Cannabis involvement and SI are both influenced by genetic factors to a similar degree (h2=40–60%; Maciejewski et al., 2014; Verweij et al. 2010) with evidence for non-additive genetic influences on SI. However, little is known of the extent to which shared genetic factors contribute to their comorbidity. One study (Lynskey et al., 2004) found that CUD was associated with SI and suicide attempts, even in identical twin pairs who shared 100% of their genetic background. The twin with cannabis dependence was at 2.9 and 2.5 greater odds of SI and suicide attempt relative to their genetically-related nondependent co-twin, suggesting that individual-specific environmental factors that are correlated across cannabis involvement and STB but are not shared by members of a twin pair were important. However, the extent of the genetic and environmental overlap between cannabis involvement and STB was not examined.
