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From the archives: This article was originally published December 15, 2015

Legalizing marijuana is now an official goal of the federal government, and not just a campaign promise. Ontario's premier even suggested this week that the province's liquor stores could sell weed next to wine. The natural worry for parents and policy makers is what this might mean for teenagers. Here's what you need to know.

What does the science say about a teenager's brain on pot?

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Researchers have definitely found some worrisome links. A University of Ottawa study found an increase in brain activity when performing cognitive tasks in teenagers who smoked pot at least once a week for three years – in other words, their brains had to work harder on tests measuring areas such as working memory and sustained attention. A 2014 study published in the Journal of Neuroscience also found structural changes in brain regions affecting emotion and reward processing in teens who smoked up at least once a week. A New Zealand study published in 2012 that followed subjects from birth into their 40s, linked long-term marijuana use to lower IQs – particularly for teens who continued using into adulthood. Experiments with rats have also found memory deficits in rodents given marijuana extract in adolescence. Other research has suggested a link, especially in teenaged boys, between marijuana use and schizophrenia. And compared with their non-stoner peers, teenagers who smoked pot daily, according to a 2014 study published in the Lancet medical journal, were 60 per cent less likely to finish high school or get a university degree than those who didn't smoke. They were also eight times more likely to use other illicit drugs as adults, and seven times more likely to attempt suicide.

That sounds pretty conclusive. Is it?

It's safe to conclude that marijuana isn't doing the teenage brain any favours – and, in some cases, it's at least a factor in serious harm. Still, a shortcoming of many of these studies is that they reveal a "correlation" between pot and certain deficits, but not whether pot directly "caused" the brain changes, or whether those differences existed before the pot use. Further confounding the studies is the fact that teenaged pot smokers – especially chronic users – often differ in many ways from their weed-averse teens; for one, they typically drink more alcohol and use other illicit drugs. Scientists also speculate that the negative effects of marijuana may be influenced by genetics, potentially explaining, for instance, the connection to schizophrenia. For years, one theory has been that teens starting to experience the symptoms of schizophrenia were self-medicating with marijuana. That's been largely debunked by new scientific research. But it is still unclear whether marijuana changes the brain in ways that lead to schizophrenia, or whether it triggers schizophrenia in teenagers already genetically disposed to the mental illness.

Will legalization lead to a nation of teenaged stoners?

In the summer, an American study analyzed the self-reported pot use among one million teenagers in 48 states between 1991 and 2014. In the 21 states that had passed a medical-marijuana law, researchers found that, while the teens in those states tended to be higher users in general, there was no increase in use after the law was passed. If legalization did translate into higher teen use, you might expect states such as Colorado and a nation such as the Netherlands to have the world's most stoned adolescents. But, based on 2013 data, the percentage of Colorado teens who reported using pot in the previous month fell slightly since the law changed. And according to the United Nations, 17 per cent of Dutch teenagers said they had smoked pot in the last year – compared with 28 per cent in Canada.

So what's the Canadian story?

Teenagers here already have the highest rate of cannabis use in the developed world. According to the Canadian Centre on Substance Abuse, about 10 per cent of Grade 12 students smoked up daily. It's pretty clear that motivated teenagers have figured out a way to score an illegal joint long before politicians were talking about selling pot in liquor stores. What's more, the street pot they're smoking is a lot stronger than the kind that made the rounds at Woodstock. In the 1960s, marijuana contained 1 per cent tetrahydrocannabinol, the psychoactive ingredient. Today's weed has a THC concentration of at least 10 per cent and, according to Health Canada, as much as 30 per cent. Regulating marijuana would arguably make a safer product.

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So what should I tell my teenager?

Quote the science. Adolescence is a critical stage of development for the brain, which continues to build new connections even into adulthood. Parents can tell their kids that, given the preponderance of evidence suggesting negative and possibly permanent effects, and the fact that science cannot yet predict who may be most vulnerable to harm, smoking pot during a brain-boosting time of life is not a good idea. (Incidentally, parents should share the same message about binge drinking.) In the New Zealand study, for instance, while subjects who started smoking only as adults didn't show IQ deficits, teenagers who eventually gave up pot did not restore their IQs to the level of their non-smoking peers. Many researchers suggest that educating teenagers about the scientific findings may be the biggest deterrent to drug use – whatever the law of the land.