About one in five Irish 15-16-year-olds report using cannabis. That means one in five adolescents are using a drug that can have a multitude of demonstrably negative effects on their short- and long-term mental health at a time when their brains are at the peak of their development.

Until relatively recently, cannabis use among Irish adolescents was in decline. In 2003, 17 per cent of 16-year-olds said they had used the drug in the past month; that number dropped to 7 per cent in 2011. The reverse began soon after. While there’s no single reason for this, one possible explanation is that the percentage of teenagers who view cannabis as “low risk” increased from 10 per cent in 2011 to almost 20 per cent in 2015.

As a society, we have come to view cannabis through rose-tinted glasses. Contrary to increasingly common belief, cannabis is not a harmless panacea, and it can be addictive. An estimated one in six who start smoking or ingesting cannabis before the age of 18 become dependent.

While the effects of alcohol and tobacco on physical health are well known, the public do not seem to be aware that cannabis can be toxic to young people’s developing brains. Over the past couple of decades, numerous scientific studies have shown that cannabis use in young people significantly increases the risk of developing mental health problems.

Heavy use of cannabis in adolescence can have long-term effects on memory and cognition

There is strong evidence that cannabis use increases the risk of depression and anxiety in young people. The risk of youth suicide increases three-fold. There are a large number of studies now showing that cannabis use causes psychosis.The risk of developing a psychotic illness such as schizophrenia is particularly high in people who start using high potency cannabis during adolescence; it is estimated that 50 per cent of all new cases of first-episode psychosis in Amsterdam are due to cannabis.

Cannabis use in adolescence and young adulthood can have serious effects on a person’s lifetime prospects. The comic stereotype of the lazy stoner is, in reality, no laughing matter: heavy use of cannabis in adolescence can have long-term effects on memory and cognition and can lead to a drop of up to six IQ points.

The cannabis of 2019 is not the cannabis of the Woodstock era. In the 1970s, a typical joint contained about 1-2 per cent tetrahydrocannabinol (THC), the psychoactive compound that gives the user a high. Today’s joints can be 10 times stronger in THC content. Young people are inhaling and ingesting a far more powerful and addictive substance than their parents ever did.

Strange paradox

There is now a strange paradox in society. Our politicians, media and celebrity influencers are increasingly conscious of mental health, particularly among the youth, and that it is a priority area. However, those very same people are likely to view cannabis as harmless or beneficial, despite the growing body of scientific evidence to the contrary.

Colorado now has more 'pot shops' than McDonalds outlet

As former New York Times journalist Alex Berenson wrote in his new book, Tell Your Children: The Truth About Marijuana, Mental Illness and Violence, much of the American public’s shift in perception can be traced back to successful campaigns (which are often funded by organisations with links to the cannabis industry) to legalise cannabis products for a range of “medicinal” uses whose benefits are largely unproven in clinical trials. Once cannabis has been legalised for “medicinal” use, it is then easier to persuade the public and legislators to proceed with full legalisation of cannabis for recreational use.

Advocates of legalisation promise a Shangri-La of moderate and sensible drug use, and an end to the black market and the “war on drugs”. However, the initial results from countries where cannabis restrictions have been lifted do not make for comfortable reading.

Black market

In Canada and the US states where cannabis is legal for recreational use, the black market still thrives. In Colorado, the first US state to legalise for recreational use, cannabis-related presentations to emergency departments have trebled, and the percentage of drivers involved in fatal car crashes who tested positive for cannabis has doubled.

Children are increasingly being seen in emergency departments with accidental overdoses of “edible” cannabis products such as gummy bears and chocolate. There are also emerging reports of deaths in young people from vaping THC products. Colorado now has more “pot shops” than McDonalds outlets.

Few Irish policymakers seem interested in factoring the costs of increased cannabis use into our overburdened health system, or our education, justice and welfare systems.

Experts and medical professionals concerned about cannabis have been, until relatively recently, largely silent in pointing out the harms of cannabis use, perhaps hoping that the scientific and clinical evidence will speak for itself. Unfortunately, that isn’t how it works.

The public and legislators need to hear the other side of the story, free from the influence of cannabis

industry lobbyists. We cannot sell our young people’s mental health for a quick buck.

Mary Cannon is a consultant psychiatrist and professor of psychiatric epidemiology and youth mental health at RCSI.