Cannabis has not shown “any evidence of increasing schizophrenia” in the UK

One of my common one-liners in response to the claim across the pond that smoking the dreaded “skunk” will lead to psychosis and schizophrenia is to sarcastically say, “Yes, that’s why there was such a spike in schizophrenia around 1979 in the US… oh, no, wait, there wasn’t; schizophrenia remains a relatively stable phenomenon that affects less than 1% of the population worldwide.”

Looks like some scientists in the United Kingdom decided to look for just such a correlation:

Assessing the impact of cannabis use on trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005

A recent systematic review concluded that cannabis use increases risk of psychotic outcomes independently of confounding and transient intoxication effects. Furthermore, a model of the association between cannabis use and schizophrenia indicated that the incidence and prevalence of schizophrenia would increase from 1990 onwards.

The model is based on three factors:

• a) increased relative risk of psychotic outcomes for frequent cannabis users compared to those who have never used cannabis between 1.8 and 3.1,
• b) a substantial rise in UK cannabis use from the mid-1970s and
• c) elevated risk of 20 years from first use of cannabis.

This paper investigates whether this has occurred in the UK by examining trends in the annual prevalence and incidence of schizophrenia and psychoses, as measured by diagnosed cases from 1996 to 2005. Retrospective analysis of the General Practice Research Database (GPRD) was conducted for 183 practices in England, Wales, Scotland and Northern Ireland. The study cohort comprised almost 600,000 patients each year, representing approximately 2.3% of the UK population aged 16 to 44.

Between 1996 and 2005 the incidence and prevalence of schizophrenia and psychoses were either stable or declining. Explanations other than a genuine stability or decline were considered, but appeared less plausible. In conclusion, this study did not find any evidence of increasing schizophrenia or psychoses in the general population from 1996 to 2005.

Ah, but the 1970s cannabis, which would cause the spike 20 years later in the 1990s, was just that 1% to 2% THC that the hippies smoked!  It’s not the 3000% stronger “skunk” of today, which will cause schizophrenia and psychoses to manifest sometime in 2025!  Just you wait!

It’s bull, of course.  1970s cannabis was every bit as strong as what you’ll find today.  Those who were consuming cannabis regularly – the ones you’d expect to “go schizo” – were always finding or growing the good stuff.  This “Woodstock Weed” idea of low-THC joints is what the casual smoker with no connections would smoke, and not very often.