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Strong link between heavy marijuana use and mental illness, Danish study finds

A new study from Denmark has found that heavy use of cannabis is linked to as much as a fifth of schizophrenia cases among young men.

The massive Danish study was published by the Cambridge Journal of Psychological Medicine, and examined 6.9 million people aged 16-49, and more than 45,000 schizophrenia cases dating from 1972 to 2021.

The study, published on 4 May, found that young males may be “particularly susceptible” to the effects of cannabis on schizophrenia. The new research has been described by Scientific American as “likely the largest epidemiological investigation conducted to date that directly focused on the cannabis-psychosis question.”

Researchers found that the adjusted incidence risk ratio for males aged 16 to 20 was more than twice that for females of the same age group, concluding that up to 30 percent of psychosis diagnosis in young men – about 3,000 in total – could have been prevented if those individuals had not used marijuana heavily.

The comparable prevention percentages for the broader age range of 16 to 49 were 15 percent for men – and 4 percent for women.

“At a population level, assuming causality, one-fifth of cases of schizophrenia among young males might be prevented by averting CUD,” researchers wrote.

Authors said that the results highlight the importance of early detection and treatment of CUD and policy decisions regarding cannabis use and access, particularly for 16–25-year-olds.

“Under the assumption of causality, in 2021, approximately 15% of recent cases of schizophrenia among males would have been prevented in the absence of CUD, in contrast to 4% among females,” the authors said. 

“For younger males, the proportion of preventable CUD-associated cases may be as high as 25% or even 30%. This increase in PARF [population attributable risk fraction] is related to both increasing associations, likely caused by more potent cannabis, and increasing the prevalence of CUD with time.”

The Danish epidemiology study comes as marijuana use and potency has risen significantly in Denmark – from 13 percent THC content in 2006 to 30 percent in 2016 – which coincides with an increased rate in schizophrenia diagnosis. 

The study’s lead aythor, associate professor at the Mental Health Services in the Capital Region of Denmark and the University of Copenhagen, Carsten Hjorthøj, commented:

“While this isn’t proving causality, it’s showing that the numbers behave exactly the way they should, under the assumption of causality”.

“We found that the proportion of cases of schizophrenia that were attributable to cannabis use disorder, and those that might have been prevented, was much higher in males than females and, in particular, younger males in whom the brain is still maturing,” Hjorthøj said, adding: “And we saw that this increase was taking place over time, completely in parallel with the increasing potency of cannabis.”

Wilson M. Compton, deputy director of the National Institute on Drug Abuse (NIDA) – which collaborated with the Mental Health Services in the Capital Region of Denmark to design the research – said that the size of the study was significant:

“This is the first time we’ve seen a large-scale study across an entire population that addresses the relationship of cannabis and schizophrenia across different age and sex groups,” he said.

In Ireland, while cannabis is illegal, there are ongoing calls to legalise the drug, and the establishment of a Citizens’ Assembly on Drug Use. The Assembly began examining the issue in April, chaired by former HSE boss Paul Reid, with letters sent to Irish households inviting the public to participate.

In March, Tánaiste Micheál Martin admitted he was “worried” about the impact of legalising cannabis – saying he finds it difficult to comprehend the “habitual” use of drugs by young people.

As reported by the Irish Examiner, Mr Martin said that there is a difference between decriminalisation, which he is in favour of, and “making everything legal”. 

In an interview with the Irish Examiner in New York, Mr Martin said medical personnel have informed him that cannabis products are much more potent now and they believe it is a contributing factor in schizophrenia, particularly if people are using cannabis in their teenage years, the paper reported.

“I’d be very worried if you legalise it. You create a kind of an idea that it’s fine,” said Mr Martin. “But I don’t think we should criminalise people either; there’s a difference between decriminalisation, which I’m in favour of, and making everything legal, but I’m open to the argument.

“I’m open to the debate and that’s why I think this Citizens’ Assembly will be interesting and important that we need to hear all sides here,” he said.

Meanwhile, as reported by The Christian Institute this month, a group of doctors here have accused the Government and the HSE of “failing to counter unrelenting pro-cannabis messaging” in the media.

According to the medics, an estimated 22,000 people are addicted to cannabis in Ireland – with 2020 seeing more than 1,000 hospital admissions due to use of the drug.  The group of medical specialists accused Irish authorities of turning a blind eye to the “huge amount of avoidable misery” caused by cannabis use.

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