Risk for initiation of substance use as a function of age of onset of cigarette, alcohol and cannabis use: findings in a Midwestern female twin cohort.

OBJECTIVE: We investigated whether use and early-onset use of cigarettes, alcohol and cannabis contributed an increase in risk for initiation of subsequent psychoactive substances in women (N = 3729, mean age = 21.7).
METHODS: Drugs were ordered in ascending order of initiation: cigarettes, alcohol, cannabis and other illicit drugs, with early-onset use of each drug used as a time-varying covariate to predict initiation of subsequent drugs. Interactions of use and early-onset use, with age of initiation of subsequent drugs, were evaluated using discrete-time Cox survival analyses.
RESULTS: The association between each substance and the early-onset of subsequent drug use was strong (ORs 1.54-19.9). Women who initiated cigarette, alcohol or cannabis use at an early age were at elevated risk for early experimentation with each subsequent drug class. Furthermore, early-onset of more than one substance contributed greater risk for initiation of subsequent drugs.
CONCLUSION: Prevention efforts should target risk factors that contribute to early initiation of cigarette and alcohol use and may subsequently correlate with hard drug involvement.