Table 1 shows the prevalence of drug use for all respondents and broken down into individuals from pairs concordant for early cannabis use, the early-users from discordant pairs, the later/never-users from discordant pairs, and individuals from pairs concordant for not using cannabis early. The percentages are consistent with a model in which level of early exposure to cannabis is a risk factor for other substance use: looking across the types of pairs, those pairs concordant for cannabis use before age 18 were the most likely to have tried other drugs, the early-users from the discordant pairs had slightly lower other drug use rates, the discordant cotwins of the early-users were next, and the individuals from pairs concordant for not using cannabis early were least likely to have tried other drugs. All four groups differed from each other significantly. That 65% of the non early-using twins from discordant twin pairs did use cannabis later and only 38% of those individuals in pairs concordant for not using cannabis early tried cannabis later suggests some familial influence, either genetic or environmental. For regular alcohol use, the members of discordant pairs did not differ from each other, but all other group differences were significant.
