Simultaneous use of alcohol and cannabis and attenuated age-related declines in alcohol and cannabis use disorder symptoms across young adulthood

Most young adults naturally mature out of high-risk substance use patterns, but it is important to identify factors that may impede normative declines. Use of alcohol and cannabis simultaneously (i.e., simultaneous alcohol and marijuana/cannabis [SAM]) is cross-sectionally associated with alcohol and cannabis concerns, and SAM use increases acute risks at the daily level. However, less is known about long-term risks of SAM use and, specifically, how SAM use relates to maturing out of alcohol and cannabis use. Using four consecutive years of survey data from young adults who reported SAM use ( N =409; 1636 responses; aged 18 -25 at baseline), we first estimated age-related changes in symptoms of alcohol and cannabis use disorder (AUD/CUD) using multilevel growth modeling. Findings supported a maturing out process, as both AUD and CUD symptom severity significantly declined across young adulthood, on average (4 % and 5 % per year respectively, with significant acceleration). Cross-level interactions tested whether participants ' mean SAM use frequency across all four timepoints moderated age-related trajectories in AUD/CUD symptom severity. Significant interactions indicated that, relative to less-frequent SAM use, participants with more frequent SAM use showed less steep declines in AUD (1 % decrease per year vs. 6 % per year) and CUD symptoms (0 % decrease per year vs. 7 % per year); thus, SAM frequency was associated with slower/delayed maturing out of hazardous alcohol and cannabis use. Findings highlight that SAM use may be a correlate or risk-factor for prolonged high-risk substance use during young adulthood that relates to deviations from maturing out processes.