Results for first alcohol use are shown in Table 4. For all models, parent and cotwin alcohol and cannabis dependence could be equated in Groups 1 and 3, and Group 2 from age 14 onwards. In Model I, offspring of alcoholic or cannabis dependent parents (Group 1) were at 1.43 times higher risk of early drinking, compared to controls (Group 4). Through age 13, offspring of unaffected parents with an alcoholic identical cotwin (Group 2a) were at nearly three times greater risk of early drinking (HR=2.82), with little to no risk observed of offspring of unaffected parents with a cannabis dependent identical cotwin (Group 2b), nor offspring of unaffected parents with an identical cotwin who is either alcohol or cannabis dependent (Group 2c), the latter from age 14 onwards. Risk to Group 3 was also small and nonsignificant. In post-hoc tests, risk to Group 2a was greater than Groups 1, 2b, 2c, and 3, with nonsignificant differences among Groups 1, 2b, 2c and 3.
