Using e-cigarettes could be a gateway to real smoking, according to a new study.
The study found teens who vape are more likely to smoke, and smoke heavily, six months later.
E-cigarettes are used by 37 percent of US 10th-grade teenagers and are now more popular amongst teens than traditional cigarettes, according to the Centers for Disease Control and Prevention.
The University of Southern California Keck School of Medicine study, published in the Journal of the American Medical Association, looked into the smoking habits of more than 3000 American 10th graders from public high schools in Los Angeles County, California.
The students took part in two surveys six months apart. At the start, 67 percent of them said they had never vaped before.
The study found the more teenagers had vaped, the more likely they were to later smoke cigarettes, with more frequent vapers more likely to smoke heavily.
The research did not look into why the participants were using e-cigarettes, but acknowledged that some use the devices as cessation aids.
The study's authors, led by Adam M Leventhal, said that though "further research was required", the "transition from vaping to smoking may warrant particular attention in tobacco control policy".
E-cigarettes are a contentious issue among health officials - some doctors believe the devices are effective in helping smokers quit, while a Massey University study in October said vaping could help people to battle obesity.