In 1975, Kandel posited that cannabis use, particularly with adolescent onset, was a “gateway” to the use of other illicit or hard drugs (38). The gateway process involves sequence (onset of gateway drug prior to use of hard drugs), association (increased likelihood of hard drug use in those who use cannabis) and, controversially, causation (39). Accordingly, researchers have demonstrated that cannabis use does occur prior to use of harder drugs, such as cocaine and heroin (40) and that, relative to non-users, cannabis users are considerably more likely to subsequently report use of hard drugs (41). However, the evidence for causation, or that cannabis use exerts a causal influence on the likelihood of using other illicit drugs, has been less unequivocal. Evidence for causation draws primarily from prevention and intervention studies that have noted that delaying or ceasing the onset of cannabis use has notable effects on use of harder drugs (42; 43). However, these studies also note that these prevention strategies are not drug-specific and hence, generally reduce exposure to risk influences that correlate with early use of cannabis and
