A common argument in favour of legalising cannabis is that alcohol is legal and is more harmful to people and society than cannabis, and therefore cannabis should be legal too.
This is a somewhat spurious argument along the lines of “Would you rather be eaten by a lion or a bear” or “Would you rather be run over by a truck or a bus”. Nevertheless, it provides a good opportunity to examine the differential harms posed by both substances.
A recent article by a Professor of Psychiatry and Addiction Medicine (NZ Herald, July 28) stated, “both cannabis and alcohol are known to cause psychotic conditions, but in both instances, these are rare events.”
This statement is true in one regard. Psychotic disorder related solely to alcohol is indeed a rare event. It is generally in the form of an uncommon condition called Alcoholic Hallucinosis, which occurs in older individuals who have been using alcohol heavily for many years. I have seen only one or two cases in my clinical career.
Individuals who are withdrawing from alcohol can suffer with hallucinations and acute paranoia as part of the withdrawal reaction but this is not a psychotic disorder. Cannabis, on the other hand, is strongly associated with psychotic symptoms and psychotic disorders such as schizophrenia.
In fact, cannabis use is now the most powerful single environmental risk factor for psychotic disorder.
Many well-designed studies have examined this association, and the majority have shown that cannabis is significantly related to psychotic disorder with the remainder showing a strong trend in that direction.
In the early 2000s, I was involved in a research team investigating the link between cannabis use in adolescence and the risk of psychotic disorder in adulthood. This research was carried out on a group of approximately 1000 young people from Dunedin, who had been followed up since the 1970s (and are still being followed up to this day).
We found that young people who started using cannabis before age 15 had had a four times increased risk of being diagnosed with schizophrenia or schizophreniform disorder by age 26. To put this in perspective, 10 per cent of the young people who had been using cannabis by age 15 developed a psychotic disorder in young adulthood compared with 3 per cent of the remainder of the group – a one in 10 chance is certainly not a rare occurrence.
The association between cannabis and psychosis appears to be getting even stronger in line with the increase in strength of cannabis (the THC content is now regularly over 20 per cent, whereas it was only about 1-2 per cent in the 1960s and 70s.)
Recent studies from Europe have examined the risks associated with high-potency cannabis (defined as greater than 10 per cent THC) and have found that…