Cannabis is the most commonly used illicit substance in the world with 143–190 million people in 2007 having used the drug at least once worldwide (1). An estimated 14.4 million Americans aged 12 or older reported cannabis use over the past month (2). Of these individuals, ~7% develop cannabis dependence (CaD) defined by DSM-IV criteria (3). With the expansion of legalization in the United States (generally for “medical” use), availability and use of cannabis is rising. Cannabis use is often accompanied by dependence on alcohol and other drugs (4), and is associated with serious consequences, including cognitive and psychomotor impairments (5, 6). The use of cannabis is associated with roughly twofold increased risk of schizophrenia; there is interindividual variability in susceptibility to cannabis-induced psychosis that could be, in part, genetic in origin (7, 8). Thus, it is important to identify factors that influence individual vulnerability to the development of CaD.
