1.Why regulating cannabis is unlikely to increase psychosis cases – numerical evidence. A concern frequently raised in discussing the cannabis control bill is the issue of cannabis-use triggering psychosis.

2.Evidence suggests that in those pre-disposed to psychosis cannabis can + the likelihood of developing a psychotic disorder. Some of the factors that increase psychosis triggering are: early onset, strength, frequency + limited amounts of CBD.

3.While this seems bad on the surface it is important to quantify the risk. One important consideration is the fact that the base rate of those predisposed to psychosis is very low in the population.

http://4.To  quantify risk we can calculate the “Number Needed to Prevent” – how many youths we would need to stop smoking cannabis to prevent one case of psychosis. Using UK data (we lack comprehensive NZ base rate data) this has been estimated as

5.1360 heavily using young men and 2480 in heavily using young women. So we would need to stop ~1700 heavy cannabis users to prevent 1 extra psychosis case. Being conservative 1000 more young people starting heavy cannabis = 1 extra psychosis case.

http://6.In  NZ there are ~ 500,000 young people (age 15-24) and it has been estimated about 10% smoke cannabis heavily (~50,000). So, after legalisation it would take a 2% increase in cannabis use in this demographic to develop a single extra psychosis case.

7.This leads to the question – will usage spike? Certainly this is possible, but evidence from Canada (the best international comparison) shows that in youth there was not much change in youth usage patterns after legalisation – in fact a decrease was seen.

8.Rather increases were actually seen in the elderly population. It is also worth noting that the Cannabis Control Bill attempts to mitigate the risk of psychosis development by limiting THC content and having an age-limit of 20.

9.Legalisation is unlikely to change psychosis levels much. In fact, we know Schizophrenia is quite heritable. Arguably, an education-based approach where abstinence is encouraged for at risk people might have better outcomes.

10.Finally, this whole discussion rests on the assumption that as a society we feel that is right to restrict people’s leisure time activities in order to prevent their brain being damaged. But is this true?

http://11.You  might be interested to know that 1% of rugby players in New Zealand will make an ACC claim for concussion/brain injury every year.

12.We are only just beginning to understand the long-term negative effects on brain and cognition of these kinds of sports injuries but initial data suggest the damage can be long-term and significant.

13.References: (because I am a pedantic geek)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3544397/
https://link.springer.com/epdf/10.1007/s40279-019-01176-9
https://www150.statcan.gc.ca/n1/en/pub/82-003-x/2020002/article/00002-eng.pdf