A team of Australian and Dutch researchers (Gillespie et al), writing in the June 2019 issue of the British medical journal The Lancet, criticized a March 2019 study by British researchers (Di Forti et al) in the same journal that suggested variations in cannabis use and potency may be responsible for variations in psychotic disorders. Opponents of cannabis legalization have used the Di Forti study as evidence to support their position.
The criticism rests on the fact that the authors of the March study “assume that cannabis causes psychosis or psychotic symptoms without acknowledging compelling, alternative hypotheses.”
Gillespie and colleagues point out that most studies looking at associations between cannabis and psychosis don’t adjust for “confounding” that arises from correlated genetic and environmental individual differences. They point to their own findings as well as those of other researchers showing cannabis use may be higher among individuals with a genetic liability that predisposes them to both cannabis use and the development of psychotic disorders.
To address the shortcomings of the March study, they specifically point to the results of their recent meta-analysis of the largest genome-wide association study of lifetime cannabis use to date. The study indicated that genetic risk factors for cannabis use and schizophrenia are positively correlated. The meta-analysis applied bidirectional randomization and found a “consistent pattern of evidence supporting a causal effect of schizophrenia risk on lifetime cannabis use.” The study “found little evidence for any causal effect of cannabis use on schizophrenia.” While conceding their analyses were not based on cannabis use frequency or potency but rather genetic risk factors, they felt confident making the following statement:
Nevertheless, our findings strongly suggested that associations between measures of cannabis use and psychosis or psychotic disorders are far more nuanced than Di Forti and colleagues assume. In addition to correlated genetic liabilities, indirect and bidirectional processes are likely to affect the associations between cannabis use, misuse, and psychotic disorders. By not acknowledging the alternative, compelling and plausible mechanisms, Di Forti and colleagues’ conclusion regarding the harmful effect of high-potency cannabis use on mental health is likely to be overestimated.