of SUD consistently report that substance initiation is significantly influenced by genetic as well as shared and unique environmental factors. This is consistent across populations that initiate tobacco, alcohol, or cannabis use (Agrawal et al., 2010; Huizink et al., 2010). In contrast, additive genetic influences are greater for substance progression; often defined as regular use as well as dependence. There is no longer a significant influence of shared environmental factors for either regular use or dependence in adulthood, although shared environmental influences remain significant during adolescence (Maes et al., 2017;; Rose et al., 2009; Bergen et al., 2007; Hopfer et al., 2003;). Additive genetic influences remain significant for regular use and dependence even when adjusting for genetic influences specific to substance initiation (Maes et al., 2004; Sullivan et al., 2001). Measurement of progression and dependence varies and can reflect the amount of substance used within a specific time frame or symptoms related to SUD diagnosis.
