There is a critical need to understand marijuana use trajectories among youth residing in socio-economically disadvantaged communities. Marijuana is the most commonly used illicit drug used during adolescence (Substance Abuse and Mental Health Service Administration, 2014b), with use adversely affecting neuromaturation of the vulnerable adolescent brain (Gruber et al., 2012; Lopez-Larson et al., 2011; Schepis et al., 2011). Marijuana use during adolescence places youth at increased risk for motor vehicle crash (Maxwell, 2012; Naimi et al., 2003; Quinlan et al., 2005; Wilhelmi and Cohen, 2012), and other drug use (Center for Disease Control and Prevention, 2012; Center for Lawful Access and Abuse Deterrence, 2011; Schillie et al., 2009; Substance Abuse and Mental Health Service Administration, 2013; White et al., 2013a) The emergency department (ED) provides a useful setting to identify at-risk youth and initiate substance use interventions (Cunningham et al., 2009).

Based on social ecological and resiliency theories (Bronfrenbrenner, 1998; Garmezy, 1991; Rutter, 1987; Stokols et al., 1996), marijuana use during adolescence and emerging adulthood is associated with evolving interactions between individual and social risk and promotive factors that can decrease or accelerate problem trajectories (Abadi et al., 2011; Bernstein et al., 2011; Brown et al., 2008; Cadoret et al., 1995; Cleveland and Wiebe, 2008; Fowler et al., 2007; Henry and Thornberry, 2010; Hussong et al., 2008; Mrug et al., 2010; Wagner, 2008; Windle et al., 2008; Zucker et al., 2000; Zucker et al., 2008). For example, violence is associated with substance use, particularly among those residing in socio-economically disadvantaged communities (Goldstick et al., 2015; Goldstick et al., 2016; Walton et al., 2009; White et al., 2009), likely due to shared risk and promotive factors for some substances (e.g., marijuana), and/or acute pharmacological effects of substances (e.g., alcohol) (Chermack and Giancola, 1997; White et al., 2009). Although parental influences are important during younger ages, peers are the most robust influence for substance use during the transition to adulthood (Abadi et al., 2011; Allen et al., 2012; Brechwald and Prinstein, 2011; Burk et al., 2012; Chein et al., 2011; Dziak et al., 2014; Simons-Morton and Farhat, 2010; Wolfe et al., 2012), increasing exposure to deviant social contexts and distancing youth from protective influences.

Latent trajectory analysis is a useful approach to identifying heterogeneity in trajectories of substance use (Nagin and Tremblay, 2005; Skardhamar, 2010), with groupings providing an opportunity to examine characteristics that can inform interventions (Brame et al., 2012). Prior research examining marijuana use trajectories are relatively rare. For example, a longitudinal study of youth from a socio-economically disadvantaged community (Brook et al., 2011) found four groups of marijuana users (low users, decreasing, increasing, and chronic), whereas Schulenberg and colleagues (2005) used Monitoring the Future (MTF) data to identify six marijuana use trajectory groups (i.e., chronic, decreased, increased, fling, rare, and abstain). Chronic users are more likely to be male, with the greatest constellation of risk factors including externalizing (e.g., delinquency, impulsivity, and substance use) and internalizing problems (Brook et al., 2011; Chassin et al., 2004; Jackson et al., 2008; Meier et al., 2013). Baseline factors that distinguish chronic users from decreasing or late onset users are less evident, with one study finding greater negative affect among the late onset group (Brook et al., 2011; Meier et al., 2013). Other researchers find that decreasing users are initially similar to chronic users, but over time had less substance use, delinquency and peers who used marijuana (Schulenberg et al., 2005); in contrast, increasing users are most likely to use other substances, use marijuana to cope, have drug-using friends, and report delinquency (Schulenberg et al., 2005). Although aggression is related to alcohol use during adolescence (White et al., 2013b; White et al., 2009), few studies have examined marijuana use in relation to violence, with one study finding that increasing users show more persistent physical aggression (Schulenberg et al., 2005). It is unclear how these findings generalize to youth presenting to urban EDs, with generally much lower socio-economic status and greater minority representation.

