More threads by David Baxter PhD

David Baxter PhD

Late Founder
Cannabis / Marijuana (and other street drugs) Have Been Linked to Significant Increases in a Person's Risk for Schizophrenia

Use of street drugs (including LSD, methamphetamine, marijuana/hash/cannabis) have been linked with significantly increased probability of developing 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).
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