Cannabis and ‘psychosis’ study in the Lancet Psychiatry Journal

Recently there has been quite a bit of media coverage of the following report in the Lancet Psychiatry Journal: ‘The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study’.

The study found that,

Daily cannabis use was associated with increased odds of psychotic disorder compared with never users…, increasing to nearly five-times increased odds for daily use of high-potency types of cannabis… The PAFs [population attributable fractions] calculated indicated that if high-potency cannabis were no longer available, 12·2% of cases of first-episode psychosis could be prevented across the 11 sites, rising to 30·3% in London and 50·3% in Amsterdam.

This ought not to come as a surprise to anyone, but several aspects of the study are, nevertheless, worth analysing.

At the heart of the study is the risk of ‘high-potency’ cannabis, which the authors define as having a THC concentration of greater than 10%. In ascertaining the potency of the cannabis the participants smoked the authors relied in large part on the apparent objectivity of ‘skunk’.

I wrote recently that ‘skunk’, a slang term for marijuana that contains a high concentration of THC, has no objective meaning, and has, like many slang terms, fallen out of use in the last few years. The Lancet study suggests this is not so.

In the media, at least, ‘skunk’ does appear to have become less trendy. Listed below in chronological order are all the stories on my site that mention the word ‘skunk’: 

• Cannabis abuse blamed for “very serious attack” (15/04/05)
• Cannabis user jailed over attack (06/02/06)
• Life for double killer hooked on cannabis (21/03/07)
• ‘Skunk killed my beloved son’ (26/03/07)
• Skunk addicted schizophrenic fulfils sick fantasy by killing a black woman (03/04/07)
• Son twisted by ‘skunk’ knifed father 23 times (23/07/07)
• Mother’s pleas for help ignored then ‘skunk’ smoker killed friend (02/08/07)
• Mother blames cannabis for suicide of promising violinist daughter (05/09/07)
• ‘I’d like to execute the thugs who kicked my husband to death’, says the grief-stricken widow of Garry Newlove (12/02/08)
• Doctors knew my son’s killer was a mentally ill skunk user (12/02/09)
• Cannabis-smoking father jailed for life after fatal stabbing (27/03/09)
• Cannabis-smoking mother stabbed young sons to death after begging social services to collect her children (28/04/09)
• Cleared Bristol man in cannabis warning over stabbings (11/06/10)
• Cannabis blamed for former Marlow man’s suicide (09/02/12)
• Family of decapitated woman tell of despair as killer cleared of murder (23/06/15)
• Teenager knifed best friend in Newcastle Aldi store following drug-induced psychosis (09/09/17)

As you can see, use of ‘skunk’ peaked in 2007, and has dropped significantly since 2010, perhaps not coincidentally the year Labour, which in 2002 downgraded cannabis from a class C drug to a class B (a decision it would reverse six years later, and which remains in effect), was removed from office after 13 years in power. In some cases it is used in single or double quotation marks, acknowledging a lack of objectivity, but in others it is not. Take, for example, the case of Tom Palmer (‘Life for double killer hooked on cannabis’). One day in 2005, 18-year-old Palmer stabbed to death two friends, aged 16 and 14, while they were walking on a woodland footpath in Berkshire, attacking one of them so savagely that he was nearly decapitated. According to the news report,

Palmer told prison doctors that he had tried, and failed to kick his cannabis habit in the year before the killing.

After trying the drug at the age of 14 he was smoking it daily by the time he was 15.

He was not smoking on the day of the killings but he told doctors he had been using the skunk form of the drug regularly in the preceding weeks. [My emphasis]

This suggests that, despite his young age, Palmer knew, or had a good idea of, the strength of the cannabis he was smoking. How would he know this? From the smell, you might think, given that ‘skunk’ is named after the black and white New World weasel that sprays a foul-smelling liquid from its rear when threatened, but this is not so. Some strains of cannabis smell somewhat differently to others, but this is governed more by terpenes than by THC. None, though, resemble the spray of a skunk, which I know from having spent my adolescence in the USA, where I smelt both skunk spray and cannabis on numerous occasions.

Regarding young Mr Palmer, then, one of the following is true: he was told the cannabis was strong by the person who sold it to him; he had smoked enough cannabis to be able to discern a difference in strength; he grew the cannabis himself and knew that it was strong (unlikely, given his age, and in any case not mentioned in any news report); or he had little, if any, idea, and claimed it was ‘skunk’ in mitigation. If one of the first two is true, the claim that hapless smokers don’t know what they are smoking is false, at least partly so, which leads back to the Lancet study.

The study did not test any cannabis, but instead relied on the participants’ own appraisal of the strength, which again is telling, if inexact: 

‘The high-potency category (THC=>10%) included all the… types reported by the study participants in their original language street names such as: UK home-grown skunk/sensimilla UK Super Skunk, Italian home-grown skunk/sensimilla , Italian Super Skunk, the Dutch Nederwiet, Nederhasj and geimporteerde hasj, the Spanish and French Hashish (from Morocco), Spanish home-grown sensimilla, French home-grown skunk/sensimilla/super-skunk and Brazilian skunk.’

This information, which further suggests that cannabis smokers do know what they are smoking, was supplemented by the following studies of the potency of cannabis in the UK,

• Potency of Δ⁹ -tetrahydrocannabinol and other cannabinoids in cannabis in England in 2016: Implications for public health and pharmacology
• Home Office Cannabis Potency Study 2008

These found that the average potency of seized cannabis in England and Wales is around 15% THC.

Citing this interesting study, ‘Just say ‘know’: how do cannabinoid concentrations influence users’ estimates of cannabis potency and the amount they roll in joints?’, the authors note:

Our findings need to be appraised in the context of limitations. Data on cannabis use are not validated by biological measures, such as urine, blood, or hair samples. However, such measures do not allow testing for use over previous years. Moreover, studies with laboratory data and self-reported information have shown that cannabis users reliably report frequency of use and the type of cannabis used. [My emphasis]

To bring all this together, then. A London resident with mental illness is interviewed for the study. He is asked, amongst other things, what type of cannabis he smokes. If he says ‘skunk’, he is believed, and thus is notched in the ‘high potency’ column. If he simply says ‘cannabis’, he is also notched in the ‘high potency’ column, because he comes from an area where the average THC content of cannabis seized by the police is over 10%.

Of course, the legalisation lobby will say that this is further evidence of the need for legalisation; that only by ‘regulating’ the market can we eliminate these potent strains. The trouble for them is that this has not happened in any country or state that has legalised cannabis. There are several reasons for this, notably the tenacity of the illicit market, and, more importantly, people’s desire for a stronger drug. Furthermore, it is far from certain that ‘low potency’ cannabis is harmless.

In any case, bear these studies in mind next time you hear the pitiful refrain ‘young people don’t know what they’re smoking’.