Reverse gateways? Frequent cannabis use as a predictor of tobacco initiation and nicotine dependence

Aims To examine the risk posed by cannabis use in young people for tobacco use disorders. Specifically we examined whether cannabis use in non-smokers predicted later initiation of tobacco use and whether cannabis use predicted later nicotine dependence in tobacco users. Design A 10-year eight-wave cohort study. Setting State of Victoria, Australia. Participants A community sample of 1943 participants initially aged 14-15 years. Measuerments Self-report of tobacco and cannabis use was assessed in the teens using a computerized interview assessment and in young adulthood with a CATI assessment. The Fagerstrom Test for Nicotine Dependence was used to define nicotine dependence. Findings For teen non-smokers, at least one report of weekly cannabis use in the teens predicted a more than eightfold increase in the odds of later initiation of tobacco use (OR 8.3; 95% CI 1.9-36). For 21-year-old smokers, not yet nicotine-dependent, daily cannabis use raised the odds of nicotine dependence at the age of 24 years more than threefold (OR 3.6, 1.2, 10) after controlling for possible confounders, including level of tobacco use and subsyndromal signs of nicotine dependence. Conclusions Weekly or more cannabis use during the teens and young adulthood is associated with an increased risk of late initiation of tobacco use and progression to nicotine dependence. If this effect is causal, it may be that a heightened risk of nicotine dependence is the most important health consequence of early frequent cannabis use.