In this instance (Figure 1D), cannabis might be viewed as an environmental trigger of genetic vulnerability to an outcome, say psychotic illness (30; 31). Also referred to as a gene-environment interaction, a few studies have found that a variant in the gene encoding catechol-o-methyltransferase (COMT) is associated with a higher likelihood of psychotic features during adulthood but only if individuals had used cannabis during adolescence (32). As the variant itself did not correlate with the use of cannabis during adolescence, this effect was not due to correlated liabilities nor was the variant alone associated with psychotic illness.
