Onset of drinking, drinking to intoxication, smoking and regular smoking, and use of cannabis and other illicit drugs (e.g., cocaine, heroin, hallucinogens) were assessed in each of Waves 1, 3 and 4 twin interviews. At Wave 5, new (past two-year) onsets of cannabis use were also assessed. Regular smoking was defined as having smoked 100 or more cigarettes lifetime, or between 21-99 cigarettes at least (i) 3-4 days per week for 3 weeks (Waves 1 and 3) or (ii) 1-2 days per week for 2 months (Wave 4). For each substance use variable, youngest reported age of onset was coded; however, youngest and first reports were highly correlated (r>0.90, p<0.05).
