Decreased drinking in adults with co-occurring cannabis and alcohol use disorders in a treatment trial for marijuana dependence: Evidence of a secondary benefit?

The purpose of the present study was to investigate whether cannabis dependent users who met criteria for a secondary diagnosis of alcohol use disorder (AUD) would increase their use of alcohol in response to decreasing their use of marijuana in a behavioral treatment trial for cannabis use disorder (CUD). This phenomenon is commonly known as "substance substitution." Participants were randomly assigned to one of four 9-session treatment conditions with cannabis and alcohol use measured at baseline, posttreatment, and at 4 follow-ups through 14 months. Of those enrolled (n = 198), 27 (13.6%) also met criteria for AUD. Linear mixed models were used to analyze alcohol use over time with cannabis use and time as predictors. Findings demonstrated that there were no associations between declines in cannabis use and changes in alcohol consumption in the full sample. However, among those with CUD who also had AUD, declines in cannabis use significantly predicted concurrent declines in alcohol use (p < .05). This study did not find evidence of substance substitution among individuals receiving treatment for CUD. Contrary to expectations, the results indicated that individuals with AUD were more likely to decrease, rather than increase, their alcohol use when they reduced their marijuana use. Treatment for CUD in this study appeared to result in improvements in substance use generally, at least for those with comorbid AUD.