Cannabis use often begins in adolescence and young adulthood, when anxiety, depression, psychosis, and bipolar disorder typically first develop. Young people aged in their mid-teens to mid-twenties who engage in daily cannabis use and develop a cannabis use disorder have a higher prevalence of these mental disorders. We assessed the coherence of evidence from epidemiological, genetic, experimental, and preclinical studies to assess relationships between daily cannabis use and the increased incidence, prevalence, and persistence of psychosis, bipolar disorder, anxiety, depression, and suicidal behaviours. We found credible evidence that daily cannabis use is a contributory cause of psychosis. There were fewer high quality epidemiological and genetic studies of bipolar disorder. The role of cannabis use in depression, anxiety, and suicidality was less certain because associations were modest and could reflect self-medication, shared risk factors, or bidirectional relationships. A high priority should be given to developing effective interventions for cannabis use disorders in people with mental disorders.