Cannabis use and dependence in a New Zealand birth cohort.

AIM To describe the development of cannabis use in a birth cohort studied to the age of 21 years. METHODS The data were gathered during the Christchurch Health and Development Study. In this study a cohort of 1265 children born in the Christchurch urban region in mid 1997 has been studied to the age of 21 years. Information was gathered on patterns of cannabis use and dependence during the period from fifteen to 21 years. RESULTS By age 21 years, 69% of the cohort had used cannabis. The extent varied widely with 24% reporting use of cannabis on less than ten occasions while 5% reported use on over 400 occasions. Over 9% of the cohort met DSM-IV criteria for cannabis dependence by the age of 21 years. Cannabis use and dependence were higher in males and in Maori. Key predictors were gender and measures of adolescent risk taking behaviours including cigarette smoking, adolescent conduct problems, affiliations with delinquent peers and novelty seeking. CONCLUSIONS Levels of cannabis use in this cohort reached a point where it was considered 'normal'. For the majority of users cannabis use did not lead to problems of dependence. Nonetheless, nearly 10% of the cohort showed clear symptoms of cannabis dependence by the age of 21 years, especially males who were prone to other forms of risk taking behaviours.