Fewer Grade 8 students in the U.S. said they saw a 'great risk' from occasionally smoking pot. Fewer Grade 8 students in the U.S. said they saw a 'great risk' from occasionally smoking pot. (Kevin Frayer/Canadian Press)

More U.S. teens appear to be smoking marijuana compared to last year, suggests a survey of students released Monday.

The annual survey on drug use by students in grades 8, 10 and 12 was conducted by researchers at the University of Michigan's National Institute on Drug Abuse for the U.S. government.

Marijuana was the most commonly used illicit drug. Its use had been decreasing, but that decade-long trend seemed to end this year, the researchers said in the report, titled "Monitoring the Future."

According to the study of 47,097 students, in 2009, marijuana use in the prior 12 months was reported by about 12 per cent of Grade 8 students, 27 per cent of Grade 10 students and a third of Grade 12 students.

Marijuana use peaked in the late 1990s.

'Upwarding trend'

Among students in Grade 12 this year, 20.6 per cent said they used it within the past month, compared to 19.4 per cent in 2008 and 18.3 per cent in 2006.

Among Grade 10 students, marijuana use in the past month rose to 15.9 per cent this year from 13.8 per cent in 2008.

Marijuana use in North America and Europe

An international study on the prevalence of cannabis use among 15-year-olds published in the Archives of Pediatric and Adolescent Medicine in February suggested that in Canada, 30 per cent of boys and 28 per cent of girls smoked the drug in 2006. In 2002, 43 per cent of boys and 37 per cent of girls said they had done so.

The largest decreases in marijuana use were found in England, Portugal, Switzerland, Slovenia and Canada. Increases occurred in Estonia, Lithuania, Malta and among Russian girls, Emmanuel Kuntsche of the Swiss Institute for the Prevention of Alcohol and Drugs Problems in Lausanne and colleagues found.

A decline in evenings out with friends, prevention efforts or changes in teens preferences were suggested as possible reasons for the decline, but it's possible that some other reason was behind the trend, the study's authors noted.

"The upward trending of the past two or three years stands in stark contrast to the steady decline that preceded it for nearly a decade," said Lloyd Johnston, who has directed the annual survey since it started in 1975.

Fewer Grade 8 students said they saw a "great risk" from occasionally smoking, 44.8 per cent this year, down from 50.5 per cent in 2004.

"When the perception of the danger goes down, in the following years you see an increase in use," said Nora Volkow, director of the National Institute on Drug Abuse.

The proportions saying they used any illicit drug in the past year were 15 per cent, 29 per cent and 37 per cent, in grades 8, 10 and 12, respectively.

Teens may mistakenly believe prescription drugs are less dangerous "because they're endorsed by the medical community," without realizing prescription narcotics such as OxyContin and Vicodin are highly addictive and can act as gateways to heroin, which is cheaper, Volkow said.

Alcohol continue to be the most widely used substance among U.S. teens, with 43.5 per cent of Grade 12 students reporting taking a drink in the past month, although use appears to be down substantially. In 1997, 52.7 per cent said they had consumed alcohol in the past month.

There were also declines this year in the use of LSD and other hallucinogens, cigarettes, and methamphetamine.

With files from The Associated Press