Researchers say tackling the use of millions of under 18s should be a priority

Smoking cannabis in your teenage years raises the risk of depression and suicide in later life, a landmark new study has found.

Researchers from the US and UK have revealed the drug could impair a child's brain to the extent it triggers mental health disorders later in life.

In the largest research of its kind, experts from Oxford University and McGill University estimated that over half a million adults in the UK and US could be saved from mental health disorders by avoiding the drug as a teenager.

The teams have now warned that cannabis, legal in several US states and used by millions of young people is a significant public health risk with 'devastating consequences'. They have urgently called for officials to make tackling use of the drug a priority.

The link between depression and juvenile cannabis use has in part been attributed to the increased strength of marijuana on the streets today - as opposed to the relatively mild strains available in the 1980s and 1990s,

Smoking cannabis in your teenage years could raise the risk of depression and suicide in later life, the largest study of its kind has found

'It's a big public health and mental health problem, we think,' co-author Professor Andrea Cipriani, from the University of Oxford, said.

'The number of people who are exposed to cannabis, especially in this vulnerable age, is very high and I think this should be a priority for public health and the mental health sector.'

The researchers, at McGill University and the University of Oxford, analysed data from 11 studies involving more than 23,000 individuals.

The study, described as the largest meta-analysis to date in this field, included teenagers who had used cannabis at least once before the age of 18.

About seven per cent of cases of adult depression may possibly not occur if teenagers stopped smoking cannabis, according to the study published in journal JAMA Psychiatry.

This means at any one time up to 60,000 cases among 18 to 34-year-olds in the UK and 400,000 in the US could be attributable to use of the drug during adolescence, they suggest.

However, a link was not found between cannabis exposure and anxiety in adulthood.

WHAT ARE UK AND US LAWS ON CANNABIS? In the UK, cannabis, and any cannabis products containing the psychoactive chemical THC, are illegal. Being caught in possession of marijuana – a class C drug – for recreational use can carry a prison sentence of up to two years and/or an unlimited fine. If you are caught dealing the drug you could face 14 years behind bars. Medicinal cannabis was legalised on November 1, 2018, in the UK but is only available for select medical conditions and must be prescribed by a specialist doctor. CBD oil, although derived from the cannabis plant, is legal because it does not contain the psychoactive chemical THC, which makes people high. In the US, the law on cannabis varies from state to state. It is legal for medical and recreational use in Washington, Oregon, California, Nevada, Alaska, Colorado, Michigan, Vermont, Maine and Massachusetts, according to Business Insider. While the drug is only available for medicinal purposes in Arizona, Utah, New Mexico, Oklahoma, Arkansas, Louisiana, Missouri, Illinois, Minnesota, North Dakota, Montana, Ohio, West Virginia, Pennsylvania, Florida, Maryland, Connecticut, New Hampshire, New York and New Jersey. It is outlawed in the remaining states. Advertisement

While the risk of depression is modest, the researchers said the common use of cannabis among teenagers makes it a concern.

They said it highlighted the importance of educating teenagers about the risks of using cannabis.

Professor Cipriani said: 'Although the size of the negative effects of cannabis can vary between individual adolescents and it is not possible to predict the exact risk for each teenager, the widespread use of cannabis among young generations makes it an important public health issue.

'Regular use during adolescence is associated with lower achievement at school, addiction psychosis and neuropsychological decline, increased risk of motor vehicle crashes, as well as the respiratory problems that are associated with smoking.'

The study did not distinguish between the frequency of use in participants.

Ian Hamilton, a lecturer in mental health and addiction at the University of York, told MailOnline: 'It is difficult to know how much cannabis you would need to use before you develop a problem like depression, what we do know that dose and frequency of cannabis use increase the risk.

'Researchers usually define regular use as more than fifty times using cannabis or more than once in the last month. But it is possible that the increasing strength of cannabis will increase the risk of developing a problem.'

Cannabis is the most commonly-used drug in the UK, with 6.5 per cent of people aged between 16 and 59 taking it in the past year, which makes up around 2.1million individuals.

In England, about four per cent of adolescents aged 11 to 15 years old in England are estimated to have used the drug within the last month.

In the US, 44 percent of those aged 12 or over have used cannabis at some point in their lives.

Mr Hamilton said although some schools provide drugs education, it often has undesirable effects.

He said: 'When this has been investigated by researchers it has been shown to backfire, in that this type of well intended education raises interest in cannabis that wouldn't have happened if the education session hadn't taken place.'

Recreational use of the class B drug can make a user feel relaxed, and even alleviate depression, anxiety and stress, according to scientific studies.

But its use has been linked to disorders including bipolar, schizophrenia, depression, anxiety, and a negative outlook of social interactions.