The current study examined longitudinal marijuana use trajectories among youth presenting for care at an urban ED. The ED is an important contact point for medical care for youth at high risk for substance use and violence, especially the underinsured and uninsured, with ED patients over-representing socially disadvantaged minority groups (Albert and McCaig, 2014). Further, the ED provides a location to access adolescents, who are often missed in school-based studies because of poor attendance (Grove et al., 2000; Ramirez et al., 2012; Wilson and Klein, 2000). Most patients (>90%) seeking ED care are treated and released (Albert and McCaig, 2014), which is consistent with studies demonstrating the receptivity of patients to ED-based interventions (Walton et al., 2012; Cunningham et al., 2015).

The purpose of this study was to examine factors associated with greater marijuana use trajectories to inform intervention development for drug-using youth presenting to urban EDs, with this study enrolling patients with past 6 month drug use, oversampling those presenting with assault injuries (Bohnert et al., 2015). Prior papers showed that assault-injured youth had increased rates of ED visits for violent injury, which was related to having a drug use disorder (Cunningham et al., 2015). First, this paper presents findings from latent trajectory analysis to identify subgroups of marijuana users two-years post-ED visit. Second, we characterize risk and promotive factors associated with these groupings at baseline, and at 12- and 24-months to inform interventions. For example, if the relationship between time-varying factors is consistent over time, then baseline screening could indicate the need for initiating interventions in the ED, with extended interventions addressing influences over time. In contrast, if the factors associated with trajectories varied over time, then adaptive interventions may be indicated to address these changing factors (Lei et al., 2012; Rich et al., 2014). Hypothesized factors associated with greater marijuana use trajectories included: male gender and older age; greater involvement with violence (assault injury, physical aggression, physical victimization, community violence) and negative peer influences; less involvement with positive peers, substance use treatment, and the criminal justice system. Also, we hypothesized that greater marijuana use trajectories would be: consistently related to peer influences over time, reflecting their importance during adolescence and emerging adulthood, and community violence, reflecting the urban setting in which youth likely remain; less strongly associated with interpersonal violence (aggression and victimization) over time, reflecting developmental trends for decreased involvement with violence during the transition to emerging adulthood; and, inconsistently related to incarceration and drug treatment over time, particularly given expected low rates of receipt of treatment services.

This study presents novel data regarding marijuana use trajectories among youth from socioeconomically disadvantaged communities who present to the ED for care. Specifically, 30.4% of youth were chronic users, reporting almost daily use of marijuana over time. Conversely, 28.2% of youth showed low rates of marijuana use at baseline, with almost no days of use two years later. Similar to increasing or late onset groups in other studies, 16.2% were intermittent users, with marijuana use increasing at 24-months. The moderate and high decline groups comprised 25.2% of the sample, showing decreases in marijuana use. Note that the trajectory groups identified represent statistical probabilities of group membership, as opposed to definitive individual outcomes (Nagin and Tremblay, 2005; Skardhamar, 2010); nonetheless, this grouping is useful for understanding characteristics associated with greater marijuana use to inform ED-based intervention efforts (Brame et al., 2012).

Consistent with prior studies, males were more likely than females to be in the greater marijuana use groups (Brook et al., 2011; Schulenberg et al., 2005). Overall, age did not distinguish trajectory groups; however, the limited numbers of adolescents could have precluded detection of effects found in other studies (White et al., 2013b; White et al., 2009). For example, it could be that the moderate declining group may mature into the high declining group. Although prior analysis from the screening sample of this study among drug-users and non-drug users showed that youth presenting with assault injuries had higher rates of substance use than those presenting for other reasons (Cunningham et al., 2014), when examining drug using youth, assault injury (as compared to other reason) was not related to substance use trajectory groups. These findings support the notion of universal screening and initiation of substance use interventions in the ED regardless of chief presenting complaint. The baseline factor consistently associated with greater marijuana use trajectories was physical aggression, which is consistent with findings for externalizing behaviors (Brook et al., 2011; Chassin et al., 2004; Jackson et al., 2008; Schulenberg et al., 2005), likely due to shared risk and promotive factors (e.g., peer influences) and/or exposure to situations that increase aggression (e.g., community violence). The relationship between greater marijuana use and aggression was also observed at 12-months post-ED visit, but dissipated by 24 months, likely reflecting developmental maturation of the sample.

