David Menkes is alarmed at how the current debate has scarcely mentioned cannabis-induced psychosis. He explains why. 

Writing in last week’s New Zealand Medical Journal, Dr Sam McBride and others used public health and social justice arguments as their basis for endorsing a ‘Yes’ vote in the upcoming cannabis referendum. While many of their points are valid, several key problems remain that should give us pause before supporting the current proposal to legalise recreational cannabis.

As frontline hospital doctors, my colleagues and I are alarmed at how the current debate scarcely mentions cannabis-induced psychosis. This is a prevalent and serious clinical problem, unmissable in acute psychiatric wards around the country, and doubly worrying because of the profound and sometimes irreversible social and occupational disability that can result.

Epidemiological data from New Zealand and elsewhere strongly implicate a causal role for tetrahydrocannabinol (THC) in schizophrenia, an often lifelong psychotic illness. Additional concern arises from the impact of regular cannabis use on brain maturation during adolescence up to the age of 25 or so, associated with impaired cognitive function and educational outcomes.

These problems are exacerbated by commercial pressure, regardless of legal status, for producers to develop strains of cannabis with ever higher THC content. Urban myths notwithstanding, THC drives recreational use as well as reported symptom relief and side-effects of ‘medicinal’ cannabis.

Another cannabis constituent, cannabidiol (CBD), may offset the psychosis-inducing effects of THC to some extent, but the claim that sufficiently high levels of the former relative to the latter could be legally mandated must be regarded as fanciful. Comparably optimistic are the claims that cannabis-associated mental health problems, set to increase with legalisation, can be effectively managed by already stretched mental health services.

McBride and colleagues make no mention whatsoever of lung disease, which has been convincingly linked to cannabis smoking, although most cannabis users also smoke tobacco, making it difficult to separate the effects of cannabis from those of tobacco. Nonetheless, it seems odd to argue from a public health perspective that recreational (and, let’s face it, generally smoked) cannabis should be legalised while we are, as a country, attempting to eliminate tobacco smoking altogether.

From the standpoint of respiratory health, surely it would be better to discourage the inhalation of combustion products of any plant material, particularly when other modes of drug delivery are available, such as ingestion.

If recreational cannabis were to be legalised in NZ, based on overseas experience we can expect rapid commercialisation, increased use in the adult and adolescent populations, followed by an elevated incidence of a variety of health problems, including psychosis and lung disease.

Another problem facing proposed regulation of the cannabis market is that cheap, illicit* imports from Thailand and elsewhere could bypass these regulations altogether, rather limiting the potential benefits of the legislation. Hoped-for benefits to rural NZ would be similarly undermined.

McBride and colleagues argue convincingly that multiple social problems do indeed arise from current cannabis laws, and that these disproportionately harm Māori. They also make a good case that legalisation would help to reduce cannabis-linked gang activity, prosecution, and incarceration. Crucially, however, the authors fail to consider the alternative strategy of decriminalisation, which would achieve many of the social benefits of legalisation, without the attendant risks of harm from anticipated increased use of the drug.

Unfortunately, this referendum asks the wrong question. The current binary choice misses the point that decriminalisation offers a useful compromise between the options on offer. Considering all the evidence, there is a strong case to be made that decriminalisation would do the best job of addressing social justice while protecting public health. Our confidence in the next government’s ability to address these two priorities should thus determine how we regard the current, flawed referendum.

*In an earlier version of this article, it was not clear the author meant: “illicit importation of cheap, high-potency cannabis, an ongoing issue and one likely to continue, driven by market forces, irrespective of the outcome of the referendum”. 

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