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.