High rates of marijuana use during the teen and preteen years when the brain is still developing places young people at risk, the National Institute on Drug Abuse says. (Kevin Frayer/Associated Press)

America's teens are using more marijuana and less alcohol, according to an annual government study of eighth-, 10th- and 12th-graders across the country.

Some 6.1 per cent of high-school seniors reported using marijuana almost daily in 2009, according to the Monitoring the Future survey released by the U.S. National Institutes of Health.

Near daily use of marijuana — that is, 20 times in the month before the survey — by 10th-graders climbed from 2.8 per cent to 3.3 per cent, and for eighth-grade students it edged up from 1.0 per cent to 1.2 per cent.

"These high rates of marijuana use during the teen and preteen years, when the brain continues to develop, place our young people at particular risk," said Dr. Nora Volkow, director of the U.S. National Institute on Drug Abuse.

On the other hand, binge drinking is on the decline. While 23.2 per cent of high school seniors reported having five or more drinks in a row, that's down from 25.2 per cent a year earlier. The binge rate for this age group peaked at 31.5 per cent in 1998.

On other topics the survey found:

• Use of ecstasy, which had declined in the early 2000s, is increasing again.
• There was a small increase in teens injecting heroin, but only among 12th-graders.
• Use of cocaine remained low after declining from levels in the 1980s and 1990s.

The survey, conducted by the University of Michigan, covered 46,482 students in 396 schools.

An international study on the prevalence of cannabis use among 15-year-olds published in the Archives of Pediatric and Adolescent Medicine last 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.

Among Canadians aged 15 to 24 years, use of at least one of five illicit drugs — cocaine or crack, speed, hallucinogens, ecstasy, and heroin — in the preceding year decreased from 11.3 per cent in 2004 to 5.5 per cent in 2009, Statistics Canada said.