Which of these genes are drug-specific in their action and which genes might contribute to the genetic covariance between cannabis and tobacco involvement? It is expected that genes involved in metabolism, such as CYP2A6 (for nicotine) and FAAH (for cannabis), are highly likely to have drug-specific effects. We are unaware of any published association results for CYP2A6 and cannabis involvement or for FAAH and tobacco involvement. In contrast, given the widespread evidence for receptor cross-talk, it is intriguing to speculate that chronic use of cannabis may modify nicotinic receptor activity and vice versa. Supporting this possibility, for CNR1, Chen and colleagues (48) found multiple polymorphisms to be associated with nicotine dependence but not with smoking initiation.
