(HealthDay)—Many folks are trying e-cigarettes, but not everyone is sticking with them, a new survey finds.

E-cigarettes, which contain nicotine, have been marketed as a way to help tobacco smokers quit smoking and as an alternative to cigarettes. Although the number of U.S. adults who tried them between 2014 and 2016 soared, those who continued to use them dropped, researchers found.

"The decline in current use, but increase in those who have tried e-cigarettes, may suggest that some individuals are trying but not continuing use of e-cigarettes," said lead researcher Dr. Wei Bao. He's an assistant professor of epidemiology at the University of Iowa.

It might be too soon, however, to draw conclusions about e-cigarette trends from just three years of data, he added.

"E-cigarette use among U.S. adults is changing over time," Bao said. "To understand the health impact of the changes in e-cigarette use, continued surveillance is needed."

To gauge the changes, Bao and his colleagues used data from the U.S. National Health Interview Survey from 2014 to 2016.

Bao's team collected data on more than 100,000 men and women who were asked about their use of e-cigarettes, also called "vaping."

The number of people who tried e-cigarettes grew by nearly 13 percent in 2014, 14 percent in 2015 and 15 percent in 2016, the researchers found.

But over the same time period, the number of those still using e-cigarettes dropped by 3.7 percent in 2014, 3.5 percent in 2015, and 3.2 percent in 2016.

The decrease was significant among adults aged 65 and older, women, whites, those with low incomes and those who smoked regular cigarettes, the findings showed.

However, the use of e-cigarettes increased among former smokers and adults who had never smoked, Bao said.

Greg Conley, president of the American Vaping Association, believes the findings provide evidence that e-cigarettes are helping people quit tobacco.

"This study should give great pause to those who have used shoddy science to claim that vaping does not help smokers quit," he said.

According to Conley, more than 2.6 million ex-smokers are vaping.

"This translates into hundreds of millions of packs of cigarettes that are not being smoked each year. This number would be even higher but for the relentless campaign to mislead the public about vaping products," Conley said.

One specialist who has researched e-cigarettes disagreed.

"It's good that e-cigarette use is starting to drop," said Stanton Glantz, a professor of medicine at the University of California, San Francisco's Center for Tobacco Control, Research and Education.

But, "the net effect e-cigarettes are having is to keep people smoking cigarettes," Glantz said.

The increase in e-cigarette use among former smokers and those who have never smoked is troubling, Glantz added. "It's just expanding the tobacco epidemic," he said.

Glantz also questions whether e-cigarettes help people quit smoking. "For most people, e-cigarettes make it harder to quit smoking," he said.

Some people have used e-cigarettes and successfully quit tobacco. But among all adults trying to quit, using e-cigarettes makes it 20 percent less likely they will quit, compared with those who don't use them, Glantz said.

"So, while they are promoted as a smoking-cessation aid, they actually make it harder to quit smoking," he said.

E-cigarettes are also popular among many teens, leading health experts to worry that the devices could be creating a new generation of smokers.

The report was published May 15 in the Journal of the American Medical Association.

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