Cannabis and Schizophrenia: Are We Really Giving Informed Consent?

WRITTEN By Sophie Vreeburg and EDITED BY Anthony Rogers*

Like all New Zealanders who are enrolled to vote, several weeks ago I received an Electoral Commission envelope in the mail. In it was a letter asking me if my enrolment details were correct, a general guideline on voting and finally, two brochures - one on the End of Life Choice Referendum and one on the Cannabis Legalisation and Control Referendum.

For the average New Zealander, a quick flick through these brochures is the extent of their research on the proposed changes under the Cannabis Legalisation and Control Bill. For those with slightly more political interest, this may include engaging with public debate on the pros and cons of allowing the possession and consumption of cannabis.

While such debates have raised key concerns surrounding the health risks of cannabis use, particularly for young people, one fundamental piece of information appears to be strikingly absent from general public knowledge: the link between cannabis and schizophrenia.

Schizophrenia is a form of psychotic illness which affects the way we think. It is characterised by a loss or re-interpretation of reality through delusions, hallucinations, and other bizarre thinking. Episodes of the illness are referred to as psychosis. Tetrahydrocannabinol (THC) is the chemical component which is responsible for the psychological effects a user experiences when consuming cannabis. New Zealand public health data currently displays significant links between THC and schizophrenia, also known as “cannabis-induced psychosis”.  

According to Robin Murray, Professor of psychiatric research at the Institute of Psychiatry at London’s Kings College, studies show that if the risk of schizophrenia for the general population is about one per cent, the risk for those who take ordinary cannabis is two per cent.  Someone who smokes regularly might push this up to four per cent; and if they smoke high strength cannabis every day the risk increases to eight per cent.

Professor Graham Mellsop, a leading New Zealand psychiatrist, has further found that the proportion of patients diagnosed with schizophrenia is significantly higher in instances of prolonged substance versus alcohol abuse. Interestingly, New Zealanders do not generally tend to associate drug use with impaired driving. However, THC also significantly impairs its users’ motor skills. It is the reason why we are instructed not to use drugs and drive. It is also the incentive behind the NZTA’s “The Unsaid” campaign in which people share their experiences of the harm caused by drug impaired driving.

So why are the links between cannabis and schizophrenia not a greater talking point of the referendum?

One of the most significant factors in favour of legalisation circulating public debate at present is the undeniable link between the criminalisation of Māori and cannabis use. Of particular concern is the high proportion of Māori rangatahi (youth) who are convicted of cannabis possession. Studies show that Māori are 1.8 times more likely than non-Māori to face legal consequences as a result of their cannabis use. While the legalisation of cannabis presents a promising response to the racially biased criminalisation of Māori, the high rates of cannabis use amongst Māori also puts them at a significantly increased risk of developing cannabis-induced psychosis or schizophrenia.

It should be noted in particular that since 2019, the decision to prosecute for the possession or consumption of cannabis is discretionary and such a prosecution should not occur unless it is in the public interest. The test for prosecuting drug users now requires consideration of whether a health-centred or therapeutic approach is more beneficial to the public interest. These amendments to the Misuse of Drugs Act have have been described as a form of decriminalisation of cannabis (though concerns remain about the effect of unconscious bias against Māori by police when exercising this discretion).

However, the proposed Bill goes also beyond mere legalization, to commercialisation. As British journalist Patrick Cockburn notes, legalisation incorrectly implies to the general population that our Government believes cannabis must be relatively harmless. Using Professor Murray’s estimates “if you have one hundred thousand teenagers smoking high strength cannabis on a daily basis the risk of schizophrenia is for eight thousand of them”.

The Bill will establish a Cannabis Regulatory Authority who in turn will be able to set limits on THC. However, the strength of cannabis available in New Zealand has increased significantly over the last 30 years, and with it the risk of developing schizophrenia. Brendan Kelly, Professor of psychiatry at Trinity College, Dublin and a consultant psychiatrist at the Tallaght Hospital in Dublin agrees with Professor Murray’s figures but adds:

“…just to be absolutely clear, if you have one hundred thousand teenagers smoking high strength cannabis on a daily basis the risk of schizophrenia is for all of them; i.e. they are all 'at risk'.  Yes, eight thousand of them will actually develop schizophrenia, but the problem is, it is not possible to pick out which eight thousand that will be. So, all are at risk. Cannabis multiplies the risk of schizophrenia eight-fold. That is a huge multiplication of risk.”

Commercial pressure not only spurs widespread accessibility of cannabis but it also pressures producers to develop strains with an ever-higher THC content. In several overseas studies, the risk of developing schizophrenia increased significantly where the cannabis is high potency or “skunk-like”.  The UK in particular has had considerable issues with “skunk-like” cannabis, which naturally contain higher levels of THC. Researchers from King’s College London have found that this high-potency cannabis contains approximately 14% THC. Its consumers are three times more likely to develop a psychotic illness such as schizophrenia than non-users.

In New Zealand, Ministry of Health data further suggests that high potency cannabis use increases the rate of schizophrenia in those genetically predisposed to its onset approximately five or six times compared to non-users. At this stage, the New Zealand Government has suggested cannabis available for commercial sale have an initial maximum potency of 15% THC. Evidently, this presents a significant risk to uninformed users and those who are genetically pre-disposed.

The significance of this medical evidence renders down to one very simple principle;:the right to give informed consent. In accordance with the Cartwright Report principle, the general public who will vote in the upcoming referendum are entitled to be informed of the link between cannabis and schizophrenia and the proportion of users who may develop schizophrenia.

It is evident that at present voters are not adequately informed. Not only is there no reference to the link between cannabis and schizophrenia on the referendum brochures every enrolled New Zealander recently received in the mail, public debates and forums periodically gloss over the health effects of cannabis in favour of decriminalisation and legalisation. At the same time, it is unlikely that many people will read through all 65 pages of the bill to understand the full extent of the legalisation before they vote for their preferred referendum statement. So can we say we are truly giving informed consent?

While displaying the amount of THC is an important means to inform purchasers, this is largely ineffective if users are unaware of the psychiatric effects of particular levels of THC. Should cannabis be legalised come October 17, the Government must acknowledge that cannabis is potentially harmful to a significant proportion of ill-informed New Zealanders and that there is a risk that users may develop cannabis-induced psychosis. Just like cigarette packets carry graphic and written warnings of the effects of smoking, cannabis users should have the right to know that “Cannabis use can increase the risk of schizophrenia”.

*Anthony Rogers is a Senior Barrister and former member of the Auckland District Law Society Mental Health and Disability Committee who specialises criminal law, traffic accident prosecutions, parole hearings and Mental Health Act applications. He has extensive experience working with clients exposed to the psychiatric effects of cannabis consumption.

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