To examine whether the associations for cannabis involvement were attributable to the comorbidity between smoking initiation and ever using cannabis (for rs6265) and between nicotine dependence (FTND; see Rice et al., 2012) and DSM-IV cannabis abuse/dependence (for rs1451240), analyses were repeated while including the tobacco measures as covariates. In both instances, while effect sizes remained consistent (odds-ratios for rs6265 and rs1451240 were 0.85 and 0.88 respectively), the p-values were no longer significant (p-value of 0.07 and 0.15 respectively). Regardless of adjustment for tobacco smoking, rs6265 (adjusted R2=0.003) and rs1451240 (adjusted R2=0.004) explained only modest proportions of variance in cannabis use and abuse/dependence respectively.
