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A rise in marijuana use among U.S. teens over the past 20 years has no significant tie to the legalisation of marijuana for medical use in many states, according to a new research paper.
Comparing surveys of marijuana use by adolescents conducted annually by the federal Centres for Disease Control and Prevention, researchers found the probability that a high schooler had used pot in the last 30 days was no more than 0.8 percent higher in legal states compared to states that had not approved medical marijuana.
"Our results are not consistent with the hypothesis that the legalisation of medical marijuana caused an increase in the use of marijuana among high school students," D. Mark Anderson of Montana State University, Daniel Rees of the University of Colorado and Benjamin Hansen of the University of Oregon wrote.
Twenty-one states and the District of Columbia have legalized the use of marijuana for medical purposes, while two states, Colorado and Washington, now allow recreational use. Alaska and Oregon are set to vote on legalisation for recreational use in November, while supporters of full legalisation in the nation's capital say they have enough signatures to put the measure on the ballot.
Marijuana remains illegal under federal law.
Some opponents of legalisation are concerned it will increase use among teens. According to Monitoring the Future, an organisation funded by the National Institute on Drug Abuse that has surveyed drug use among teens since 1975, 36 percent of high school seniors surveyed in 2013 said they used pot in the last year, while 6.5 percent said they used it almost every day.
Use among twelfth graders peaked in 1979 at 51 percent and fell to a low of 22 percent in 1992. Use slowly increased after 1992, but it has leveled since 2011.
A study published in the Annals of Epidemiology in 2011 found use among adolescents in medical marijuana states had risen, but concluded more research was needed to draw a causal conclusion. Further, it found that between 2002 and 2008, use among teens was highest in states where the drug was legal, but it was also already high in those states prior to legalisation.