Researchers in the US say they're surprised at how many 8th, 10th and 12th graders reported using electronic cigarettes this year.

More US teenagers are trying out e-cigarettes than the real thing, according to the government's annual drug use survey.

Researchers were surprised at how many 8th, 10th and 12th graders reported using electronic cigarettes this year, even as regular smoking by teenagers dropped to new lows.

Nearly nine per cent of 8th graders said they had used an e-cigarette in the previous month, while just four per cent reported smoking a traditional cigarette, said the report released on Tuesday by the National Institutes of Health.

Use increased with age: Some 16 per cent of 10th graders had tried an e-cigarette in the past month, and 17 per cent of high school seniors.

Regular smoking continued inching down, to seven per cent of 10th graders and 14 per cent of 12th graders.

"I worry that the tremendous progress that we've made over the last almost two decades in smoking could be reversed on us by the introduction of e-cigarettes," said University of Michigan professor Lloyd Johnston, who leads the annual Monitoring the Future survey of more than 41,000 students.

The survey didn't ask about repeat use, or if teenagers were just experimenting with something new.

But between four per cent and seven per cent of students who tried e-cigarettes said they'd never smoked a tobacco cigarette, noted University of Michigan professor Richard Miech, a study senior investigator.

"They must think that e-cigarettes are fundamentally different," he said.

E-cigarettes often are described as a less dangerous alternative for regular smokers who can't or don't want to kick the habit.

The battery-powered devices produce vapour infused with potentially addictive nicotine, but without the same chemicals and tar of tobacco cigarettes.

AAP.