Illicit drug use and dependence in a New Zealand birth cohort

Objective: To describe the patterns of illicit drug use in a birth cohort studied to the age of 25 years. Method: The data were gathered during the Christchurch Health and Development Study. In this study a cohort of 1265 children born in the Christchurch, New Zealand urban region in mid-1977 have been studied to the age of 25 years. Information was gathered on patterns of illicit drug use and dependence during the period 15-25 years. Results: By age 25 years, 76.7% of the cohort had used cannabis, while 43.5% had used other illicit drugs on at least one occasion. In addition, 12.5% of the cohort met DSM-IV criteria for dependence on cannabis, and 3.6% of the cohort met criteria for dependence on other illicit drugs at some time by age 25. There was also evidence of substantial poly-drug use among the cohort, with hallucinogens and amphetamines being the most commonly used illicit drugs (excluding cannabis). Illicit drug use and dependence was higher in males, in Maori, and in those leaving school without qualifications. Key risk factors for illicit drug use and dependence included adolescent risk-taking behaviours including cigarette smoking and alcohol consumption, affiliation with substance-using peers, novelty-seeking, and conduct problems in adolescence. Other key risk factors included parental history of illicit drug use and childhood sexual abuse. Conclusions: Levels of cumulative illicit drug use in this cohort were relatively high, with the majority of respondents having tried illicit drugs by age 25. For the majority of illicit drug users, drug use did not lead to problems of dependence. Nonetheless, nearly 15% of the cohort showed symptoms of illicit drug dependence by the age of 25 years, with cannabis dependence accounting for the majority of illicit drug dependence.