Title: Co-Use of Alcohol and Cannabis in Young Adulthood: An Examination of the Effects of Co-Use on Substance Use and Mental Health Symptoms.

Introduction: The advent of cannabis legalization raises concerns about the potential interactive health effects of cannabis with other drugs. Cannabis and alcohol are the two most commonly used substances among young adults and these substances are commonly used together, either concurrently (use of both substances at different times) or simultaneously (use of both substances at the same time). While our understanding of the interactive effects of alcohol and cannabis co-use is not entirely understood, using cannabis and alcohol at the same time results in greater impairment than ingesting either one alone and cross-sectional studies suggest co-use is associated with a variety of high-risk behaviours including heavy drinking and alcohol-related harm. In the current study, we examined the longitudinal impact of concurrent and simultaneous co-use of alcohol and cannabis on substance use patterns, substance related harms and mental health symptoms two years later.

Method: Data were drawn from Waves 5 (n = 464; 46% male) and 6 (n = 478; 45% male) of the Victoria Healthy Youth Survey, a random community sample of young adults ages 20-26 at Wave 5 and 22-29 at Wave 6. At baseline there were 662 participants ages 12-18 and were assessed biennially between 2003 and 2013. Path analysis was used to examine whether co-use status at Wave 5 predicted substance use, substance-related harms, and mental health symptoms at Wave 6.

Results: At Wave 5, 43% of participants were alcohol only users, 14% were concurrent users, 41% were simultaneous users, 1% were cannabis only users, and 3% did not use either cannabis or alcohol. Compared to alcohol only users, both concurrent and simultaneous users reported greater use of illicit drugs, smoking and more alcohol use disorder symptoms at Wave 6, and simultaneous users reported higher levels of alcohol use, risky driving and social harms from alcohol. Compared to concurrent users, simultaneous users reported higher levels of heavy drinking, illicit drugs, cannabis frequency and quantity, risky driving, and symptoms of both alcohol and cannabis use disorders. For mental health symptoms, both concurrent and simultaneous users reported higher levels of conduct symptoms at Wave 6 compared to alcohol only users, concurrent users reported higher anxiety symptoms, and simultaneous users reported higher levels of psychosis and ADHD symptoms. There were no differences in mental health symptoms between concurrent and simultaneous users.

Conclusions: Cannabis is rarely used in isolation and simultaneous use is much more common than concurrent use. Both types of co-use increased substance use and related harms compared to alcohol only users but simultaneous users report higher levels of substances and experience substance related harm compared to concurrent users.