A small U.S. study raises new questions about whether electronic cigarettes lead people to quit smoking, adding to the debate over how tightly the products should be regulated.

The study, which looked at the habits of 88 smokers who also used e-cigarettes, was published as a research letter in the journal JAMA Internal Medicine. It found that smokers who also used e-cigarettes were no more likely to quit smoking after a year, compared to smokers who didn't use the devices.

Outside experts say the small number of respondents, and a lack of data on whether they intentionally used e-cigarettes to help them quit smoking, mean the findings from the Center for Tobacco Control Research and Education at the University of California, San Francisco can't take the place of much more rigorous study on the subject.

E-cigarettes were first introduced in China in 2004 and have since grown into a $2 billion industry. The battery-powered devices let users inhale nicotine-infused vapors, which don't contain the harmful tar and carbon monoxide in tobacco.

At issue is how strictly U.S. health regulators should control the products. Advocates say e-cigarettes can help smokers quit. Public health experts fear they can serve as a gateway to smoking for the uninitiated, particularly teenagers.

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