Introduction. 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.

Method. 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.

Results. 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.

Conclusions. 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.