... and in 'legal' Canada, youth are still abusing drug By Ben Spencer for the Daily Mail The number of young people in Canada using cannabis has not changed since the country legalised the drug in October, despite officials saying the law would 'keep marijuana out of the hands of children and youth'. Canada prime minister Justin Trudeau had promised that the policy would 'protect young Canadians'. He pledged that an end to the chaotic 'war on drugs' would make it easier to control the substance. But as calls grow for Britain to follow Canada's lead and legalise cannabis, figures suggest the legislation is not stopping young people from using the drug. Professor Gabriella Gobbi, of McGill University in Canada, said: 'Just three days ago we had the first data from Statistics Canada. 'It shows adolescents in Canada continue to smoke – and in Quebec we have had an increase of 10 per cent of adolescents that smoke. Legalisation is not enough to prevent young children and adolescents from smoking if we don't have something else – prevention, education.' The figures showed that 27 per cent of Canadians aged 15 to 24 – more than any other age group – used cannabis in the last three months of 2018. Professor Andrea Cipriani, of Oxford University, said: 'One of the arguments in favour of legalisation is to reduce the number of young people who use cannabis. 'This is not happening.' Data from Colorado, which legalised cannabis in 2013, shows use of the drug is the single most common reason for school expulsions. Use among high school students in the state barely changed – from 19.7 per cent in 2013 to 19.4 per cent in 2017. The new laws in Canada gave adults the green light to use cannabis as freely as they use alcohol. But Mr Trudeau said harsh penalties for adults caught selling to minors would stop use among under-19s. 'We have a war on drugs that isn't working,' he said. 'We're having kids access pot easier than they access alcohol. We need to realise prohibition just isn't working.' Advertisement

Grieving mum: This confirms all my fears By Ben Spencer for the Daily Mail Mrs Hamilton had no idea James (pictured) was smoking cannabis until he was sectioned aged 20, A retired teacher whose son's death was caused by mental illness brought on by cannabis said yesterday she felt 'vindicated' by the research. Janie Hamilton, 67, has been convinced the drug can negatively impact the brain since her son James, who was a heavy user in his teens, developed psychosis and depression. 'I welcome this research and I feel vindicated,' she said. 'It confirms what I have known all along – that cannabis does affect young people's mental health. 'It is true that some people will use it and get away with it. But when young people experiment with the drug, they won't know until it's too late whether it will affect them. They are playing Russian roulette with their mental health.' Mrs Hamilton had no idea James was smoking cannabis until he was sectioned aged 20, and later diagnosed with paranoid schizophrenia. He was in and out of mental health wards, suffering with both psychosis and depression, until he developed testicular cancer in his 30s. Because of his psychotic state of mind, he refused life-saving treatment and died at 36. Advertisement

Smoking weed just ONCE could change a teenager's brain

In a recent study scientists revealed just one or two joints could be enough to change the structure of a teenager's brain.

The research, by the University of Vermont, scanned the brains of teenagers from England, Ireland, France and Germany to study marijuana's effects.

They found differences in the volume of grey matter in the amygdala and the hippocampus areas of the organs.

This means the tissue in certain areas is thicker, and it was found to be in the same areas as the receptors which marijuana affects.

These sections are involved with emotions, fear, memory development and spatial skills – changes to them suggests smoking cannabis could affect these faculties.

Experts said thickening of brain tissue is the opposite of what usually happens during puberty, when teenagers' brain matter gets thinner and more refined.

Researchers did scans of teenagers' brains and discovered those who had been exposed to small amounts of marijuana (top row) had thicker regions of the brain (indicated by more orange and yellow tissue) than those who had never smoked cannabis (bottom row)

Scientists said theirs is the first evidence to suggest there are structural brain changes and cognitive effects of just one or two uses of cannabis in young teenagers.

And it suggests as teenagers' brains are still developing, they may be particularly vulnerable to the effects of THC.

Researchers found 14-year-old girls and boys exposed to THC – the psychoactive chemical in cannabis – had a greater volume of grey matter in their brains.

This means the tissue in certain areas is thicker, and it was found to be in the same areas as the receptors which marijuana affects.

Experts said thickening of brain tissue is the opposite of what usually happens during puberty, when teenagers' brain matter gets thinner and more refined.

'Consuming just one or two joints seems to change grey matter volumes in young adolescents,' said study author Professor Dr Hugh Garavan.

'The implication is that this is potentially a consequence of cannabis use. You're changing your brain with just one or two joints.

'Most people would likely assume that one or two joints would have no impact on the brain.'

What changes the increased brain volume directly causes is unclear, but the researchers said it is important to understand cannabis's effects in detail.

This is especially so in the US, where more states are legalising the drug and a view of it being harmless is spreading, they said.

Professor Garavan said cannabis use appears to produce the opposite effect on brain matter of what usually happens during puberty.

He said a typical adolescent brain undergoes a 'pruning' process in which it gets thinner, rather than thicker, as it refines its connections.

'One possibility is they've actually disrupted that pruning process,' he said.