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New study makes strong link between use of potent cannabis and psychosis

A new study published in The Lancet has found how cannabis use affects the incidence of psychosis.

INDIVIDUALS WHO USE cannabis every day are three times more likely to suffer from a psychotic disorder than those who don’t use the drug at all, a new study has revealed.

The study, which examines how cannabis use and strength affects the incidence of psychosis, found that the odds of having the disorder among daily users of the drug were 3.2 times higher than among non-users.

It also found that users of high-potency cannabis were approximately 1.6 times more likely to have a psychotic disorder than those who never used the drug.

The study, published today in The Lancet Psychiatry journal, claims to be the first to show the impact of cannabis use on population rates of psychosis, and highlights the potential public health impact of changes to the legal classification of cannabis.

To carry out the study, researchers compared more than 2,000 people between the ages of 18 and 64 at 11 sites in Europe and one in Brazil between 2010 and 2015, in order to understand the risk factors associated with the disorder.

Of these, 901 patients had presented to mental health services following their first episode of psychosis, while a further 1,237 were studied as “healthy matched controls”.

Comparing the two groups, researchers collected information about participants’ history of the use of cannabis and other recreational drugs.

They used published data on levels of delta-6-tetrahydrocannabinol (THC) – the psychoactive compound in cannabis – to evaluate the potency of cannabis used by participants.

They also separated cannabis used by participants into two categories: cannabis classed as high potency, where its THC content was over 10%, and cannabis classified as low potency (under 10%).

No longer available

The research found that daily cannabis use was more common among patients who had their first episode of psychosis (266 of 901 participants), compared to the “healthy” control group (84 of 1,237 participants).

Meanwhile, high-potency cannabis usage was more likely among patients who had their first episode of psychosis (334 of 901 participants) than those in the “healthy” control group (240 out of 1,237 participants).

The study also discovered that psychosis was five times more likely among those who used high-potency cannabis on a daily basis, and estimated that one in five new cases of psychosis across the 12 sites in the study were linked to daily cannabis use.

In two sites, Amsterdam and London, high potency cannabis was found to be a strong predictor of psychotic disorders.

In Amsterdam, four in 10 (43.8%) new cases of psychosis were estimated to be linked to daily cannabis use, and five in 10 (50.3%) new cases were linked to high potency use.

Meanwhile, rates of psychosis in London were 21% among daily users, and 30.3% among those who used the drug with high rates of THC.

The researchers suggested that if high potency cannabis was no longer available, the incidence of psychosis in Amsterdam would be expected to halve from 37.9 to 18.8 per 100,000 people per year, while in London, it would fall from 45.7 to 31.9 per 100,000 people per year.

At-risk individuals

Dr Marta Di Forti of King’s College London, the lead author of the study, claimed the findings were consistent with other studies linking high cannabis potency with adverse effects on mental health.

“As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties,” she said.

The research follows observational studies which have supported a causal link between cannabis use and psychosis, although none has previously shown that patterns of cannabis use can influence rates of psychosis on a national level.

Commenting on the findings, Dr Suzanne H Gage of the University of Liverpool suggested that the findings indicated that a new approach to treating psychosis among cannabis users was required.

“Given the changing legal status of cannabis across the world, and the associated potential for an increase in use, the next priority is to identify which individuals are at risk from daily potent cannabis use, and to develop educational strategies and interventions to mitigate this,” she said.

People Before Profit TD Gino Kenny suggested that regulating cannabis for recreational use would allow Irish users to consume the drug more carefully.

Kenny’s Cannabis for Medicinal Use Regulation Bill is currently moving through the Dáil, although the government delayed the Bill with a money message last year.

“We need to have a grown-up conversation about this,” he told TheJournal.ie.

“There are forms of unregulated street cannabis that have very high forms of THC, which can be sold by people who are sometimes unscrupulous.

“Regulation would take it out of their hands and allow users to make a choice about the strength of cannabis they use, even if it wouldn’t stop varieties with high levels of THC.”

He added that the findings did not affect his call for the legalisation of cannabis for medicinal use, which contains low levels of THC.

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    Mute mary cooke
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    Oct 6th 2023, 11:27 PM

    Good man Lula, way to go!!!

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    Mute John finn
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    Oct 6th 2023, 11:37 PM

    Just stop buying Brazilian beef…. They will always be deforestation there. Europe imports of Brazilian beef have increased 23 precent 2023. While China has banned it. Well done European Union.

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    Mute Brendan O'Brien
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    Oct 6th 2023, 11:40 PM

    @John finn: Individual countries within the EU, perhaps, which can take the blame. Not the EU per se.

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    Mute David
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    Oct 7th 2023, 3:30 AM

    @Brendan O’Brien: Under the umbrella of the EU and the EU allow it, so yes, it is the EU.

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    Mute ItWasLikeThatWhenIGotHere
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    Oct 7th 2023, 6:15 AM

    @Brendan O’Brien: Individual corporations, within the EU.

    Within the EU which controls trade, and allows it.

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    Mute Timo
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    Oct 7th 2023, 2:48 PM

    @John finn: it must be fairly toxic of China banned it

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    Mute Brendan Godley
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    Oct 6th 2023, 11:32 PM

    Does this mean we can’t import as much Amazon rain forest mulsh to generate our green energy this year

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    Mute Seanfhear míshásta
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    Oct 7th 2023, 2:02 AM

    Well done Lula!

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    Mute Pato
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    Oct 7th 2023, 9:37 AM

    Good man Lula. Stop being distracted by beef imports, a bigger problem is soy imports to Ireland to feed our famous “grass fed” cattle. Deforestation is used to clear space to grow soy.

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    Mute eoin fitzpatrick
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    Oct 7th 2023, 9:43 AM

    @Pato: and our pigs and chickens

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    Mute Steve O'Hara Smith.
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    Oct 7th 2023, 12:11 PM

    Sometimes I think it’s a little hypocritical to complain about deforestation in other countries, after all the only reason we’re not doing the same thing is that here in Europe we ran out of forest to de centuries ago.

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    Mute Timo
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    Oct 7th 2023, 2:45 PM

    Amazing, a desire by government to import beef from Brazil and Argentina who cut down the rainforests to make way for Grazing cattle and growing soy crops.
    Cull 50% of the Herd in Ireland, and the population growing like wildfire. Let’s eat grass
    You will have 1845 back again before you know it.

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    Mute Paul Moloney
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    Oct 7th 2023, 10:30 AM

    Just cut the whole thing down and move on to the next forestry

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    Mute Monetpenny
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    Oct 7th 2023, 10:34 PM

    I have seen reports that as they remove amazon forests they are finding the ruins of ancient cities. This is evidence that there were civilisations previously where there are now forests. There are more forests in these areas than there was previously.
    A north American example is the ancient native American ruin (Cahokia) outside St. Louis in Missouri that dates from approx 1050-1350. At it’s height it was the largest human settlement in the world. The ‘Americas’ were much more ‘civilised’ & settled hundreds of years. These were more recently overrun by forests & the deforestation that is taking place is returning to the state that existed hundreds of years ago.

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