Research paperProbability and predictors of the cannabis gateway effect: A national study
Introduction
The gateway hypothesis holds that substance use progresses in sequential stages beginning with alcohol and tobacco use, followed by cannabis use and, later, the use of other illicit drugs (Kandel, 1975, Kandel, 2003, Kandel et al., 1992, Kandel et al., 2006). According to the gateway hypothesis, individuals rarely use certain substances, such as heroin or cocaine, without having first used “gateway” substances, such as legal drugs or cannabis. The validity of the gateway hypothesis has been the topic of intense debate since the early 1970s. Although some studies have found that use of legal drugs or cannabis are not a requirement for the progression to other illicit drugs (Golub and Johnson, 1994, Mackesy-Amiti et al., 1997, Malone et al., 2010, Morral et al., 2002, Tarter et al., 2012, Tarter et al., 2006), most studies have supported the “gateway sequence” (Degenhardt et al., 2009, Fergusson et al., 2006, Fergusson and Horwood, 2000, Grau et al., 2007, Makanjuola et al., 2010, Mayet et al., 2012, Rebellon and Van Gundy, 2006, Van Ours, 2003, Yamaguchi and Kandel, 1984).
In recent years there has been a growing interest in the effects of cannabis on mental health and psychosocial functioning (Blanco et al., 2014, Copeland et al., 2013, Moore et al., 2007, van Gastel et al., 2013, Van Ours and Williams, 2012), including the extent to which cannabis acts as a ‘gateway drug’ (Fergusson et al., 2006, Vanyukov et al., 2012). Cannabis would meet the conditions for gateway drug if (a) its use was initiated prior to the onset of other illicit drug use; and, (b) cannabis use increased the likelihood of using other illicit drugs (Fergusson et al., 2006).
While most of the studies have shown a high degree of association between cannabis use and use of other illicit drugs (Agrawal et al., 2004, Fergusson and Horwood, 2000, Khan et al., 2013, Lynskey et al., 2003, O’Donnell and Clayton, 1982, Van Ours, 2003), the predictors of progression from cannabis to other illicit drugs remain largely unknown (Kandel et al., 2006, Van Gundy and Rebellon, 2010). Identification of those predictors is a crucial step in understanding the etiology of substance use disorders that could help in the development of more effective treatment and preventive interventions.
Prior research has indicated that genetic predisposition (Agrawal et al., 2004), higher frequency of cannabis use (Fergusson and Horwood, 2000, Mayet et al., 2012) and early onset of cannabis use (Fergusson et al., 2006, Van Gundy and Rebellon, 2010) are associated with increased risk of progression to other illicit drug use. Presence of depressive symptoms (Yamaguchi & Kandel, 1984), stress and unemployment (Van Gundy & Rebellon, 2010), peer influence (Wagner & Anthony, 2002) or drug availability (Degenhardt et al., 2010) have also been linked to increased risk of progression to other illicit drug use. Despite this body of knowledge, important questions remain regarding predictors of progression from cannabis use to use of other drugs (Kandel et al., 2006). For example, several sociodemographic, psychopathologic and substance use related predictors previously reported for other types of drug use transitions (Florez-Salamanca et al., 2013, Lopez-Quintero et al., 2011b, Ridenour et al., 2005) have not been examined. With the exception of one study that examined depression, no published study has investigated the effect of psychiatric comorbidity (i.e., anxiety, conduct or personality disorders) on progression from cannabis use to use of other drugs.
We sought to build on prior work by drawing on data from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC), a large nationally representative study of the United States (US) adult population (Grant & Kaplan, 2005). The main goals of this study were to: (1) estimate the cumulative probability of progression to illicit drug use among individuals with lifetime history of cannabis use; and, (2) assess the association between several sociodemographic characteristics, psychiatric comorbidity and substance use-related variables and the risk of progression from cannabis use to other illicit drugs use.
Section snippets
Sample
The NESARC target population at Wave 1 (2001–2002) was the civilian non-institutionalized population 18 years and older residing in households and group quarters. The final sample included 43,093 respondents. Blacks, Hispanics, and adults 18–24 were oversampled, with data adjusted for oversampling, household- and person-level non-response. The overall survey response rate was 81%. Data were adjusted using the 2000 Decennial Census, to be representative of the US civilian population for a
Probability of progression from cannabis use to any other illicit drug use
Lifetime cumulative probability estimates indicated that 44.7%of individuals with lifetime cannabis use progressed to other illicit drug use at some time in their lives. During the second year after first cannabis use, the probability of other illicit drug initiation was 8.7%. The estimated cumulative probability of other illicit drug initiation a decade after the onset of cannabis use was 36% (Fig. 1).
Sociodemographic characteristics
Sociodemographic characteristics of the study sample are presented in Table 1. Despite
Discussion
To our knowledge, this is the first study to identify subgroups of the population at increased risk for progression from cannabis use to use of other illicit drugs in a large, nationally representative sample of U.S. adults. The cumulative probability of transition from cannabis use to other illicit drug use was 44.7%. Several sociodemographic and psychiatric variables and indicators of substance use severity predicted progression, including being male, urban residence, being never married,
Role of funding source
Work on this manuscript was supported by NIH grants DA019606, DA020783, DA023200, DA023973 and CA133050 (Dr. Blanco), by the New York State Psychiatric Institute (Dr. Blanco) and by the University of Oviedo (Drs. Secades-Villa and García-Rodríguez).
Conflict of interest
No conflict declared.
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