National epidemiological samples have demonstrated that cannabis is the most commonly used illicit drug [Anthony et al., 1994; Stinson et al., 2005, 2006], and that its use has substantially increased over the last decade [Compton et al., 2004]. Chronic use of cannabis is associated with both physical and mental health problems. Persistent use poses health problems similar to those of tobacco [Taylor et al., 2000; Mittleman et al., 2001; Fisher et al., 2005; Hashibe et al., 2005; Tashkin, 2005]. Cannabis use has also been implicated in a syndrome characterized by apathy, loss of goal-directed behavior, and cognitive impairment termed the “amotivational syndrome” [Sharma, 1975; Pope et al., 2001; Solowij et al., 2002; Schuckit, 2006]. Cannabis use is associated with psychotic illness and depression [Degenhardt et al., 2003; Hall et al., 2004], as well as impaired educational and work performance [Kandel and Chen, 2000; Lynskey and Hall, 2000; Swift et al., 2001; Schuckit, 2006]. Use of cannabis, particularly by adolescents and young adults, may also facilitate progression to other illicit drug use (the “gateway” drug hypothesis) [Fergusson and Horwood, 2000; Lynskey et al., 2003].
