Abstract Objective Reductions in substance involvement into adulthood are thought to represent a normative maturing out of substance use. However, patterns and predictors of maturing out of alcohol and cannabis co‐use remain largely unstudied. Therefore, the current study tested developmental trajectories of alcohol and cannabis use from late adolescence into adulthood and whether late adolescent personality traits predicted trajectory class membership. Methods Data come from a longitudinal study of family history of alcohol disorder (N?=?458). Age bands were created to model trajectories of drinking quantity, negative alcohol consequences, and cannabis use frequency from late adolescence (age 18–22) to young adulthood (age 23–28) and adulthood (age 29–36). Participants reported on their sensation seeking, conscientiousness, and neuroticism during late adolescence and their typical drinking quantity, negative alcohol consequences, and cannabis use frequency at each age band. Results Three trajectory classes were derived from an initial Parallel Process Growth Mixture Model: (1) low‐risk maturing out of alcohol‐only use, (2) high‐risk maturing out of co‐use, and (3) high‐risk switchers who increased their cannabis use into adulthood. Late adolescent sensation seeking was associated with higher odds of being in both co‐use trajectories, whereas a lack of conscientiousness was associated with higher odds of being a co‐use switcher. Conclusions We identified heterogeneity in trajectories of co‐use, which suggests that a lack of maturing out of alcohol involvement may be accompanied by increased cannabis use. Moreover, late adolescent personality traits may predispose individuals toward riskier developmental trajectories of substance use into adulthood.
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