It is also worth noting that, for both tobacco smoking and cannabis involvement, SNPs associated with initiation (or lifetime use) were not associated with later stages of CPD or abuse/dependence. Prior twin analyses have suggested substantial overlap across genetic factors influencing initiation and problem use of both tobacco and cannabis (e.g., Gillespie et al., 2009; Neale et al., 2006). Nonetheless, evidence for genetic factors that only influence later stages of tobacco dependence have also been identified (Kendler et al., 1999; Neale et al., 2006) and is consistent with our results and those from the prior smoking meta-analyses (Table 1).
