Are There Secondary Effects On Marijuana Use From Alcohol Interventions for College Students?

Introduction: Marijuana is the most prevalent illegal drug used on college campuses and is often used in the context of drinking. While brief alcohol interventions for college students have been found to be efficacious for reducing heavy drinking, little is known about whether they also impact marijuana use. Given that marijuana and alcohol use covary in terms of correlates and precursors and that both substances are often used concurrently, it is reasonable to hypothesize that brief alcohol interventions may have secondary, contagion effects on marijuana use. The purpose of the current study was to: 1) examine the effects of brief alcohol interventions on marijuana use, 2) test whether interventions that also included a focus on drugs had better marijuana use outcomes than those that did not, and 3) determine if reductions in alcohol use also resulted in reductions in marijuana use.

Methods: Data came from 11 separate, randomized, college alcohol intervention trials that collected data on marijuana frequency at baseline and follow up (N=7639). Three of the eleven interventions also focused on drugs. We used t-tests (for continuous measures) and Wilcoxon's Two Sample tests (for ordinal measures) to examine intervention versus control group differences in marijuana use frequency at both short-term (1-3 months post intervention) and long-term (6-12 months) follow ups. In addition, to examine within-group changes in marijuana use, we utilized paired t-tests and Wilcoxon Signed Rank tests.

Results: Across studies, there were no significant differences between intervention and control groups in short-term marijuana effects and only one significant long-term effect indicating that the control group exhibited a lower frequency of marijuana use than the intervention group. However, within-group analyses showed that intervention groups were twice as likely as control groups to experience significant reductions in marijuana use, regardless of whether the intervention specifically targeted drug use. Within studies, students were divided by thirds into those who reduced their alcohol use, those who remained the same, and those who increased. For the most part, those students who reduced their drinking were more likely to decrease their marijuana use, whereas those who increased their drinking were more likely to increase their marijuana use, especially over the long term.

Conclusions: It appears that interventions that reduce alcohol use may be beneficial also for reducing marijuana use. There is still a need, however, to develop and test new interventions for college students that focus specifically on marijuana use.