prior to age 17 had significantly elevated rates of illicit drug use and illicit drug dependence, suggesting that the observed associations between early onset cannabis use and later illicit drug use cannot be solely attributed to genetic and shared environmental liabilities (Lynskey et al., 2003). Further, in a replication of this study employing sample of Dutch twins discordant for early onset use, we again found elevated rates of subsequent illicit drug use in individuals who reported early onset cannabis use, relative to their nonearly using co-twin (Lynskey et al., 2006) indicating that the relative tolerance toward marijuana use in the Netherlands did not modify the links between cannabis and other illicit drug use.
