An intriguing aspect of cannabis and tobacco co-use is that, for both drugs, the predominant route of administration is via inhalation. The aero-respiratory adaptations afforded by smoking one drug may facilitate the use of the other such that cigarette smokers may be more likely to continue smoking cannabis past a preliminary phase of experimentation, partly because the sensation of inhaled smoke is less unpleasant than it would be in non cigarette smokers. For instance, in a prior study of a representative U.S. sample (64), even after accounting for a lifetime history of nicotine dependence and other covariates, those using smoked forms of tobacco were more likely, compared with those using non-smoked forms, to report cannabis use and, importantly, meet criteria for cannabis use disorders. However, vanLeeuwen and colleagues (65) argue that if route of administration (i.e. smoking) were important, then early onset of tobacco use would predict the subsequent use of cannabis more strongly than early use of alcohol, which is not the case in their cohort of Dutch adolescents. We posit that route of administration may be an additional (not in lieu of) adaptive mechanism that contributes to sustained cannabis use in tobacco smokers.
