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Marijuana / Cannabis and Schizophreni

Marijuana / Cannabis and Schizophrenia

  • Overview
  • Internet Video Report on Cannabis and Schizophrenia
  • Helpful Actions
  • Supporting Scientific Evidence
  • Most Recent Scientific Research on Schizophrenia and Cannabis Link
  • Books on Marijuana / Cannabis Link to Schizophrenia

Overview: Use of street drugs (including LSD,methamphetamine,marijuana/hash/cannabis) and alcohol have been linked with significantly increased probability of developing psychosis and schizophrenia. This link has been documented in over 30 different scientific studies (studies done mostly in the UK, Australia and Sweden) over the past 20 years. In one example, a study interviewed 50,000 members of the Swedish Army about their drug consumption and followed up with them later in life. Those who were heavy consumers of cannabis at age 18 were over 600% more likely to be diagnosed with schizophrenia over the next 15 years than those did not take it. (see diagram below).Experts estimate that between 8% and 13% of all schizophrenia cases are linked to marijuna / cannabis use during teen years.

Many of these research studies indicate that the risk is higher when the drugs are used by people under the age of 21, a time when the human brain is developing rapidly and is particularly vulnerable.

People with any biological predisposition towards schizophrenia are at the highest risk — unfortunately its impossible to accurately identify this predisposition beforehand ( a family history of mental illness is just one indicator of such a predisposition). [see causes and prevention of schizophrenia for more information on all risk factors linked to a person developing schizophrenia]

Researchers in New Zealand found that those who used cannabis by the age of 15 were more than three times (300%) more likely to develop illnesses such as schizophrenia. Other research has backed this up, showing that cannabis use increases the risk of psychosis by up to 700% for heavy users, and that the risk increases in proportion to the amount of cannabis used (smoked or consumed). Additionally, the younger a person smokes/uses cannabis, the higher the risk for schizophrenia, and the worse the schizophrenia is when the person does develop it. Research by psychiatrists in inner-city areas speak of cannabis being a factor in up to 80 percent of schizophrenia cases.

Professor Robin Murray (London Institute of Psychiatry) has recently (2005) completed a 15-year study of more than 750 adolescents in conjunction with colleagues at King’s College London and the University of Otago in New Zealand.

Overall people were 4.5 times more likely to be schizophrenic at 26 if they were regular cannabis smokers at 15, compared to 1.65 times for those who did not report regular use until age 18.

Many researchers now believe that using the drug while the brain is still developing boosts levels of the chemical dopamine in the brain, which can directly lead to schizophrenia.

Professor John Henry, clinical toxicologist at Imperial College London said research has shown that people with a certain genetic makeup who use the drug face a ten times (1000%) higher risk of schizophrenia. (for example – if your risk of schizophrenia was 6% (due to a family history of mental illness) prior to taking cannabis, it could be 60% — or more likely than not – after taking cannabis). Every person is different (i.e. has different genes and different environments) – so this “10 Times Higher Risk with cannabis use”- is just a generalization, and it may or may not apply to a given person.


The increased risk applies to people who inherit variants of a gene named COMT and who smoked cannabis as teenagers. About a quarter of the population have this genetic make-up and up to 15 per cent of the group are likely to develop psychotic conditions if exposed to the drug early in life. Neither the drug nor the gene raises the risk of psychosis by itself.

A recent Dutch study showed that teenagers who indulge in cannabis as few as five times in their life significantly increase their risk of psychotic symptoms.

The increase in evidence during the past decade could be tied to the increased potency of marijuana. A review by the British Lung Association says that the cannabis available on the streets today is 15 times more powerful than the joints being smoked three decades ago.

Schizophrenia can sometimes be triggered by heavy use of hallucinogenic drugs, especially LSD; but it appears that one has to have a genetic predisposition towards developing schizophrenia for this to occur. There is also some evidence suggesting that people suffering from schizophrenia but responding to treatment can have an episode as a result of use of LSD. Methamphetamine and PCP also mimic the symptoms of schizophrenia, and can trigger ongoing symptoms of schizophrenia in those who are vulnerable.

Melbourne University’s Professor David Castle stated in a February, 2005 interview that heavy drug use during formative times of life, such as the years at school, could affect the way a teenager or young adult thought, impairing cognitive ability and having a long-term impact on job prospects. Victorian studies had revealed that regular use of cannabis by adolescent girls could trigger long-term depression. And for those vulnerable to a psychotic disorder, even a small amount of cannabis could pose a threat.

Professor Castle, author of the book Marijuana and Madness, has said that those people with this “psychotic proneness” were those who had a family history of mental illness or who had had a bad response on their first use of cannabis or to a tiny amount. Others at risk included those who had experienced a psychotic episode where they had paranoid thinking or heard a voice calling their name. Professor Castle said experiencing such a one-off episode was far more common than people thought.

