The structure of genetic correlations between liability to addiction and other behavioral traits indicates that sources of CLA are also likely to be unrelated to specific drug action as such, including a specific role of a drug designated as a “gateway”. It is possible, for instance, that marijuana use leads to sensitization, increasing the response of neurobiological systems involved in addiction to other drugs. Sensitization has been proposed as “probably the most relevant interpretation underlying the Gateway Hypothesis” (Kandel et al., 2006, p. 471). Apart from the data suggesting the lack of marijuana sensitization (Ellgren et al., 2004), however, a possibility of sensitization clearly exists for any other psychoactive drug. Therefore, it would not support the specific “gateway” role of marijuana as the initiator of the sequence and thus does not concern the GH, or it would support any drug's “gateway” role, rendering the concept superfluous. Moreover, the sensitization mechanisms would have to be by definition shared in common between marijuana and other drugs.
