As the debate over legalising medical cannabis burns on, the arguments for potential benefits need to be weighed against the arguments for potential risks, particularly to mental health and society's view of the drug generally.
The use of cannabis as a medicine has a long tradition with a wide variety of ailments identified as potentially being ameliorated by it. There is also growing interest in its use for largely neuropsychiatric conditions, such as spasticity and seizures.
Although there may be benefits in the use of cannabis and cannabis-based products, there is also a potential negative impact on mental health, both in terms of individual use and how society sees cannabis more generally. This is important as cannabis is widely used in New Zealand, despite our current prohibitionist stance.
Cannabis is generally used for its pleasurable psychoactive effects, although there is a clear overlap between those who report use for pleasure and for "medical" purposes.
Use in New Zealand is high by international standards and it is interesting to see that in the United States, where legislation on legal use of cannabis varies by state, there appears to be increasing rates of addiction problems where cannabis is legally allowed as medicine.
The link between legislation of cannabis for medical uses and a growth in drug use disorder is great.
With increasing use of cannabis in the community, the mental health problems directly related to use are likely to increase. These are well documented and include psychosis, anxiety disorders, cognitive problems and as mentioned above, addiction.
Psychotic mental disorder, like schizophrenia, is a significant mental health and public health concern and the links with cannabis and psychosis are clearest for use in youth.
Any potential for the use of cannabis as a medicine in youth and early adulthood is therefore risky.
Bearing in mind the considerable pressure for the use of cannabis as an analgesic, this poses a major challenge. It would not be difficult to see clinical scenarios where young patients, with pain disorders, insist on a trial of cannabis irrespective of the risk.
The link between cannabis and anxiety disorders is also most pronounced in teenagers.
The difficulty with cognitive problems are more complex to disentangle but may relate to a lower life satisfaction that is found with chronic cannabis use.
A further social challenge associated with cannabis as a medicine is the potential change in acceptability for use and how this may impact on mental health issues. Increasing cannabis use has been considered likely to be associated with increasingly liberal policy.
As cannabis dons the jacket of medical acceptability, even if it is licensed only for use in specific conditions and only in later adulthood, this is likely to increase use and reduce the perceived risks in all age groups.
This problem is compounded in those with severe mental illness, for whom cannabis significantly worsens an outcome. Despite these documented poor outcomes, cannabis use in this group is prevalent and identifying cannabis as a medicine opens the door to this group arguing cannabis is "my medicine".
Particularly in conditions like schizophrenia, where the correlations between cannabis, onset and functional impairment are clear, patients may prefer the short term benefits of a "high", despite the long term difficulties.
This raises the potential for psychiatrists and other physicians to become gatekeepers for cannabis use, trying to weigh the harm minimisation of prescribing cannabis to mental health patients to ensure a safe and known product as opposed to their patients sourcing illicit cannabis.
This appears similar to the use of methadone for opioid dependence, but no clear policy has been considered around this.
Medical practitioners are likely to be those who are left facing these dilemmas as Parliament marches towards opening the door to medical cannabis.
• Associate Professor Giles Newton-Howes is in the University of Otago's Wellington department of psychological medicine.