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Many policymakers and theorists believe that marijuana use leads to use of harder drugs. If this supposition is true, efforts to prevent marijuana use and to intervene with marijuana users are crucial. To find evidence to support or refute marijuana's role as a gateway drug, researchers surveyed 311 Australian monozygotic or dizygotic same-sex twins who were discordant for early marijuana use.
Most twins who reported use of other illicit drugs had used marijuana first. Early users of marijuana were more likely than late- and never-users to use other illicit drugs and to meet diagnostic criteria for illicit drug abuse or dependence or for alcohol dependence. In adjusted analyses that considered family, social, and individual factors (includin…