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Do more teens smoke when pot is legal? We still don't know

tanjila ahmed via Flickr Creative Commons

Californians' vote in November to legalize recreational marijuana for adults has raised the question of whether it will lead to greater use among teenagers. A new survey of two states that have already legalized the drug doesn't provide a clear answer: it finds that use among some teenagers increased in one state but not in the other.

The study, published Tuesday in JAMA Pediatrics, compares perceptions of marijuana and recent pot use among youth in Colorado and Washington, where recreational pot was legalized in 2012, and in states that have not legalized marijuana.

The researchers find that recent pot use among Washington eighth and 10th grade students increased between 2013 and 2015, while use decreased among that age group in states where recreational pot is illegal.

They also find a significant decrease in the number of Washington eighth and 10th graders who perceive marijuana as harmful, compared with teens from states that have not legalized recreational use.

In Colorado, meanwhile, youth reported no real change in their perceptions or use of marijuana, according to the study.

The differing results from Colorado and Washington "may be related to the different degree of commercialization of marijuana prior to legalization in Washington and Colorado," write the authors.

They note that Colorado had a well developed medical marijuana dispensary system with substantial advertising prior to legalization, while Washington had substantially less commercialization and advertising for its medical pot stores.

"In addition, rates of perceived harmfulness in Colorado were already lower and rates of marijuana use were already higher than rates in Washington and [states where recreational pot remains illegal] prior to legalization," they write.

"While the authors offer a number of plausible explanations for the observed differences between the two states, it is fair to say that we are still uncertain how to explain the results," JAMA Pediatrics associate editor Alain Joffe writes in an accompanying editorial.

He concludes with an appeal for more research on how marijuana legalization affects use by those under 21.

"We want to emphasize our commitment to publishing high-quality research that helps inform public policy," writes Joffe. "We cannot afford to lose this opportunity, not when the public health consequences are potentially so significant."

There is a paucity of research on whether teen pot use increases when recreational marijuana is legalized. Advocates of legalization point to research concluding that teen use has not increased in states that have legalized medical marijuana.

The JAMA Pediatrics study was based on data from about 254,000 eighth, 10th and 12th grade students, collected through the national Monitoring the Future survey.

It finds that between 2013 and 2015, marijuana use in Washington increased by 2 percent among eighth graders, and by roughly 4 percent among 10th graders. In contrast, use among this age group in states without recreational pot decreased by about 1 percent. 

Pointing to this data, the authors speculate that teen use in Washington would also have decreased if the state hadn't changed its laws.

The study also finds that perceived harmfulness of marijuana in Washington decreased by about 14 percent among eighth graders and by roughly 16 percent among 10th graders. That compares with states without recreational pot laws, where perceptions of harm dropped by nearly 5 percent among eighth graders and just over 7 percent among tenth graders.

The report highlights the need for sophisticated health education about the effects of marijuana, writes lead author Wayne Hall, of the University of Queensland in Australia, in an accompanying editorial.

Health educators will be in a bind as more states legalize recreational pot and the industry tries to grow its market and downplay the drug's health risks, he writes.

"The challenge for health educators will be in acknowledging that the acute adverse effects of marijuana use are modest by comparison with those of alcohol or heroin, while persuading young people that they can experience adverse effects, especially if they begin use in their teens and use daily throughout young adult life," says Hall.