No abstract

Opinions about marijuana use within the US are becoming increasingly favorable, making it important to understand how psychosocial influences impact individuals’ use within this context. Here we use the Theory of Planned Behavior (TPB) to examine the influence of initial attitudes, norms, and efficacy to resist use on initial intentions, and then the effect of initial intentions on actual marijuana use measured one year later using data drawn from a community with relatively high use. We expanded the traditional TPB model by investigating two types of normative influence (descriptive and injunctive) and two types of intentions (use intentions and proximity intentions), reasoning that exposure to high use in the population may produce high descriptive norms and proximity intentions overall, but not necessarily increase actual use. By contrast, we expected greater variability in injunctive norms and use intentions and that only use intentions would predict actual use. Consistent with hypotheses, intentions to use marijuana were predicted by injunctive norms (and attitudes) and in turn predicted marijuana use one year later. By contrast, descriptive norms were relatively high among all participants and did not predict intentions. Moreover, proximity intentions were not predictive of actual use. We also found that increasing intentions to use over a one year period predicted greater use. Given the greater efficacy of theory-based as compared to non-theory-based interventions, these findings provide critical information for the design of successful interventions to decrease marijuana-associated harms.

Increases in marijuana use in recent years highlight the importance of understanding how marijuana affects mental health. Of particular relevance is the effect of marijuana use on anxiety and depression given that marijuana use is highest among late adolescents/early adults, the same age range in which risk for anxiety and depression is the highest. Here we examine how marijuana use moderates the effects of temperament on level of anxiety and depression in a prospective design in which baseline marijuana use and temperament predict anxiety and depression one year later. We found that harm avoidance (HA) is associated with higher anxiety and depression a year later, but only among those low in marijuana use. Those higher in marijuana use show no relation between HA and symptoms of anxiety and depression. Marijuana use also moderated the effect of novelty seeking (NS), with symptoms of anxiety and depression increasing with NS only among those with high marijuana use. NS was unrelated to symptoms of anxiety and depression among those low in marijuana use. The temperament dimension of reward dependence was unrelated to anxiety and depression symptoms. Our results suggest that marijuana use does not have an invariant relationship with anxiety and depression, and that the effects of relatively stable temperament dimensions can be moderated by other contextual factors.

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Objective: The relationship between marijuana and alcohol use among late adolescents was examined as whether marijuana use was related to quantity of alcohol consumed that day, and whether changes in marijuana and alcohol use frequency over 3 years were related. Method: College students (n = 375) reported marijuana and alcohol use for 28 days over 3 years. Results: Within-day analyses showed that more alcohol was consumed on days on which marijuana was used. Co-use varied by alcohol use problems, day of week, and year in study; the relation between marijuana and alcohol use was stronger among those who reported less problematic alcohol use, on weekdays, and as time increased. Daily level co-use relations also differed marginally by gender. At the annual level, there was a marginal relation between changes in use such that increasing use of one substance over time was weakly associated with increased use of the other substance over the same time period. Conclusions: Results add to the emerging conclusion of complementary marijuana and alcohol co-use within a single day, showing it occurs for both women and men, across a full range of marijuana use, and increases with time while being affected by alcohol use problems and day of week. The overall strong co-use relationship highlights risky behavior among late adolescents and supports an emphasis on common underlying substance use causes. However, the different pattern when assessed yearly demonstrates the importance of tailoring the timescale of analysis to the specific substance use question. Public Health Significance StatementResults show that late adolescents making the transition to college co-use marijuana and alcohol within individual days, with marijuana use on a particular day associated with more drinks consumed the same day. The relationship between daily marijuana and alcohol use was weaker among those with more problematic alcohol use, on weekends, and marginally so among men. Marijuana use also showed a stronger relation to alcohol use across the measurement period, suggesting co-use risk increased across the college transition. The consistent evidence of within-day co-use suggests substance use interventions need to explicitly consider motivations for and target co-use.