Monitoring the Future, a nationally representative annual survey of eighth- 10th- and 12th-graders conducted by the University of Michigan, said for the three grades combined, the annual prevalence of marijuana use rose in 2011 to 25 percent from 21.4 percent in 2007.
"In addition, one in every 15 high school seniors today is smoking pot on a daily or near daily basis," Lloyd Johnston, the principal investigator of the study, said in a statement. "And that's the highest rate that we have seen over the past thirty years -- since 1981."
In addition, "synthetic marijuana," which until this year was legally sold and goes by such names as "K2" and "spice," was added to the study's coverage in 2011. The survey said 11.4 percent of high-school seniors reported using this drug in the prior 12 months.
The federal government used temporary emergency powers in February to declare a number of chemicals used in synthetic marijuana to be Schedule I drugs -- unsafe, highly abused substances with no legitimate medical use -- for at least a year. At least 18 states have banned synthetic marijuana, the researchers said.
One possible explanation for the resurgence in marijuana use is that in recent years fewer teens
report seeing much danger associated with even regular use, the researchers said.
Alcohol use and importantly, occasions of heavy drinking, continued a long-term gradual decline among U.S. teens, reaching historically low levels in 2011, while energy drinks are being consumed by about one-third of teens, with use highest among younger teens.
The number of those who said they smoked tobacco at all in the prior 30 days fell significantly for the three grades combined, from 12.8 percent in 2010 to 11.7 percent this year.
One reason smoking has declined so sharply is that the proportion of students who ever try smoking has fallen dramatically -- 49 percent of eighth-graders in 1996 had tried cigarettes, while in 2011 it was 18 percent.
Reasons given for not smoking tobacco included increased preference for dating non-smokers, strongly disliking being around people who are smoking, thinking that becoming a smoker reflects poor judgment and believing that smoking is a dirty habit.
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