Cannabis use and mental health

Cannabis use has been implicated as a risk factor for mental health problems, (subclinical) psychotic symptoms in particular. If cannabis use was a cause of these problems, cessation would lead to improved public mental health. If cannabis use was a mere consequence of a predisposition for mental health problems, cessation would not result in less problems. Prevention and intervention strategies aimed at a better public mental health would then do better to incorporate cannabis use in screening instrument, where it could serve as a ‘red flag’, calling for raised awareness. This thesis investigated the association of cannabis use with mental health problems in general, and psychotic symptoms in specific. This association was studied in young adolescents (12-16 years) and young adults (18-30 years). Cannabis use and mental health problems in general The result of studies investigating the association between cannabis use and mental health problems indicate that cannabis use should be viewed as an indicator of risk for mental health problems. For young adolescents, the relationship was explained by common risk factors for both cannabis use and poor psychosocial functioning, including use of other substances, frequent truancy, an unfavourable school evaluation and frequent absence. For young adults, the association was independent of age at onset of cannabis use. Also, young women who used cannabis had a higher risk of mental health problems than men who did. Cannabis use and psychotic-like experiences The results of this thesis imply that age at onset and frequency of use are crucial in elucidating the nature of the relationship between cannabis use and psychotic-like experiences (PLE). Moderate cannabis use (monthly or less) and use that started no earlier than 15 years of age were associated with PLEs, but results indicate that this is most likely the result of pre-existing vulnerability. This is illustrated by the finding that cigarette smoking and cannabis use are equally strong predictors of PLEs in young adults. Early and heavy use however, do seem to causally increase the risk of PLEs. Young adolescents who had started cannabis use at an early age but who had ceased it completely for at least a year, reported as many PLEs as adolescents who used cannabis heavily at the time of the survey. Also, young adults who started to use cannabis before the age of 12 years had a threefold risk of PLE compared to those who first used cannabis after the age of 15. Furthermore, a dose-response relationship between cannabis use and PLEs was found for young adults: participants who used cannabis daily had a threefold increased risk of PLEs and those who used every other day a twofold risk. Taken together, the results suggest that mentally vulnerable individuals are more inclined towards cannabis use. This implicates that cannabis use per se is best viewed as an indicator of risk for mental health. However, the results also point towards additionally increased psychosis proneness when cannabis use is initiated before the age of 15 years or the frequency of use exceeds once per week