Together, these findings suggest that ED-based interventions should address both substance use and violence, among youth reporting these multiple risk behaviors. Prior studies examining brief interventions for marijuana use are sparse, with a handful of studies showing positive outcomes among youth (D'Amico et al., 2008; Walton et al., 2013; Walton et al., 2014; Woolard et al., 2013), whereas adult studies show null findings (Blow et al., under review; Bogenschutz et al., 2014; Roy-Byrne et al., 2014; Saitz et al., 2014; Woodruff et al., 2014). These interventions, however, have not addressed violence. Similarly, although violence interventions have shown promise among assault injured youth (Becker et al., 2004; Cheng et al., 2008a; Cheng et al., 2008b; Cooper et al., 2006; De Vos et al., 1996; Dicker, 2005; Fein et al., 2013; Karraker et al., 2011; Zun et al., 2006; Zun et al., 2003), these approaches do not typically address substance use. Given that a motivational interviewing (Miller and Rollnick, 2002; Miller and Rollnick, 2012) based brief intervention addressing alcohol and violence showed sustained though modest effects (Cunningham et al., 2012; Walton et al., 2010), interventions may need to be extended, adapting to address increasing and decreasing risk and promotive factors for marijuana use over time (Lei et al., 2012; Rich et al., 2014).

Findings from this study suggest that such adaptive substance use interventions approaches consider also addressing negative peer influences and community violence, which were associated with greater marijuana use trajectories over time. For example, the chronic group showed less positive peer influences at all time points, and more negative peer influences and community violence at 12- and 24-months than the low group. Corresponding to increasing rates of marijuana use, the intermittent group reported greater negative peer influences and community violence at 24-months. Caution is required, however, when making causal conclusions, as the direction of this relationship can't be fully ascertained in this study; in fact, it is likely that these relationships are reciprocal. For example, greater exposure to community violence could lead to greater marijuana use to cope with negative affect, and/or greater marijuana use could lead to greater exposure to situations where violence is likely (e.g., buying and selling drugs). In this regard, multi-level interventions may be beneficial, including individual approaches that facilitate connections with pro-social peers, activities, and services potentially using technology (e.g., text messaging, remote therapy) to reduce barriers, in concert with community-level approaches (Hawkins et al., 2012).

Participation in substance use treatment was not related to marijuana use trajectories, likely because access to treatment services is generally low for youth, particularly in resource poor environments (Center for Behavioral Health Statistics and Quality, 2015), which is reflected in low rates of treatment utilization in this sample despite nearly daily use of marijuana in the chronic group. Next, incarceration was not a useful correlate of marijuana use trajectories, with the exception that the moderate decline group was more likely (at 12 months) to be incarcerated and the chronic group was less likely (at 24 months) to be incarcerated than the low group. Given data showing disproportionate rates of incarceration among African-American youth, with substance use being a salient risk factor for incarceration, early interventions are needed to reduce these health disparities (Mukku et al., 2012; Substance Abuse and Mental Health Service Administration, 2014a).

Findings underscore the need for interventions to interrupt marijuana use trajectories among youth residing in urban communities, with a third of youth likely remaining chronic users of marijuana. Interventions for marijuana-using youth should be initiated at the point of the ED visit, particularly among those involved with physical aggression and/or exposed to community violence, which are associated with greater marijuana use. Peer influences are particularly important in distinguishing marijuana use trajectory groups over time, suggesting that interventions are needed to help young people increase involvement with positive peers, activities, and services, adapting to evolving needs over time.