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Background and aims: Little is known about how cannabis use over the life‐course relates to harms in adulthood. The present study aimed to identify trajectories of cannabis use from adolescence to adulthood and examine both the predictors of these trajectories and adverse adult outcomes associated with those trajectories. Design A latent trajectory analysis of a longitudinal birth cohort (from birth to age 35 years). Setting and participants: General community sample (n = 1065) from New Zealand. Measurement Annual frequency of cannabis use (ages 15–35 years); childhood family and individual characteristics (birth to age 16 years); measures of adult outcomes (substance use disorders, ages 30–35 years; mental health disorders, ages 30–35 years; socio‐economic outcomes at age 35 years; social/family outcomes at age 35 years). Findings A six‐class solution was the best fit to the data. Individuals assigned to trajectories with higher levels of cannabis use were more likely to have experienced adverse childhood family and individual circumstances. Membership of trajectories with higher levels of use was associated with increased risk of adverse outcomes at ages 30–35 years. Adjustment of these associations for the childhood family and individual predictors largely did not reduce the magnitude of the associations. Conclusions: In New Zealand, long‐term frequent cannabis use, or transition to such use, appears to be robustly associated with diverse harms in adulthood. [ABSTRACT FROM AUTHOR]