CannTeen is a behavioural and magnetic resonance imaging (MRI) study. The study aimed to investigate whether cannabis use was associated with a different trajectory in teenage cannabis users (relative to their age-matched controls) compared with adult cannabis users (relative to their age-matched controls). Participants consisted of two age groups: teenagers aged 16 to 17 years and young adults aged 26 to 29 years, and included cannabis and non-cannabis users. All participants completed the behavioural arm of the study, and a subset of these also completed the MRI arm. There were 274 participants in total, including 140 participants in the MRI arm of the study. In the behavioural arm of the study, participants attended five sessions at 0, 3, 6, 9, and 12 months. Participants in the MRI arm also participated in an additional session at 0 and 12 months.