*The following is excerpted from an online article posted on WebMD.
Teens who regularly "vape" e-cigarettes are more likely to become frequent and heavy cigarette smokers, new research finds.
A survey of students at 10 Los Angeles County public schools found that teens who vape frequently are more than twice as likely to start smoking "on about a weekly basis," said lead researcher Adam Leventhal.
Further, these teenagers are twice as likely to smoke more cigarettes on days when they do smoke, Leventhal added.
"The more you vape, the more likely in the future you're going to be smoking (cigarettes). You're going to be smoking more frequently and you're going to smoke more cigarettes per day on your smoking days," Leventhal said.
He is an associate professor of preventive medicine and psychology at the University of Southern California Keck School of Medicine in Los Angeles.
However, the study only found an association between vaping and smoking, not cause and effect.
Previous research has also found a connection between e-cigarette use by teens and later tobacco use.
The e-cigarette industry criticized the new study for defining "frequent" vaping or smoking as three days or more each month.
"Why? Because despite having a sample size of over 3,000, the authors were only able to identify a fraction of students who had progressed onto any cigarette smoking, let alone actual frequent or heavy smoking," said Gregory Conley. He's the president of the American Vaping Association.
For their study, Leventhal and his colleagues surveyed nearly 3,100 10th graders in the Los Angeles area -- first during the fall of 2014 and then six months later.
"It was a short period, but it was an important period," Leventhal said. "Teens who start smoking and become regular smokers at this age, around 16, are more likely to become chronic smokers throughout adulthood."
The researchers agreed that not many kids admitted to either smoking or vaping during the previous 30 days. About 95 percent of kids said they were non-smokers, and about 98.5 percent said they'd never vaped, the study findings showed.
But the kids who did vape were more likely to try cigarette smoking, and more frequent vaping was associated with more frequent and heavier smoking, Leventhal said.
The study was published in the Journal of the American Medical Association.