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.