As a secondary analysis, polygenic risk score for schizophrenia risk alleles with p<0.01 (the threshold with the greatest association in the primary analysis) were examined within 990 twin pairs (608 dizygotic and 382 monozygotic) where data on cannabis use of both twins was available. Taking the mean polygenic risk score within each twin pair, an ordinal regression was performed to predict whether neither (n=272), one (n=273), or both twins (n=445) were cannabis users. After correcting for age, sex, and zygosity, a significant association was observed (p=0.001). Those twin pairs where both reported using cannabis had the greatest burden of schizophrenia risk alleles, pairs with only one user were found to have an intermediate level, and the lowest burden was found in pairs where neither twin reported use (see Figure 2).
