The researchers found that students who used e-cigarettes were more likely to have smoked conventional cigarettes in the past (defined as at least 100 cigarettes, or 5 packs, in their lifetime) and to be current smokers. In addition, among current conventional smokers, those who were also currently using e-cigarettes tended to smoke more than those who didn't use e-cigs. An earlier study of young people in Korea reached similar conclusions.

It's not possible to conclude whether using e-cigarettes actually caused teens to smoke more, or whether teens who are heavier smokers are more likely to also use e-cigarettes. But the findings are important either way because they suggest that e-cigarettes aren't effective as smoking-cessation aids, as they've been touted by some proponents, but rather are linked with higher use of regular cigarettes.

Read our ongoing coverage of e-cigarette safety and regulation. Have you tried e-cigarettes? Please consider sharing your experience with us.

—Jamie Kopf