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Students Tori Petersen, 18, right, and Georgia McNab, 20, left, share a joint. A large crowd filled Dundas Square in Toronto as pot smokers of every kind gathered on April 20, 2011 to celebrate, and make a political statement at 4:20pm on the twentieth day of the fourth month - the 4/20.

Peter Power/The Globe and Mail

Because their brains are still developing, adolescents may be particularly vulnerable to the negative effects of marijuana.

During this crucial period, brain connections are strengthened through myelination – growth of fatty insulation around the neurons – as well as a "pruning" of inefficient neural connections. It's a lengthy process, stretching past the post-secondary years to at least age 25.

Hundreds of studies have been conducted in recent decades to determine how cannabis affects youth.

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While not all research has shown harms, study after study – including a large review released October 7 in the journal Addiction – has linked regular pot use in adolescence to detrimental effects ranging from worse education outcomes to cognitive impairments and losses in IQ.

Some pot advocates suggest that research has been skewed by anti-drug organizations that only fund studies looking for harms. Others argue that marijuana is less harmful to teens than opiate drugs or alcohol, and that negative effects in heavy cannabis users have been generalized to light recreational users.

Still others point to inconsistent results in some studies. One explanation, researchers say, is that participants' marijuana use was largely self-reported and the dosage of the psychoactive ingredient tetrahydrocannabinol (THC) might have varied widely depending on which strains they were smoking.

Still, scientists have been unable to prove without a doubt that marijuana is the direct cause of a teen's low motivation or learning problems. And they may never have ironclad proof since randomized controlled trials, the gold standard of scientific research, are not possible in marijuana studies involving youth for the simple reason that scientists can't ethically assign a group of children to take regular doses of cannabis, compare them to a drug-free control group, and see how they turn out.

Instead, scientists have relied on long-term studies in which participants, followed since birth, serve as their own controls.

Now, scientific consensus that marijuana is harmful to the developing brain is so strong, concerns about teen cannabis use can no longer be dismissed as a modern-day version of "reefer madness" propaganda, substance abuse experts say.

To determine how marijuana use affects the minds of young Canadians, we analyzed a long list of peer-reviewed studies and Health Canada publications, weighed arguments against demonizing cannabis use, and scanned reports in other media. After careful deliberation, we based the bulk of our reporting on rigorous studies cited again and again by specialists in the fields of psychology, pharmacology and addiction research.

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Here is a list of our sources:

National and international surveys

Health Canada and Statistics Canada reports

  • Information for Health Professionals: Cannabis (marihuana, marijuana) and the cannabinoids (2013). Health Canada.
  • Canadian Alcohol and Drug Use Monitoring Survey (2012). Health Canada.
  • Mental Health Profile, Canadian Community Health Survey (2012). Statistics Canada.

