Despite evidence for genetic overlap, few studies have examined whether genetic variants implicated for one substance also influence the other. Even though cannabis and tobacco have unique pharmacological profiles, it is possible that pleiotropic effects may be responsible for shared aspects of their use and misuse, such as experience of reward or attenuation of negative mood states. One approach for identifying such genetic variants is to focus on those that have been repeatedly identified in meta-analyses of genomewide association studies (GWAS; Duncan and Keller, 2011). As GWAS simultaneously examines a million or more single nucleotide polymorphisms (SNPs), in these analyses only SNPs with p-values of 5 × 10−8 are considered statistically significant. Consequently, the identification of such SNPs typically necessitates large sample sizes. While there have not been such meta-analyses for cannabis involvement, three large meta-analyses of cigarette smoking have identified such significant SNPs (Liu et al., 2010; Thorgeirsson et al., 2010; Tobacco and Genetics Consortium, 2010). In the present study, we selected 11 genomewide significant SNPs associated with CPD, smoking initiation and smoking cessation/current smoking and examined: (a) whether
