Smoking cannabis as teenager increases risk of depression by 40 per cent, Oxford study finds

Professor Andrea Cipriani, from the University of Oxford, said: "It's a big public health and mental health problem."

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

The study by researchers from McGill University in Canada and the University of Oxford, is the largest meta-analysis in the field.  It included teenagers who had used cannabis at least once before the age of 18 and did not distinguish between the frequency of use.

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.

Animal studies have suggested a link between exposure to cannabinoids, the active component of cannabis, and the onset of depressive symptoms in adulthood.

Prof Cipriani said: “For teenagers there may be an effect which is biological, with some consequences which can be devastating.

“It is a vulnerable period of development of the brain - [so] exposing young teenagers to cannabis is likely to increase the risk of depression.”

The father of two teenagers said parents needed to know the facts about the risks of drug use. He said the evidence did not support having a relaxed attitude towards teenage experimentation.

“My bottom line message is… to avoid using cannabis,” he said.

“I think we should stress the importance of education and prevention programmes.”

The studies were observational, meaning they could not prove that use of the drugs triggered the later depression, researchers said. It it possible that those who were struggling in adolescence were more likely to turn to drugs, they said.

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,” he said.

Fellow researcher Dr Gabriella Gobbi, from McGill University, said teenagers often thought cannabis was safe because it was derived from plants.

“It’s very important to inform adolescents about the risk and about the kinds of cannabis they use,” she said.

“Today, it is not as in the 1980s and 1990s, when THC (tetrahydrocannabinol) was about three per cent in joints. Now we have joints of 10, 20, 30 even more per cent and adolescents must be aware of this.”

She also highlighted the dangers of consuming ‘hash cookies’ and other foods containing drugs.

“Many cannabis today, we have edibles. Adolescents buy cookies with THC and cannabidiol with different percentages. They are not aware that edibles have some risks,” she said.

It comes as the Royal College of Psychiatrists reviews its opposition to the legalisation of cannabis, with politicians including former Tory leader William Hague among those calling for a change in the law.  

Prof Sir Robin Murray, Professor of Psychiatric Research, Institute of Psychiatry Psychology & Neuroscience, King's College London (IoPPN), said the study appeared robust, but had limitations.

He said: “Information about the patterns of cannabis use in the original studies being re-examined is not very detailed; for example they do not quantify the amount of cannabis being smoked or what kind of cannabis is being used -we know from studies of psychosis that the risk is much greater with daily use of modern high potency (high THC/Low CBD) cannabis than old fashioned low THC varieties.”

Dr Joseph Firth, Senior Research Fellow, NICM Health Research Institute at Western Sydney University, said: “Among young adults worldwide, depression is the leading cause of disability, and suicide is the most common cause of death.

"By linking cannabis as a contributing factor to both of these huge issues for public health, this latest study provides new insights into importance of reducing adolescent cannabis use.

“However, it is important to consider that not all cannabis is equal. In particular, high-THC strains of cannabis are typically associated with more severe impact on mental health – whereas another component of cannabis (known as ‘CBD’) may even attenuate some of the adverse effects.”

He called for further research on the varying affects of different types of cannabis use and the impact of policies which aimed to cut drug use.

“If such schemes actually can produce significant reductions in the incidence of mental illness and suicide among young adults, these efforts could lead to population-scale prevention strategies for tackling the severe mental health issues currently affecting many young adults all over the world,” he said.

Dr Lindsey Hines, from the University of Bristol’s Centre for Academic Mental Health said: "This study has taken the results of multiple studies of teenagers published over the past 15 years, and pulled together the results from those studies to get a clear, good quality estimate of the association between using cannabis as a teenager and mental health in adulthood.

“We know that cannabis use co-occurs with anxiety, depression and self-harm in teenagers, but this research suggests that teenage cannabis use is still related to mental health in later years. However, we don't know if cannabis use as a teenager is causing these adult mental health problems. It could be that these behaviours are all due to shared underlying risk factors, such as early adversity or genetics," she said. 

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Cannabis in numbers

• 4 per cent of adolescents aged 11 to 15 years old in England are estimated to have used the drug within the last month.

• 37 per cent increased risk of depression among young adults who had smoked cannabis in their teens

• Tripling in rates of attempted suicide among such adults

• 60,000 cases of depression might be avoided among adults in their twenties and thirties if teenagers avoided all cannabis, researchers say

• 1 million young people likely to take cannabis if it was legalised, estimates suggest

Smoking cannabis at teenage parties ended with depression at university 

Like many teenagers, Liam* started smoking cannabis at parties and at friends’ houses when their parents were out.

At first, the habit he began at 16 was occasional.

But by the time he was 18, he found himself smoking cannabis everyday with his friends, reflecting that he originally started because it “looked cool”.

He said he and his group of grammar school friends then kept it up because it “felt good”.

But it was just a year later - during the Summer before he left his home in Buckinghamshire to go to university in Sheffield - that Liam, a sociable and outgoing teenager, noticed an unfamiliar shift in his mood.

He was feeling increasingly lonely and unproductive, and experiencing low moods that he didn’t understand how to deal with over his three years at university.

“I had no idea what anxiety or even depression were. At the time I didn’t realise how common they were, therefore I didn’t realise it was affecting me,” said Liam, now 24.

“My symptoms also reflected the paranoia I experienced when I was smoking, and I had a friend going through a similar thing,” he added, describing how he now believes his symptoms, which often surfaced at night, were directly related to his use of the drug.

In his mid-twenties now and working in sales, Liam has completely stopped smoking cannabis, having taken a break from it at university - he said he has “pretty much fully recovered, although you always have that doubt.”

Although the salesman, who now lives in London, thanks exercise and good relationships for helping him out of depression, he isn’t shocked by the new findings. “To me, the link is obvious,” he said.

*Liam’s name has been changed at his request

By Laura Fitzpatrick