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Epidemiology and Comorbidity
Cannabis is probably the most commonly used illicit substance in the world, with an estimated 200 to 300 million regular users (Johnson, 1990). In the USA, cannabis is generally thought to be the most widely used illicit drug, with more than 50% of Americans reporting at least one episode of use (Johnson, 1990; Mueser et al., 1992; Chen and Kandel, 1998; Hubbard et al., 1999). As with most other illicit drugs cannabis use occurs more often in men though the difference between the sexes is decreasing (Greenfield and O’Leary, 1999). Also, like other illicit drugs can-nabis use typically begins in adolescence and is most prevalent in people between the ages of 18 to 30 years (Chen and Kandel, 1995, 1998; Johnston et al., 2001; Kandel and Chen, 2000). The age of first cannabis use in America has been decreasing; in 1997 the average age of first use was reported to be 14 by the National Household Survey on Drug Abuse (NHSDA) (SAMHSA, 1997). The annual, monitoring the future study of high school students, reported that in 2000, 15.6% of 8th graders, 32.2% of 10th grad-ers and 36.5% of 12th graders reported using cannabis in the past year. Approximately 20% of 12th grade cannabis users reported daily use (Johnston et al., 2001).
Adolescents appear more vulnerable to developing can-nabis dependence than adults, becoming dependent after using cannabis at a lower dose and frequency of use (Chen et al., 1997). Although earlier onset of use during adolescence is a predictor for continued use and the development of dependency, frequency of use is a stronger predictor (Chen and Kandel, 1998; Kandel and Chen, 2000; DeWit et al., 2000; SAMHSA, 2000). Of those adolescents who use cannabis more than once, about a third will subsequently use cannabis regularly for some period of time, with 20% using daily, and an additional 10 to 20% using near daily (Hall and Solowij, 1998; Johnston et al., 2001; Zoccollilo et al., 1999). It is estimated that a third of those who use can-nabis daily meet the criteria for cannabis dependence, and that of all adolescents who use cannabis at least once, approximately 9% will develop dependence (Anthony et al., 1994; Chen et al., 1997). However, most adolescents who use cannabis regularly will have stopped use by the time they are 30 (Chen and Kandel, 1995, 1998).
Studies have reported that individuals with cannabis use disorders have high rates of other substance abuse disorders (Miller et al., 1990) as well as other types of Axis I disorders (Regier et al., 1990; Troisi et al., 1998). It is possible, however, that these findings reflect “spurious comorbidity” (Smoller et al., 2000) because individuals with cannabis dependence and other Axis I disorders are probably more likely to present for treatment or research studies than those with cannabis dependence alone.
Conversely, studies of several psychiatric populations (Brady et al., 1991; Alterman et al., 1982; Miller et al., 1989; Cantwell et al., 1999; Menezes et al., 1996; Johns, 2001) with a number of different Axis I diagnoses other than panic disor-der (Szuster et al., 1988) have found high rates of cannabis use. The course of the Axis I illnesses is often adversely affected by cannabis use; cannabis may exacerbate psychotic symptoms in patients with schizophrenia, possibly precipitate schizophre-nia in predisposed individuals (Andreasson et al., 1987, 1989), precipitate hypomanic or manic episodes in bipolar patients (Gruber and Pope, 1994) and trigger panic reactions in patients with panic disorder (Szuster et al., 1988). Cannabis use, abuse and dependence are also commonly comorbid with conduct dis-order in children and adolescents and with antisocial personality disorder in adults (Weller and Halikas, 1985; Henry et al., 1993; True et al., 1999; Crowley et al., 1998). Despite these findings of comorbidity, cannabis use has not been shown to induce any psy-chiatric disorders de novo in nonpredisposed individuals (Hall and Degenhardt, 2000; Johns, 2001).
The prevalence of cannabis intoxication should be approxi-mately the same as the prevalence of cannabis use described at the beginning of this section. No formal epidemiological data exist regarding the prevalence of cannabis intoxication delirium, cannabis-induced psychotic disorder, or cannabis-induced anxi-ety disorder. In fact, it is not entirely certain that any of these three entities actually occurs in individuals free of preexist-ing DSM-IV-TR Axis I disorders (Hall and Degenhardt, 2000; Johns, 2001).
For example, no original reports of cannabis-induced de-lirium in the literature were found except for comments about it in review articles. Whereas cannabis use occasionally causes anxiety, or even panic reactions, especially among inexperi-enced users, there is again no known published study exhibit-ing a cohort of previously asymptomatic subjects who developed clinically significant cannabis-induced anxiety disorder. One investigator (Pillard, 1970) observed that there were five to seven cases of cannabis-associated anxiety reactions reported to a uni-versity health service per year; he hypothesized that more cases occurred but were not reported. However, he noted that reassur-ance was all the treatment necessary, suggesting that these pa-tients had not developed clinically significant anxiety disorders. Finally, although anecdotal reports and even case series of can-nabis-induced psychotic disorder have appeared, many of these have been collected outside the USA and most provide insuffi-cient evidence to assess whether the subjects studied were suffer-ing from preexisting psychotic disorders before their ingestion of cannabis (Gruber and Pope, 1994). In one US study, the in-vestigators reviewed approximately 10 000 discharges from two psychiatric units. All cases of possible cannabis-induced mental disorders were investigated by chart review. No cases of clear-cut cannabis-induced psychotic disorder or cannabis-induced anxi-ety disorder were found. Thus, it appears that these disorders, at least of sufficient magnitude to prompt a psychiatric admission, are rare or do not exist at all (Gruber and Pope, 1994; Hall and Degenhardt, 2000; Johns, 2001).
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