Psychosis five times more likely for cannabis users, study finds


A British study released Monday suggested that the risk of psychosis was five times higher for regular users of cannabis, adding to a growing body of evidence linking drug use and mental health disorders.

The six-year study published in the medical journal The Lancet reported on 780 people living in south London, 410 of whom were being treated for conditions including schizophrenia and bipolar disorder.

The report's lead author was Marta Di Forti from the Institute of Psychiatry, Psychology and Neuroscience at King's College London, who warned about the growing use of "skunk"—a powerful type of cannabis.

"Compared with those who had never tried cannabis, users of high potency skunk-like cannabis had a threefold increase in risk of psychosis," she said.

"The risk to those who use every day was even higher—a fivefold increase compared to people who never use," she added in a statement.

Psychosis is a problem and the symptoms include hallucinations and delusions.

In England, about one new case of psychosis is diagnosed for every 2,000 people every year.

"This paper suggests that we could prevent almost one quarter of cases of psychosis if no-one smoked high potency cannabis," said Robin Murray, professor of psychiatric research at King's College London and a senior researcher for the study.

"This could save young patients a lot of suffering and the NHS (National Health Service) a lot of money," he said.

The study was based on 410 patients who reported psychosis between 2005 and 2011. A further 370 healthy participants from the same area of south London were included for comparison.

Researchers said Monday that it was important for doctors to ask not just about drug use but about frequency of drug use to assess the risk.

"This gives more information about whether the user is at risk of . Awareness needs to increase for this to happen," Di Forti said.

A Home Office spokesman said the findings confirmed the government's hardline approach, adding: "Drugs such as cannabis are illegal because scientific and medical evidence demonstrates they are harmful."

Several major reports over recent years have pointed to a link between cannabis use and psychosis.

In 2010, a survey of 3,800 young adults in Australia found an increased risk of for those who started smoking cannabis at an early age and used it for several years.

Nearly four percent of adults around the world use , according to a paper in The Lancet from 2009, which cited figures from the UN Office on Drugs and Crime.

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Journal information: The Lancet

© 2015 AFP

Citation: Psychosis five times more likely for cannabis users, study finds (2015, February 16) retrieved 22 October 2019 from
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Feb 16, 2015
Also there is a link between hot days and ice cream sales, so ban ice cream !

Feb 16, 2015
Correlation is not causation. Or, to expand upon Kochevnik's point, eating ice cream is associated with an increased risk of death by drowning.

The controls in this study seem fairly flimsy - are the results adjusted for say racial or socio-economic predispositions to psychosis? What about use of other drugs - presumably heavier skunk users are also more likely to use class A's, may be more involved in crime, have compromised employment opportunities, smoke more tobacco, may drink more alcohol, may have poorer sleeping patterns, less healthy diets, less physical exercise and so on.

Do they smoke more because of their pre-existing psychological predispositions, rather than such predispositions being a consequence of smoking?

Speaking as a non-user, i nonetheless regard most anti-drugs research with a great deal of suspicion due to the inherent political bias - our govt's own drugs advisory panel were censured for finding the risks of MDMA were being misrepresented...!

Feb 16, 2015
Even here- no mention of the doses checked. As if it is not something that affects the experience or impact(s), which would make it unlike just about everything else consumed by people. Why not just try offering some honest advice on appropriate dosage, what to look for in determining this and the short and long term effects that might be expected with accidental over-dosing - in addition to the chronic over dosing which is what this research is apparently focussing on.

Feb 17, 2015
Correlation is not causation... Speaking as a non-user, i nonetheless regard most anti-drugs research with a great deal of suspicion due to the inherent political bias...
Spot on, the article's propaganda plain and simple.

There's a very good interview with British journalist Johann Hari on DemocracyNow everyone should see: Everything We Know About the Drug War & Addiction is Wrong.

Also see Ethan Nadelmann (TEDGlobal 2014): Why we need to end the War on Drugs

Feb 17, 2015
What a load of crap! Are these so called scientists in kindergarten? Demonstrating a link between cannabis and psychosis is like demonstrating a link between cars and petrol. But assuming that the link is that cannabis causes psychosis is like assuming that petrol uses a car in some way. How could anyone come to that conclusion? The premise for these delusional studies is that cannabis causes psychosis and they'll spend hours crafting deceptive studies that "prove" this so they can get more grant money for more crap research while the govs and companies that sponsor this hysteria can keep profiting from the corruption they purchase! Cannabis is the best medicine for most mental illness and many people now know the truth. When will the liars realise the game is up?

Feb 17, 2015
@adrian - You're obviously right about the preformed premises. The study's hardly blind since the researchers already knew who the users were, and how much they used. Makes you wonder what kind of checks against such confirmation bias they included, if any...

Feb 17, 2015
The head of the UK's drugs advisory panel i mentioned earlier was David Nutt. His conclusions on the need for (and conspicuous absence of) evidence-based risk assessment were scandalously branded "off message" by the government.

Given that the research here is right-on "message" i think we can be forgiven for taking it with a hefty pinch of salt..

Feb 22, 2015
The most significantly misleading assertion of this article is this:

Psychiatrists and psychologist only see those who may have a suspected psychosis. This might be 1 out of 10,000 people, or 0.01% of the population. If very potent marijuana encourages psychosis by a factor of 5, meaning 5 out of 10,000 people. then that is only 0.04% more of the population. That is an EXCEPTIONALLY small result for a recreational drug.

There are a hell of a LOT of people who shouldn't drink alcohol, and the problem those people would have switching to a higher proof drink is obvious. The marijuana dilemma they are insinuating here is excruciatingly insignificant. Even without the blatantly obvious alcohol comparison, this "problem" barely registers.

Feb 22, 2015
Its the essential oils...

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