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Jonathan Hayward/The Canadian Press

Parents seeking direction on what to tell their teens about pot might take some pointers from the President of the United States, who explained in a recent New Yorker profile how he'll put it to his two daughters: "It's a bad idea, it's a waste of time, not very healthy."

Barack Obama went on to express thoughtful support for legalization, and described his own youthful dope smoking as "a bad habit, a vice," one not that different than the cigarettes he has more recently given up, and not more dangerous than alcohol. Many mothers and fathers of his boyhood would have passed out at such presidential admissions: a powerful, anti-pot parent movement in the 1970s had brought an abrupt halt to any substantive debate over decriminalization, both in the United States and Canada, with exaggerated worries of weed-happy, zombie-like offspring unable to compete in the economy.

But even with the debate over marijuana shifting rapidly – with decriminalization or straight legalization heavily favoured in public surveys, new laws in several states and endorsed by Liberal Leader Justin Trudeau – some scientists working in the relatively new research area of pot and the teenage brain say young people need more specifics than "not very healthy."

Researchers also worry that with the focus on eliminating overzealous prosecutions, easing access to medicinal marijuana and regulating supply to reduce the black market, a public-service message about the health consequences for adolescents, who make up the bulk of users, has been overlooked. In Canada, for instance, although rates have fallen in the last decade and only 10 per cent report being regular users, teens have the highest level of dope-smoking in the developed world, according to 2013 Unicef report; using data from the World Health Organization, the study suggested that 28 per cent of children between age 11 and 15 had tried marijuana within the past year. Having gotten the nicotine message, many have tossed the cigarettes in favour of weed, which, surveys suggest, they are increasingly likely to view as safe, even beneficial.

When it comes to marijuana, adults and teenagers have different risks, a burgeoning field of research suggests. A 30-year-old can smoke up relatively safely, researchers say, but there's a crucial neurological window in adolescence – and for some young users, getting high can have potentially, life-changing long-term consequences.

Selling a "wait till you're older" message will make most teens roll their eyes. It's not like they haven't heard that one before. Considering the deaths that result from binge drinking, and the highly addictive, cancer-causing effect of nicotine, pot is comparatively benign. Obama, after all, was simply reflecting the views of many parents, who toked up, moved on, and are just fine. For most teenagers – especially occasional users who try it later in high school – the pattern will likely be similar. But scientists point out that the pot today is more potent than what was smoked in the seventies. For a small group of adolescents, those who start sooner and use more heavily, research suggests that marijuana may alter brain development at a pivotal time, causing long-term damage to working memory and even potentially trigger a gene that causes schizophrenia.

"It is a disservice not to give [teenagers] the knowledge that it is not the panacea of all positives," says Yasmin Hurd, a neuroscientist at New York's Mount Sinai School of Medicine who studies marijuana. "Some of them will be very much affected and it will change the trajectory of their lives…. And how do you know if you're going to be that person who is more vulnerable?"

For a drug that's been around for centuries, there's a lot scientists don't knowabout marijuana. Most studies, both animal and human, look at heavy users, and many of the effects are subtle, so it's still unclear how much those findings translate to more occasional use. It is known, however, that adolescence is an important period of brain development, when cross-brain connections are cemented with the temporal lobe, a central hub for working memory and reasoning. A recent review of 120 Canadian and U.S. studies on the effect of cannabis found that early marijuana use altered brain development. A McGill University study found a link between daily consumption in teens and depression and anxiety. The connection to mental-health issues such as schizophrenia isn't conclusive – teenagers with early symptoms may use marijuana to self-medicate – but many scientists theorize that pot smoking, early and in high doses, interacts with a genetic predisposition for the illness. A U.S. study found similar deficits in working memory, and changes in brain function between people with schizophrenia who didn't smoke pot, and those without the disease who were heavy marijuana users.

The role marijuana plays as a gateway drug is also the subject of debate. A U.S. study published last year suggested that it was alcohol that really started the path to such hard drugs as cocaine and heroin, with pot a pit stop along the way. Many teenagers never leave that pit stop. But in studies of juvenile rats, argues Hurd, the neurological link with addiction is beyond dispute. And its effects may be long-lasting: in joint U.S.-Canada experiment released last year, the "grandchildren" of teenaged rats given the equivalent of one joint every three days were more likely to seek out heroin, even if their own "parents" had not been exposed to marijuana.

Still, as Hurd points out, animal studies reveal the neurological effects in a lab. Human beings are more complex – environmental and social factors, as well as genes, go a long way to protecting a teenager from the risks of marijuana. But, says Hurd, "is there a potential for early marijuana exposure to change the brain in a way that will last into their adulthood [and] make them more susceptible to other drugs? Hands down, absolutely, the results shows that."

Decriminalization or legalization, however, would make the drug scene safer for teenagers – particularly if it leads to stricter controls on the type of marijuana available, and more regulation over who can access it. A 2012 U.S. study based on national survey data found that the legalization of medical marijuana did not increase use among teenagers; European countries with more lax marijuana laws report lower rates of teenage use than Canada. Benjamin Hansen, co-author of the 2012 paper, suggests that while a lower price might make pot more affordable to teens, and there would be some third-party sales to minors, regulated suppliers would restrict their sales to minors and reduce demand on the street. "Right now, the best person [for a dealer] to sell to is a 15-year-old kid," Hansen says. "How likely is that kid to be an undercover cop?"

Decriminalizing marijuana would also, experts say, make it more likely for teenagers to seek help if they run into trouble with the drug, and it would foster more research into addiction treatment, for which there are currently few options. According to Dr. Didier Jutras-Aswad, a researcher at the University of Montreal, who worked on the cross-generational rat study, only about 10 per cent of marijuana users will become addicted – compared to roughly 32 per cent of those who use nicotine or cocaine. But since pot is by far the most widely used illegal drug, the actual number of people struggling with addiction may be higher.

As the legalization debate moves forward, Jutras-Aswad suggests that society can learn from its history with alcohol, in which it took decades to develop standards for healthy use, controls on impaired driving and clear public-health messaging. "We can learn what to avoid with cannabis, and not wait for problems to happen and then react." For teenagers, that means clearing away the politics and the rhetoric, and giving them a balanced take on the best science available.

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