A twin study of early cannabis use and subsequent use and abuse/dependence of other illicit drugs

Virginia Commonwealth University, Virginia Institute for Psychiatric and Behavioral Genetics, Department of Human Genetics, Richmond, VA 23298-0003, USA.
Psychological Medicine (Impact Factor: 5.94). 11/2004; 34(7):1227-37. DOI: 10.1017/S0033291704002545
Source: PubMed


Cannabis use is strongly associated with the use and abuse/dependence of other illicit drugs. Gateway and common liabilities models have been employed to explain this relationship. We sought to examine this association using a combination of the discordant twin design and modeling methods.
We assess the relationship between early cannabis use and the subsequent use and abuse/ dependence of other illicit drugs in a population-based sample of male and female twin pairs using four analyses: (i) analysis of the association between early cannabis use and other illicit drug use and abuse/dependence in the entire sample of twins, (ii) assessment of the influence of early cannabis use in twin 1 on twin 2's use or abuse/dependence of other illicit drugs, (iii) use of twin pairs discordant for early cannabis use in a discordant twin design and (iv) a model-fitting procedure.
We found: (i) a strong association between early cannabis use and use and abuse/dependence of other illicit drugs in the sample, (ii) twin 1's early cannabis use is significantly associated with the twin 2's other illicit drug use, (iii) the role of correlated genetic factors with some evidence for a causal influence, and (iv) the correlated liabilities model fits the data well.
Early cannabis use is strongly associated with other illicit drug use and abuse/dependence. The relationship arises largely due to correlated genetic and environmental influences with persisting evidence for some causal influences.

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    • "Several sociodemographic and psychiatric variables and indicators of substance use severity predicted progression, including being male, urban residence , being never married, separated or divorced, having a broad range of psychiatric disorders or a family history of SUD, and early onset of cannabis use. Our results, in line with previous findings (Agrawal et al., 2004; Fergusson et al., 2006; Fergusson & Horwood, 2000; Lynskey et al., 2003; Van Gundy & Rebellon, 2010) suggest that a large proportion, but not all, of individuals who use cannabis go on to use other illegal drugs. Moreover, in agreement with the predictions of the GH, a minority of the total NESARC sample reported used other illicit drug before cannabis or only used other illicit drug. "
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