A longitudinal study of the effects of adolescent cannabis use on high school completion.

OBJECTIVE: To examine the extent to which weekly cannabis use during mid-adolescence may increase the risk of early school-leaving.

SETTING: A prospective study of a general population sample of adolescents studied from ages 15-21 years in Melbourne, Australia.

METHOD: Computer-assisted self-completion questionnaires and telephone interviews conducted in six waves at ages 15-18 and again at age 21 in a sample of 1601 male and female school students.

RESULTS: Weekly cannabis use, assessed prospectively, was associated with significantly increased risk of early school-leaving. This effect remained after adjustment for a range of prospectively assessed covariates including demographic characteristics, other substance use, psychiatric morbidity and antisocial behavior. There was suggestive evidence of an interaction between weekly cannabis use and age with the effects of weekly cannabis use on early school-leaving being strongest at the youngest ages and diminishing progressively with age.

CONCLUSIONS: Early regular cannabis use (weekly use at age 15) is associated with increased risk of early school-leaving. These effects of regular cannabis use may diminish with increasing age and are likely to operate through the social context within which cannabis is used and obtained.