Transitions between tobacco and cannabis use among adolescents: a multi-state modeling of progression from onset to daily use.

Use of a given substance may follow a stage process leading from onset to regular use, and use of one substance can be strongly associated with use of another. The aim of this study was to describe the transitions between tobacco and cannabis use. Data was derived from a French nationwide survey involving 29,393 teenagers. A homogenous Markov multi-state model (MSM) was fitted. The substance use pattern modeled was: no lifetime use→1 (2) substance(s) initiation→1 (2) daily substance(s) use, with pathways between tobacco and cannabis. The likelihood of first initiating tobacco appeared 17.6 times greater than the likelihood of initiating cannabis. Once a subject has experimented with one substance, the risk of another substance experiment was much greater. Transition intensity from tobacco initiation to daily use was 4.8 times higher than that from cannabis. Our results are compatible with a process mixing the gateway theory, the reverse gateway theory and the route of administration model, but do not explore a common liability to addictions, which could be explored by using a MSM on a prospective cohort with initial collection of some explanatory factors.