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Cannabis is linked to schizophrenia in predisposed individuals

Cannabis has long been thought to increase the risk of developing schizophrenia. However, the evidence is still not fully conclusive.

The first study on schizophrenia and cannabis took place in Jamaica in the 1960, when doctors noticed that Rastafarians who regularly smoked cannabis were more prone to psychotic episodes. Research continued in the 1980s when a team at the Institute of Psychiatry in London showed that schizophrenic patients were twice as likely as people without mental illness to regularly take cannabis.

In 1987, a major Swedish study based at the Karolinska Institute suggested that those who took cannabis were six times as likely to be hospitalized for schizophrenia as those who never took it. Although the study involved over 50,000 men, it only illustrated a correlation rather than cause and effect. Many ‘confounding factors’ were ignored, and the possibility was raised that another factor predisposed the men to both schizophrenia and taking cannabis.

The self-medicating theory was also raised – that the men were taking the drug to alleviate their symptoms. This theory became more popular as further research found that schizophrenic patients often report that cannabis makes them feel better. One of the chemical ingredients of cannabis, cannabidiol, has even been investigated as a treatment for schizophrenia.

Nevertheless, research on cannabis as a risk factor continued. Psychiatrists from the University of Amsterdam published data suggesting that schizophrenia patients who used cannabis showed worse outcomes than those who didn’t, and further studies confirmed this observation. Recent work following large groups over time also added to the risk theory – and these were careful to include all relevant factors.

Mary Cannon, of Ireland’s Royal College of Surgeons, took part in a New Zealand-based study which found that smoking cannabis before the age of 15 increased the risk of schizophrenia from 3% to 10% by age 26.

“Cannabis is part of the cause of schizophrenia,” she believes. Her team concluded that there is a minority of teenagers who are particularly vulnerable, and this is backed up by evidence from Greece, in which children with disturbed thought processes at age 11 were at a 25% risk of psychosis if they didn’t go on to take cannabis, and a 50% risk if they did.

In fact, many of these research studies indicate that the risk is higher for teenagers and young adults, probably because the brain is still developing and more susceptible to damage. What’s more, cannabis use among teenagers is rising, and the strength of cannabis is increasing. On the other hand, the rate of schizophrenia has not risen during the last 30 years, in which cannabis use has increased.

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Overall, cannabis is estimated to be responsible for 10% of all schizophrenia cases (approximately 400,000 people in the US). The UK government is currently under pressure to reverse their reclassification of the drug – in 2004 it was downgraded from a class B to a class C drug, meaning people caught with small quantities are not usually arrested.

Robin Murray of the Institute of Psychiatry is now convinced there is a small but significant minority who are predisposed to psychotic illness and should steer clear of cannabis at all costs.

So who is at risk? It seems that for those with a genetic predisposition for schizophrenia, cannabis can spell trouble, but few people have had their genes analysed. The sensible course of action is to look at your family history and avoid cannabis if a close relative has a mental illness including schizophrenia, manic depression, and to a lesser extent, depression or dementia. You are also at higher risk after an injury to the head, a brain tumor, or even a traumatic event.

Avoid cannabis if you personally have a 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. For people with schizophrenia, cannabis may make a relapse more likely and make the symptoms worse. Using cannabis also makes it more likely that an individual will not stick to their treatment plan.

The New Zealand-based researchers conclude that “cannabis use among psychologically vulnerable adolescents should be strongly discouraged by parents, teachers, and health practitioners.”

Cannabis is linked to schizophrenia in predisposed individuals

Jane Collingwood

Jane Collingwood is a longtime regular contributing journalist to Psych Central, focusing on topics of mental health and dissecting recent research findings.

APA Reference
Collingwood, J. (2020). Cannabis is linked to schizophrenia in predisposed individuals. Psych Central. Retrieved on August 12, 2020, from
Scientifically Reviewed
Last updated: 14 Jan 2020 (Originally: 17 May 2016)
Last reviewed: By a member of our scientific advisory board on 14 Jan 2020
Published on Psych All rights reserved.