“People with such a vulnerability should avoid cannabis like the plague,” he said.

Without the effects of the drug, such a person might live their whole life without ever experiencing mental health problems. It has been estimated, for example, that between 8% and 13% of people that have schizophrenia today would never have developed the illness without exposure to cannabis.

Professor Castle compared the effect to feeding sweets to a diabetic. While high sugar content foods did not cause too many problems for most people in the short term, they could be catastrophic for diabetics.

He said there was an accumulative effect when it came to cannabis use and schizophrenia. Those who used the drug more than once a week were more prone to needing hospitalisation and often suffered other associated problems such as the breakdown of relations with their family, isolation, crime and violence.

* Cannabis impacts on neurotransmitters that regulate how arousal and stress are managed in the brain. Cannabis takes a long time to metabolise, and can quickly build up to high levels in the body. Once you get to this point, there is a real risk of depression or schizophrenia being triggered.

* A Swedish study of 50,000 military conscripts found heavy use of cannabis increased the risk of suicide by four times (400%). A Victorian study of 2332 adolescents found weekly use increased the risk of suicide attempts among females by five times. Weekly use as a teenager doubled the risk of depression and anxiety. Daily use at the age of 20 boosted the risk of depression and anxiety by five times (500%).

Helpful Actions: If you want to avoid getting schizophrenia – research suggests that the number one thing you should avoid are street drugs (especially marijuana/cannabis – but because you never know what someone has put into a street drug, all of them are dangerous). By avoiding use of all street drugs research suggests that you can greatly reduce the chance (by as much as 50% to 80% if you are biologically predisposed) that you’ll develop schizophrenia. Avoiding marijuana after developing schizophrenia also helps reduce relapse rates. Some people with schizophrenia suggest that it makes them feel better, but if depression is an issue we recommend these people talk to their Psych-Doc about possible anti-depressant use rather than street drugs.

Do not use even small amounts of cannabis if you have any family history of mental illness, have had an episode of paranoid thinking or hearing voices or had a bad response when first using cannabis or when using a small amount.

Other street drugs are also very dangerous – partly because they are produced in home laboratories with virtually any possible combination of additional substances mixed in with the drugs. See:Crystal Meth & Schizophrenia

Schizophrenia and Cannabis Video Report:

A recent Internet video report on schizophrenia and cannabis has recently become available. To play the video go to the following link – and then click on “Play” button to view any of the 6 different sections of the video report: Messing with Heads: New Research into the longterm effects of Cannabis (Internet Video, 2005) from the Australian Broadcasting Company (ABC).

Supporting News (a sample):

  • Recent news on Marijuana / cannabis and schizophrenia (from schizophrenia.com newsblog)
  • Marijuana Doubles Risk of Schizophrenia – March, 2005
  • Interview with Dr. Andrew Campbell on Schizophrenia and Cannabis – Feb, 2005
  • 25% of cannabis users faces a ten-fold higher risk of mental illness – Jan, 2005
  • True Story of Cannabis-Induced Schizophrenia – January, 2005
  • Marijuana and Psychosis Link – December, 2004
  • Another Study Links Marijuana to Schizophrenia – News, April 2004
  • Psychotic Symptoms More Likely with Cannabis – New Scientist, 2004
  • The Link between Cannabis and Psychosis – Robin Murray, MD
  • Cannabis link to mental illness strengthened – New Scientist Magazine
  • If cannabis is safe, why am I psychotic? – Times Online
  • My Son and Cannabis – a anecdotal story from a parent convinced that his son’s use of cannabis caused the development of schizophrenia. BBC News, June 2005
  • Drug Abuse and Risk of Developing Schizophrenia – (News)
  • Marijuana and Schizophrenia – (News)
  • Cannabis mental health risks ‘must be taught’ – Guardian Newspaper
  • Powerpoint Presentation “Canabis: The Facts” by British Toxicology Society
  • Review shows that cannabis use is a risk factor for schizophrenia
  • Psychiatrists say “No” to Marijuana
  • The National Institute on Drug Abuse Adds Warning of Major Depressive Disorders (MDD’s) Experienced by Marijuana Users
  • Causal association between cannabis and psychosis: examination of the evidence. (British Journal of Psychiatry, 2004)
  • Cannabis use as a probable causative factor in the later development of schizophrenia
  • Cannabis and neurological soft signs in schizophrenia: absence of relationship and influence on psychopathology
  • Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study
  • Cannabis and schizophrenia: impact on onset, course, psychopathology and outcomes
  • Marijuana Mental Disturbances

Most Recent Scientific Research on Schizophrenia and Cannabis:

Cannabis-induced psychosis and subsequent schizophrenia-spectrum disorders: follow-up study of 535 incident cases. Br J Psychiatry. 2005 Dec;187:510-5

Toward a world consensus on prevention of schizophrenia.

