CHICAGO — Teens who use e-cigarettes are more likely than others to later smoke conventional cigarettes and other tobacco products, a study at 10 Los Angeles high schools suggests. The study doesn't prove that electronic cigarettes are a "gateway drug" but some doctors say it bolsters arguments that the devices should be strictly regulated as proposed by the Food and Drug Administration. Whether teens had tried just one e-cigarette or were habitual users isn't known, nor is whether they became heavy smokers or just had a few puffs.