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US teenagers are using marijuana and vaporisers more than they smoke cigarettes, a government study shows.

Some 15% of high school students said they had used marijuana within the previous 30 days, found the report for the National Institute on Drug Abuse.

And 12.1% of students said they had used a vaping device. But only 5% had smoked cigarettes.

One in 10 high school seniors said he or she had vaped marijuana at least once in the past year.

"It's much higher than I expected," said Richard Miech, the University of Michigan researcher who led , of the cannabis vaping figure.

The findings suggest cannabis use is up about 1% overall among teenagers.

Nearly a quarter of students said they had vaped, smoked or eaten marijuana in the previous year.

One in 17 high school seniors said he or she had used marijuana every day.

Dr Nora Volkow, director of the National Institute on Drug Abuse, said: "These are teens that are supposed to be learning at school.

"When you're stoned, you can't learn much."

Some students surveyed said they used vaporisers to smoke nicotine or flavouring, instead of marijuana.

The study suggests this as a possible reason why daily cigarette smoking among 17 and 18-year-old students was down to 4.2% this year from a recorded high of 24.6% in 1997.

Some 43,703 students aged between 13 and 18 years old in public and private high schools were questioned for the study.