Dialogues Clin Neurosci. 2005;7(1):53-67.

Risk for schizophrenia–broadening the concepts, pushing back the boundaries.

Schizophr Res. 2005 Nov 1;79(1):5-13.

The environment and schizophrenia: the role of cannabis use.

Schizophr Bull. 2005 Jul;31(3):608-12. Epub 2005 Jun 23.

Predictors of schizophrenia–a review.

Br Med Bull. 2005 Jun 9;73:1-15. Print 2005.

Cannabis as a risk factor for psychosis: systematic review.

J Psychopharmacol. 2005 Mar;19(2):187-94.

Cannabis use prior to first onset psychosis predicts spared neurocognition at 10-year follow-up.

Schizophr Res. 2005 Jun 1;75(1):135-7.

[Acute and chronic cognitive disorders caused by cannabis use]

Rev Prat. 2005 Jan 15;55(1):23-6; discussion 27-9. French.

Delta-9-tetrahydrocannabinol effects in schizophrenia: implications for cognition, psychosis, and addiction.

Biol Psychiatry. 2005 Mar 15;57(6):594-608.

Cannabis use and psychotic disorders: an update.

Drug Alcohol Rev. 2004 Dec;23(4):433-43. Review.

Is cannabis an anti-antipsychotic? The experience in psychiatric intensive care.

Hum Psychopharmacol. 2005 Apr;20(3):207-10.

Cannabis and risk of psychosis.

Swiss Med Wkly. 2004 Nov 13;134(45-46):659-63. Review.

[Cannabis can double the risk of schizophrenia. Increasing but still controversial knowledge of the psychological effects of the drug]

Lakartidningen. 2004 Oct 7;101(41):3126-7. Swedish.

Is the party over? Cannabis and juvenile psychiatric disorder: the past 10 years.

J Am Acad Child Adolesc Psychiatry. 2004 Oct;43(10):1194-205. Review.

Adverse effects of cannabis on health: an update of the literature since 1996.

Prog Neuropsychopharmacol Biol Psychiatry. 2004 Aug;28(5):849-63. Review.

Cannabis use and risk of psychosis: an etiological link?

Epidemiol Psichiatr Soc. 2004 Apr-Jun;13(2):113-9. Review.

[Cannabis and schizophrenia. From euphoria to psychosis]

Rev Med Liege. 2004 Feb;59(2):98-103. French.

Heavy cannabis users seeking treatment- prevalence of psychiatric disorders.

Soc Psychiatry Psychiatr Epidemiol. 2004 Feb;39(2):97-105.

Cannabis use and the risk of later schizophrenia: a review.

Addiction. 2004 Apr;99(4):425-30. Review.

Cannabis use and psychosis.Drug Alcohol Rev. 1998 Dec;17(4):433-44.

Cannabis use and age at onset of schizophrenia.

Am J Psychiatry. 2004 Mar;161(3):501-6.

Causal association between cannabis and psychosis: examination of the evidence.

Br J Psychiatry. 2004 Feb;184:110-7. Review.

[Cannabis use as a probable causative factor in the later development of schizophrenia]

Ned Tijdschr Geneeskd. 2003 Nov 1;147(44):2178-83. Dutch.

[Use of cannabis in adolescence and risk of schizophrenia]

Rev Bras Psiquiatr. 2003 Sep;25(3):131-2. Portuguese.

Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study.

BMJ. 2002 Nov 23;325(7374):1212-3. No abstract available.

[Is there a temporal correlation between substance abuse and psychosis in adolescents?]

Z Kinder Jugendpsychiatr Psychother. 2002 May;30(2):97-103. German.

Cannabis and psychosis.

Curr Psychiatry Rep. 2002 Jun;4(3):191-6. Review.

Cannabis-induced psychosis: a cross-sectional comparison with acute schizophrenia.

Acta Psychiatr Scand. 2002 Mar;105(3):173-8.

Cannabis and schizophrenia: a longitudinal study of cases treated in Stockholm County

Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study

For more science studies on this topic, go to these Search Engine Results:

Google Scholar search on “Schizophrenia cannabis” or

“schizophrenia and marijuana”

Books on the Schizophrenia and Marijuana (Cannabis) link:

Marijuana and Madness; Psychiatry and Neurobiology by David Castle and Robin Murray, Published by Cambridge University Press. 2005


This book is available for purchase from Amazon.com here.Amazon: Marijuana and Madness

Learn more about the book: Book Review of “Marijuana and Madness