In this sample of Vietnam-era veteran twins, after controlling for familial influences and measured covariates, there was limited evidence that early-onset use conferred increased risk of other illegal drug use, drug abuse/dependence, and alcohol dependence compared to later-onset use. Although early cannabis use (before age 18) was associated with significantly increased risk for other drug use/abuse/dependence when comparing early-users to later/never-users, the association was dramatically reduced when early-users were compared to later users (excluding never-users). Compared to later/never users, early cannabis users were 2.7-4.1 times more likely to have used other illegal drugs, 1.5-2.1 times more likely to have illegal drug abuse/dependence, and 2.4 times more likely to have alcohol dependence. In contrast, when compared to later-users only (excluding never-users), early cannabis users were 1.3-2.5 times more likely to have tried other illegal drugs, 1.0-1.2 times more likely to have illegal drug abuse/dependence, and 1.3 times more likely to have alcohol dependence. Eight of the nine comparisons between early cannabis users and late/never users were significant, whereas only 3 of those comparisons remained significant when early-users were compared to later-users.
