New Survey: No Clear Evidence Cannabis causes Schizophrenia

  • Dr. Musa Sami is a British researcher at King’s College London
  • Sami claims if cannabis caused psychosis increased rates of use would be reflected in more diagnoses
  • Cannabis potency has increased since the 60s, yet rates of schizophrenia have stayed the same
  • 2% of UK’s population regularly smoke cannabis (over 4 million people)
  • Only 220,000 people are diagnosed with schizophrenia in the UK

Dr Musa Sami, a psychology research fellow at King’s College London, has made claims that there is no direct correlation between using cannabis and developing schizophrenia.

The argument that cannabis can cause users to develop mental illnesses, such as psychosis, is a common one used against legalising cannabis.

Theresa May used the argument during her election campaign, claiming that due to an increase in the potency of cannabis there is a greater risk of developing mental illness:

“What we’ve seen is stronger forms now being used – I think it can have a real impact on people in terms of their mental health.” Dr. Sami, however, is of a different opinion, arguing that if there was a link between using high strength cannabis and developing schizophrenia, we would have seen an increase in the diagnosis of schizophrenia in the UK.

Discussing the results of his research survey, Dr. Sami wrote in The Conversation: “cannabis use is common, while schizophrenia is relatively rare, affecting less than one percent of the population.

“Even if cannabis use were to double the risk, over 98% of cannabis users would not develop schizophrenia.”

While the prevalence of THC in street cannabis has exponentially risen since the 1960s, the level of those diagnosed with schizophrenia in the UK has remained steady:

“Since the 1960s, cannabis potency and rates of use have risen in many Western countries with high-potency strains now dominating the market

“If cannabis were a cause of psychosis, we would expect that, as this increased, rates of schizophrenia would increase alongside it. But this has not happened.”

By surveying cannabis users, Dr. Sami hoped to find an answer to why there was not a correlating increase in those diagnosed with schizophrenia and the increase in number of users and potency of cannabis.

Even if cannabis use were to double the risk, over 98% of cannabis users would not develop schizophrenia.
– Dr. Musa Sami, research fellow at King’s College London

“We asked 1,231 cannabis users about their experiences when they used cannabis and calculated a ‘pleasurable experiences score’ and a ‘psychotic-like experiences score’.

“We then asked the participants if they were continuing to use cannabis, or if they were thinking of quitting in the future.

“Those who reported the most pleasurable experiences continued to use the drug and had no intention of quitting.

“Those with higher psychotic-like experiences had either stopped or were thinking of quitting in the future.

“The experience you have with the drug determines whether you continue to use it or not, regardless of your age, sex, mental health history or other drugs you have used.”

Essentially, psychotic episodes experienced by schizophrenics, induced by cannabis, acts as a warning to the user.

Those who enjoy using cannabis, and have a positive experience, continue to do so; Those who do not have a positive experience on cannabis tend to stop use. The study concluded that:

“Cessation of cannabis use was associated with greater psychotic-like experiences, whilst continued cannabis users were more likely to report pleasurable experiences.

“Whereas former users clearly ascribed cessation to negative experiences, continued users who expressed intention to quit less readily ascribed the intention to negative experiences.”

UNODC data suggests that cannabis is used by 6.2% of the UK, roughly 4,030,000 people.

If cannabis use caused psychosis, then surely there would be a similar number of people diagnosed with schizophrenia as there are cannabis users.

According to a leading schizophrenia charity, Living With Schizophrenia: At any one time, about 220,000 people are being treated for schizophrenia in the UK by the NHS.

The numbers clearly do not add up.

However, there is an argument to be had that causation does not lead to correlation.

For example, there are 9.4 million smokers in the UK, but it is projected that only 62,832 cases of lung cancer (32,875 in males, 29,957 in females) will be diagnosed in the UK in 2035.

Dr. Sami admitted that more research will be needed before any definitive conclusions can be made:

“It is important to remember that, at this stage, this is a hypothesis, not a fact.

“The survey was taken at a single point in time and the online sample we had may be different from the average cannabis user.

“But this group were moderate to heavy users, drawn from activist sites and social media – those that we need to engage the most in this kind of work.”

This recent survey puts yet another nail in the coffin for the argument that cannabis use causes schizophrenia, and, in turn, the argument against legalising cannabis, especially for medicinal purposes.

If cannabis does not cause mental illness, then on what grounds would a Government have for maintaining its prohibition of it?

The best study to confirm the hypothesis would be a long-term study mapping cannabis experiences to schizophrenia risk, drawn from the general population, but this would be a long and expensive study to do.

Dr. Sami and his team of researchers are continuing their investigation into cannabis use and mental illness.

If you want to help their research, you can participate by filling out their questionnaire at:

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