have never tried electronic cigarettes, according to new research published Monday.

E-cigarettes, which vaporize a flavored fluid that typically includes nicotine, are advertised as a way for smokers to wean themselves off regular cigarettes. However, "kids who experiment with e-cigarettes may be moving on to other types of tobacco products, like combustible cigarettes, which are arguably a lot more dangerous," University of Southern California researcher Jessica Barrington-Trimis, lead author of the study, told Reuters.

The study, published in the journal Pediatrics, found that out of 300 southern California high school students surveyed in 2014, approximately half reported having tried an e-cigarette at least once. In a 2015 follow-up survey, approximately 40 percent of students who had reported trying an e-cigarette the previous year had tried regular cigarettes, compared to only 11 percent of those who had not tried vaping.

After adjusting the statistics for factors such as gender, ethnicity, grade, and parental education, the researchers concluded that the teens who tried vaping were six times more likely to take up smoking than their non-vaping counterparts.

Despite the strong correlation, the study does not necessarily prove that vaping causes teens to take up regular cigarette use, some experts note, since the survey did not determine how many times the teens had vaped prior to trying cigarettes.

"What's probably happening is these kids did not become regular vapers, (and) they turned to smoking," Michael Siegel, a professor at the Boston University School of Public Health and a proponent of e-cigarettes as a way to reduce combustible cigarette use, told Reuters.

Earlier this year, a study by the Centers for Disease Control and Prevention found that e-cigarette advertisers appeal to teenagers using the same tactics that regular cigarette companies are no longer allowed to use, as the Christian Science Monitor's Lucy Schouten reported:

E-cigarette companies spent $115 million on advertising in 2014, compared to $6.4 million in 2011, the CDC's National Youth Tobacco Survey found. And they feature familiar images of sex, rebellion, and gritty independence that got yesterday's teens smoking regular cigarettes. 'The e-cigarette advertising we're seeing is like the old-time Wild West,' CDC Director Tom Frieden told Reuters. 'No rules, no regulations, and heavy spending advertising the products.'

One recent study found that the smoking rate in the US dropped faster last year than it has in more than two decades, thanks to mandatory warning labels on cigarette packs, anti-smoking campaigns, additional taxes, and cigarette bans in restaurants and other public places. In 2015, only 15 percent of American adults surveyed reported using cigarettes.

After a recommendation from the American Academy of Pediatrics that e-cigarettes be regulated as tobacco products because of their nicotine content, the US Food and Drug Administration took steps last month to reduce teen use by introducing a minimum purchase age of 18. Some communities have gone further, extending the minimum age for purchase to 21.

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"The use of e-cigarettes in kids appears increasingly likely to result in an increased risk of using regular cigarettes," said CDC Director Frieden. "They are not harmless."

This report contains material from Reuters.