Cannabis is the most widely consumed illicit drug in the world (United Nations Office on Drugs and Crime, 2008) and its prolonged use is associated with various adverse effects including anxiety, paranoia, depression, tiredness, lack of motivation and low energy (Reilly et al. 1998). Furthermore, recurrent cannabis use appears to have negative psychosocial consequences, including poor work- and school performance (Lynskey & Hall, 2000; Hall, 2009), and physical impairments like decreased infection resistance, respiratory system problems and adverse reproductive effects (Hall & Solowij, 1998; Tashkin et al. 2002; Hall, 2009). Cannabis use is addictive and cessation attempts often result in withdrawal symptoms, such as anger, aggression, anxiety, decreased appetite, irritability, restlessness and sleep difficulty (Budney et al. 2008; Pruess et al. 2010). In turn, these withdrawal symptoms make successful long-term cessation difficult (Coffey et al. 2002; Budney & Hughes, 2006; Budney et al. 2008). Although the cannabis withdrawal syndrome is not recognized in the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition (DSM-IV; APA, 2000), recent research has found consistent evidence for the existence of a reliable and valid