Published studies

  • What Has Research Over the Past Two Decades Revealed About the Adverse Health Effects of Recreational Cannabis Use? (2014). Addiction.
  • Impact of Marijuana on Response Inhibition: an fMRI Study in Young Adults (2011). Journal of Behavioral and Brain Science.
  • Effects of Marijuana on Visuospatial Working Memory: An fMRI Study in Young Adults (2010). Psychopharmacology.
  • Effects of Cannabis and Cannabinoids in the Human Nervous System [chapter in The Effects of Drug Abuse on the Human Nervous System] (2013). Academic Press.
  • Cannabis Use is Quantitatively Associated With Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users (2014). The Journal of Neuroscience.
  • Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife (2012). PNAS.
  • Rate of Cannabis Use Disorders in Clinical Samples of Patients With Schizophrenia: A Meta-analysis (2010). Schizophrenia Bulletin.
  • Cannabis Use in Adolescence and Risk for Adult Psychosis: Longitudinal Prospective Study (2002). BMJ.
  • Cannabis and Schizophrenia: A Longitudinal Study of Swedish Conscripts (1987). Lancet.
  • Cannabis Use and Psychosis: A Longitudinal Population-Based Study (2002). American Journal of Epidemiology.
  • Prospective Cohort Study of Cannabis Use, Predisposition for Psychosis, and Psychotic Symptoms in Young People (2004). BMJ.
  • Cannabis Controversies: How Genetics Can Inform the Study of Comorbidity (2014). Addiction.
  • Young Adult Sequelae of Adolescent Cannabis Use: An Integrative Analysis (2014). The Lancet Psychiatry.
  • The Epidemiology of Cannabis Dependence [chapter in Cannabis Dependence: Its Nature, Consequences and Treatment] (2006). Cambridge University Press.
  • Does Early Drug Use Increase the Risk of Dropping Out of High School? (1998). Journal of Drug Issues.
  • The Effects of Adolescent Cannabis Use on Educational Attainment: A Review (2000). Addiction.
  • Cannabis and Canadian youth: Evidence, not ideology (2014). Canadian Family Physician.
  • Dare to delay? The Impacts of Adolescent Alcohol and Marijuana Use Onset on Cognition, Brain Structure, and Function (2013). Frontiers in Psychiatry.
  • Effects of Frequent Marijuana Use on Memory-Related Regional Cerebral Blood Flow (2002). Pharmacology, Biochemistry, and Behavior.
  • Effect of Long-Term Cannabis Use on Axonal Fibre Connectivity (2012). Brain: A Journal of Neurology.
  • A Controlled Family Study of Cannabis Users With and Without Psychosis (2014). Schizophrenia Research.
  • Current and Former Marijuana Use: Preliminary Findings of a Longitudinal Study of Effects on IQ in Young Adults (2002). CMAJ.
  • Adverse Health Effects of Marijuana Use (2014). New England Journal of Medicine.
  • Long-Term Effects of Cannabis on Brain Structure (2014). Neuropsychopharmacology.
  • Cannabis-Related Working Memory Deficits and Associated Subcortical Morphological Differences in Healthy Individuals and Schizophrenia Subjects (2013). Schizophrenia Bulletin.
  • A Preliminary DTI Study Showing No Brain Structural Change Associated with Adolescent Cannabis Use (2006). Harm Reduction Journal.
  • The Role of Leisure and Delinquency in Frequent Cannabis Use and Dependence Trajectories Among Young Adults (2014). The International Journal on Drug Policy.
  • Probability and Predictors of the Cannabis Gateway Effect: A National Study (2014). The International Journal on Drug Policy.
  • Adverse Psychosocial Outcomes Associated With Drug Use Among US High School Seniors: A Comparison of Alcohol and Marijuana (2014). The American Journal of Drug and Alcohol Abuse.
  • The Health Risks and Harms Associated with the Use of Marijuana (2014). Canadian Medical Association.
  • Residual Effects of Prolonged Cannabis Administration on Exploration and DRL Performance in Rats (1982). Psychopharmacology.
  • Learning Impairment in the Radial-Arm Maze Following Prolonged Cannabis Treatment in Rats (1982). Psychopharmacology.

Experts interviews

  • Phone interview (2014) with Dr. Harold Kalant, professor of pharmacology at the University of Toronto.
  • Phone interview (2014) with Dr. Sheryl Spithoff, family physician and addictions medicine specialist at Women’s College Hospital, Toronto.
  • Phone interview (2014) with Dr. Amy Porath-Waller, interim director of research and policy at the Canadian Centre on Substance Abuse.
  • Phone interview (2014) with Dr. Bernard Le Foll, a clinician-scientist specializing in drug addiction at the Centre for Addiction and Mental Health, Toronto.
  • Phone interview (2014) with Kevin Sabet, director of the Drug Policy Institute at the University of Florida College of Medicine.
  • Phone interview (2014) with Dr. Andra Smith, associate professor of psychology at the University of Ottawa.
  • Phone interview (2014) with Zach Walsh, assistant professor of psychology at the University of British Columbia.

Other sources

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Secondary sources

  • Teens Who Smoke Weed Daily are 60 per cent Less Likely to Complete High School than Those Who Never Use (2014). The Washington Post.
  • Marijuana ‘Hash Oil’ Explodes In Popularity, And Kitchens (2014). NPR.
  • Why Marijuana Edibles Might Be More Dangerous Than Smoking (2014). ABC News.
  • This Might Be Your Brain On Drugs: Colorado’s Softer Anti-Pot Propaganda Is Still Propaganda (2014). Forbes.
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