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.
Read: Less smoking but more vaping among US teens
Previous research has also found a connection between e-cigarette use by teens and later tobacco use.
The e-cigarette industry criticised 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.
More than one-third of 10th graders in the United States report vaping e-cigarettes, the researchers said in background information.
Read: To vape or not to vape - a look at trends, analysis and debate
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.
Kids get hooked
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.
It could be that kids get hooked on nicotine and turn to tobacco for a stronger fix, Leventhal said. Also, e-cigarettes might act as a teaching aid that prepares them for smoking tobacco cigarettes, he added.
"Once they start smoking, it's not a foreign sensation to them," Leventhal said. "They've experienced the act of drawing in these vapour clouds from e-cigarettes and then exhaling them. When they puff on a regular cigarette, it could be more pleasing in comparison to someone who puffs a cigarette for the first time and never had the experience of inhaling a substance before."
Read: 10 celebs who vape
The findings give credence to the worst fears of the American Lung Association, said Dr Norman Edelman, the association's senior scientific advisor.
"That's exactly what the American Lung Association has been worried about. We're afraid this product will addict youngsters to nicotine and lead them to smoke," Edelman said. "That's contrary to what the proponents of e-cigarettes say, but this study contradicts their claims."
Far safer products
Armed with this and other research, the US Food and Drug Administration needs to step in and begin strict regulation of e-cigarettes, Edelman said.
The FDA has banned sales of e-cigarettes to minors, but can take further steps such as prohibiting any marketing that appears to target teens, he said.
Conley countered that "the rise in vaping experimentation has fuelled record-breaking declines in teen smoking".
So, he added, "It would be public health malpractice to use studies with poorly defined parameters, such as this one, as an excuse to deny adults access to these far safer products."
The study was published in the Journal of the American Medical Association.
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