Associations between cannabis use and mental health symptoms over four years in adolescents and adults

Background and AimsAdolescence may be a time when the brain and mind are more sensitive to the psychological harms caused by chronic cannabis use. However, few studies have directly compared adolescents who use cannabis with adults who use cannabis. We investigated mental health symptoms in adolescents and adults over four years, with varying cannabis use frequency. We hypothesised that there would be stronger associations between cannabis use and mental health in adolescents compared to adults.DesignFour-year matched-cohort longitudinal study, with factors of Age-Group (Adolescent/Adult), Cannabis Use, and Time.SettingLondon (United Kingdom).Participants274 adolescents (aged 16-17 years) and adults (aged 26-29 years) who either used cannabis (≥1 days/week) or did not (0 days/week).MeasurementsWe measured problematic cannabis use, depression, anxiety, psychotic-like symptoms, and apathy, every three months for one year and then again approximately four years later. We also measured spatial working memory ability in the first year. We (1) conducted linear mixed models to explore the associations between time-varying, past-3-month cannabis use frequency and mental health outcomes, (2) investigated the mental health trajectories of participants who consistently remained a cannabis-user (≥1 days/week) or a control (0 days/week) over four years; and (3) investigated the change in mental health symptoms in those who stopped or started using cannabis weekly. We adjusted for sociodemographic and environmental variables, and polygenic risk scores.FindingsWe achieved 85% retention at one-year and 60% retention at four-year follow-ups. Our adjusted models showed that adolescents were more likely to have persistent problematic cannabis use symptoms than adults (p<0.001). Cannabis use frequency predicted: severity of problematic cannabis use (p<0.001), psychotic-like symptoms (p=0.003), and worse spatial working memory performance (p=0.038). Being a consistent cannabis user predicted worse spatial working memory performance (p<0.001). Through an additive relationship, being an adolescent and using cannabis more frequently was associated with worse psychotic-like symptoms. Importantly, people who stopped using cannabis entirely before the four-year follow-up had the greatest drop in psychotic-like symptoms (p=0.026) and depressive symptoms (p=0.047).ConclusionsAdolescents who use cannabis had a persistently enhanced risk of problematic cannabis use over four years, compared to adults. Frequent cannabis use was associated with elevated psychotic-like symptoms and poorer working memory. We did not find evidence that adolescents were especially vulnerable to the harmful effects of cannabis on any outcome, apart from problematic cannabis use. However, reductions in depressive and psychotic-like symptom were observed in people who stopped using cannabis.