Aims To summarize and evaluate our knowledge of the relationship between heavy cannabis use, cannabis use disorder (CUD) and the brain. Methods Narrative review of relevant literature identified through existing systematic reviews, meta‐analyses and a PubMed search. Epidemiology, clinical representations, potential causal mechanisms, assessments, treatment and prognosis are discussed. Results Although causality is unclear, heavy and dependent cannabis use is consistently associated with a high prevalence of comorbid psychiatric disorders and learning and memory impairments that seem to recover after a period of abstinence. Evidence regarding other cognitive domains and neurological consequences, including cerebrovascular events, is limited and inconsistent. Abstinence after treatment is only achieved in a minority of cases; treatment targeted at reduction in use appears have some success. Potential moderators of the impact of CUD on the brain include age of onset, heaviness of use, CUD severity, the ratio of ∆9‐tetrahydrocannabinol to cannabidiol and severity of comorbid disorders. Conclusions Current evidence of long‐term effects of daily cannabis use and cannabis use disorder on brain‐related outcomes is suggestive rather than conclusive, but use is associated with psychiatric morbidity and with cognitive impairments that recover after a period of abstinence.

The impact of cannabis on the adolescent compared to adult brain is of interest to researchers and society alike. From a theoretical perspective, adolescence represents a period of both risk and resilience to the harms of cannabis use and cannabis use disorders. The aim of this systematic review is to provide a critical examination of the moderating role of age on the relationship between cannabis use and cognition. To this end, we reviewed human and animal studies that formally tested whether age, adolescent or adult, changes the relationship between cannabis exposure and cognitive outcomes. While the results of this review do not offer a conclusive answer on the role of age, the novel review question, along with the inclusion of both human and animal work, has allowed for the formation of new hypotheses to be addressed in future work. First, general executive functioning seems to be more impaired in adolescent frequent cannabis users compared to adult frequent cannabis users. Second, age-effects may be most prominent among very heavy and dependent users. Third, craving and inhibitory control may not decrease as much post-intoxication in adolescents compared to adults. Lastly, adolescents’ vulnerability to reduced learning following cannabis use may not persist after sustained abstinence. If these hypotheses prove correct, it could lead to important developments in policy and prevention efforts. Electronic supplementary material The online version of this article (10.1007/s00406-019-00981-7) contains supplementary material, which is available to authorized users.

Background and aims Lockdown measures aimed at limiting the number of infections and deaths from the coronavirus disease 2019 (COVID‐19) have introduced substantial psychosocial stressors in everyday life. We aimed to investigate the influence of the Dutch lockdown on cannabis use and cannabis use disorder (CUD) and investigate relations with change in mental wellbeing and experienced psychosocial stressors during the lockdown. Design Explorative longitudinal baseline‐, pre‐ and during lockdown survey study. Setting The Netherlands, on‐line between January 2019 and May 2020. Participants Community sample of 120 monthly to daily cannabis users and reference group of 63 non‐using controls. Measurements Change in cannabis use and CUD symptom severity from baseline to pre‐ to post‐lockdown. Change in cannabis use motives, mental health, quality of social relationships and job status from pre‐ to post‐lockdown. Findings In cannabis users, lockdown related to increased cannabis use [B = 1.96, 95% confidence interval (CI) = 0.26–3.66, P = 0.024], but not CUD symptom severity. Cannabis users experienced 30% job loss and increased loneliness [P < 0.001, Bayes factor (BF)10 > 100], while contact with partners (P = 0.005, BF10= 8.21) and families improved (P < 0.001, BF10 = 19.73), with no differences between cannabis users and control. Generally, mental health problems (all Ps > 0.277, all BF10 < 0.139) did not change, but individual differences were significant and severity of cannabis use pre‐lockdown, COVID‐19‐related worries, change in anxiety, expansion motives, social motives and family contact all uniquely related to variance in change in cannabis use or CUD. Conclusions While cannabis use among daily cannabis users in the Netherlands increased at the group level during the period of COVID‐19 lockdown, the effect of the first months of lockdown on cannabis use disorder severity and mental wellbeing varied significantly among individual daily cannabis users.

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