• Gabriel J Merrin, Kara Thompson, and Bonnie J Leadbeater.
• University of Victoria, 3800 Finnerty Rd., Victoria, BC, V8P 5C2, Canada. Electronic address: gmerrin@uvic.ca.
• Drug Alcohol Depend. 2018 Aug 1; 189: 147-153.

BackgroundResearch indicates that many adolescents frequently use multiple substances. This study examines stability and transitions in the use and co-use of multiple substances (cigarette, alcohol, marijuana, illicit drugs) from adolescence to young adulthood.MethodsData were collected biennially from 662 youth in six assessments across ten years (2003-2013). We used latent class analysis (LCA) to classify youth by substances used at each wave and used latent transition analysis (LTA) to examine transition probabilities across waves.ResultsAt each wave, a three-class model best fit the data. Classes included a poly-use class, that had the highest probabilities of use among all substances, a co-use class, that had high probabilities of use of alcohol and marijuana, and an alcohol-dominate class that started with low probabilities of use among all substances but showed increasing probabilities of alcohol use, at later ages. LTA showed that the probability of remaining in the poly-use class was the most stable from one wave to the next, followed by the alcohol-dominate class, and the co-use class. The most transitions occurred for the co-use class, with more individuals transitioning to the poly-use class than to the alcohol-dominate class.ConclusionsStrong stability among adolescent substance use classes was found between waves. Transitions were typically to classes using more substances, although, transitions to use fewer substances was evident for some individuals. Distinguishing stable and transitioning classes of substance use and co-use provides opportunities for prevention and intervention to disrupt high-risk use classes.Copyright © 2018 Elsevier B.V. All rights reserved.

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