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Cannabis is linked to a greater risk of schizophrenia. Why isn’t that part of the legalisation debate?

We’ve heard a lot about the social benefits a law change would bring, but precious little on the mental health risks it could also pose, writes Sophie Vreeburg.

Read a response to this column by psychopharmacologist Suresh Muthukumaraswamy here.

Editor’s note: An earlier version of this article made the claim that cannabis can cause schizophrenia. Cannabis has not been proven to cause schizophrenia; rather, there is evidence of a link between some types of heavy usage and a higher risk of developing schizophrenia. The Spinoff regrets the error.

Like every New Zealander who is 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, some general guidance on voting, and two brochures: one on the End of Life Choice Referendum and the other on the Cannabis Legislation and Control Referendum.

For many, a quick flick through these brochures could be the extent of their research on the proposed changes to cannabis law; others may have delved further into the pros and cons of legalisation on sites such as The Spinoff. Still, no matter how much attention a voter has paid to the issue, it’s likely they’ve seen no mention at all of one incredibly important factor: 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. Research shows a significant connection between schizophrenia and Tetrahydrocannabinol (THC), the chemical component responsible for cannabis’s psychological effects. This form of schizophrenia is also known as cannabis-induced psychosis.

There is no evidence that cannabis can cause schizophrenia. There is, however, significant evidence that, in some situations, heavy use of high potency cannabis can increase the risk of developing schizophrenia.

According to Robin Murray, professor of psychiatric research at the Institute of Psychiatry at London’s Kings College, the risk of schizophrenia increases with the potency of the cannabis used: “if the risk of schizophrenia for the general population is about 1%, the evidence is that, if you take ordinary cannabis, it is 2%; if you smoke regularly you might push it up to 4%; and if you smoke ‘skunk’ (high-strength cannabis) every day you push it up to 8%”. Evidence shows the risk is concentrated in users of high strength cannabis in adolescence; it should be noted here that the proposed law would set an age limit of 20 to use or purchase cannabis.

Auckland University psychiatry professor Graham Mellsop has further found that the proportion of patients diagnosed with schizophrenia is significantly higher in instances of prolonged illicit substance abuse than in prolonged alcohol abuse.

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

One of the most significant factors in favour of legalisation is the criminalisation of Māori, particularly rangatahi (youth), as a result of cannabis possession. Studies show that Māori are 1.8 times more likely than non-Māori to face legal consequences of their cannabis use. While legalisation would reduce the criminalisation of Māori, it should be noted that Māori would still be at a significantly increased risk of developing cannabis-induced psychosis or schizophrenia due to high rates of usage among Māori.

If the referendum passes, the resulting legislation will establish a Cannabis Regulatory Authority that will in turn set limits on THC. We already know that the strength of cannabis available in New Zealand has increased significantly over the last 30 years. Remember those figures from Robin Murray – that 8% of users of high-potency cannabis (or around 8000 out of every 100,000) will develop schizophrenia? In an email to me, Brendan Kelly, professor of psychiatry at Trinity College in Dublin, made an important point: all 100,000 of those users are at risk:

“Yes, 8,000 of them will actually develop schizophrenia, but the problem is, it is not possible to pick out which 8,000 that will be. So, all are at risk. [High strength] 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 can also pressure producers into developing strains with an ever-higher THC content. The UK in particular has had considerable issues with “skunk” cannabis, which naturally contain higher levels of THC. This high-potency cannabis contains approximately 14% THC.

In New Zealand, there is evidence that high potency cannabis use increases the rate of schizophrenia in those genetically predisposed to its onset by approximately five or six times compared to non-users. At this stage, the government has suggested unadulterated cannabis available for commercial sale would have an initial maximum potency of 15% THC – higher than the potency shown to increase the risk of schizophrenia in UK users. Evidently, this presents a significant risk to uninformed users and to those genetically predisposed to schizophrenia.

The significance of this medical evidence can be boiled down to one simple principle: informed consent. Just as those who are being treated in the healthcare system have the right to give informed consent, those who are voting in the referendum are entitled to be informed of the link between cannabis and schizophrenia.

It is evident voters are currently not adequately informed. Not only is there no reference to the link between cannabis and schizophrenia in the referendum material recently mailed to New Zealand voters, the public debate has tended to gloss over the health risks of cannabis in favour of the social benefits of 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 yes or no. So can they say they are truly giving informed consent?

Under the proposed law, a cannabis product label would have to include the amount of THC it contained. While that information is valuable, it’s largely ineffective if users are unaware of the psychiatric effects of particular levels of THC. Should cannabis be legalised as a result of the referendum, the government must acknowledge there is a risk that some users may develop cannabis-induced psychosis, and that the risk is particularly acute for those users who are ill-informed about the effects of THC or who have a genetic predisposition to schizophrenia. Just as cigarette packets carry graphic warnings of the effects of smoking, cannabis users should have the right to know that “cannabis use can increase the risk of schizophrenia”.

Sophie Vreeburg is a fourth year law student at the University of Auckland, the co-director of the Equal Justice Project and a student representative on the Auckland District Law Society Mental Health and Disability Committee. She is passionate about mental health advocacy and issues surrounding access to justice. 

Anthony Rogers assisted in the editing of this article. He is a senior barrister and former member of the Auckland District Law Society Mental Health and Disability Committee who specialises in 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.

Read more: Everything you need to know about the 2020 cannabis referendum



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