Minimal Relationship Between Cannabis And Schizophrenia Or Psychosis

Last year the UK government reclassified cannabis from a class C to a class B drug, partly out of concerns that cannabis, especially the more potent varieties, may increase the risk of schizophrenia in young people. But the evidence for the relationship between cannabis and schizophrenia or psychosis remains controversial. A new study has determined that it may be necessary to stop thousands of cannabis users in order to prevent a single case of schizophrenia.

Scientists from Bristol, Cambridge and the London School of Hygiene and Tropical Medicine took the latest information on numbers of cannabis users, the risk of developing schizophrenia, and the risk that cannabis use causes schizophrenia to estimate how many cannabis users may need to be stopped to prevent one case of schizophrenia. The study found it would be necessary to stop 2800 heavy cannabis users in young men and over 5000 heavy cannabis users in young women to prevent a single case of schizophrenia. Among light cannabis users, those numbers rise to over 10,000 young men and nearly 30,000 young women to prevent one case of schizophrenia.

That’s just part of the story. Interventions to prevent cannabis use typically do not succeed for every person who is treated. Depending on how effective an intervention is at preventing cannabis use, it would be necessary to treat even higher numbers of users to achieve the thousands of successful results necessary to prevent a very few cases of schizophrenia.

Matt Hickman, one of the authors of the report recently published in the journal Addiction, said that “preventing cannabis use is important for many reasons — including reducing tobacco and drug dependence and improving school performance. But our evidence suggests that focusing on schizophrenia may have been misguided. Our research cannot resolve the question whether cannabis causes schizophrenia, but does show that many people need to give up cannabis in order to have an impact on the number of people with schizophrenia. The likely impact of re-classifying cannabis in the UK on schizophrenia or psychosis incidence is very uncertain.”

Journal reference:

1. Hickman et al. If cannabis caused schizophrenia-how many cannabis users may need to be prevented in order to prevent one case of schizophrenia? England and Wales calculations. Addiction, 2009; 104 (11): 1856 DOI: 10.1111/j.1360-0443.2009.02736.x

– Article from ScienceDaily on October 22, 2009.



  1. Angus Campbell on

    I’ve been looking at a lot of data about schizophrenia. here is government statistics
    just under 1% of the total population develop schizophrenia (0.8%)
    that mean around 1 in every 100 people. thats quite a lot.
    thats 100 in 10,000.. wait a second….
    if only 1 in 10,000 cannabis users get schizophrenia, but 100 out of 10,000 get it from everyone, that means 99 out of 10,000 people who DONT smoke ganja get schizophrenia while only 1 in 10,000 people do…. i think we’ve found a cure for schizophrenia lads!

  2. Anonymous on

    Why doesn’t someone pull up numbers on cases of Schizophrenia in say the 60’s and then compare the increase to say now.
    Then do the same with Cannabis, knowing how Many new users their are we should see and equal increase to the cases of schizophrenia to how many new Cannabis users their are.
    Already you can see their flaw in the reasoning.
    If Cannabis is the cause of Schizophrenia then we should see a huge increase in the number of cases over the years but we do not.
    It’s all propaganda control or at least afraid of losing control. Cannabis Family Users are thinkers and questioners where as drinkers usually are not.

  3. Anonymous on

    because the world is run by drunks not stoners.

  4. Steve on

    What gets to me is that none of these people who get these reports done MENTION the huge link between VIOLENCE and ALCHOHOL, if they did then should’nt booze be outlawed .The cost to society with this is massive compared to a couple of people who can’t handle POT. Why should everyone else be banned because a small % can’t deal with it.

  5. Anonymous on

    This study is suggesting that if 2800 men use cannabis heavily one may have or develop schizophrenia.

    It seems to me they have correlation at best , definitely not causation , and probably just new age reefer madness.

    This person don’t seem to approve of cannabis use but because he is a good scientist he is presenting the facts not what he wants the results to be.

  6. river cat on

    yeah, but giving up cannabis will make you much easier for the goverment to control. They’ll think of any excuse. They are nasty evil coke abusers that dont get jail time(rahim jaffer) yet they give a great man, Marc Emry, the man that should be prime minister, 5 years in a maximum security for selling seeds! SEEDS!!! The world is run by injustice evil doers that will try to keep power by any means. People seemed to already forgotten about Jaffer being caught with with coke. We must not forget! If Marc deservs 5 years in a maximum security prison for seeds, shouldn’t Jaffers crime deserve crucifixtion?

  7. Anonymous on

    sounds like bullshit, so they couldn’t find out if cannabis use causes schizophrenia yet they go on to say that, “Our research cannot resolve the question whether cannabis causes schizophrenia, but does show that many people need to give up cannabis in order to have an impact on the number of people with schizophrenia.” If there is indeed no impact from cannabis smoking on schizophrenia, then less cannabis users will not affect the amount of people with schizophrenia. It’s not like giving up cannabis will reduce cases of schizophrenia.