LAUSR

AIMS To examine adolescent cannabis use-both at national and individual levels-by deconstructing it into its necessary conditions of realistic use opportunities and willingness to use the drug given such opportunities. DESIGN Nationally representative, repeated cross-sectional survey. SETTING Norway. PARTICIPANTS A total of 8818 16-year-olds who participated in the European School Survey Project on Alcohol and Other Drugs (ESPAD) in 2007, 2011, and 2015. MEASUREMENTS Adolescent reports concerning their life-time 'cannabis use' and 'possibilities to use cannabis' were used to assess: (a) overall cannabis use, (b) exposure to realistic cannabis use opportunities, and (c) cannabis use among those exposed to use opportunities. Logistic regression models were used to estimate national trends since 2007 in these indicators, and to identify individual-level factors associated with cannabis use versus non-use among youth exposed to concrete use opportunities. FINDINGS Prevalence of life-time cannabis use remained stably low, averaging 6.4% across the three surveys. Life-time exposure to cannabis use opportunities decreased [odds ratio (OR)ESPAD assessment  = 0.90, 95% confidence interval (CI) = 0.84-0.97, P = 0.006], yet cannabis use among adolescents with such opportunities increased significantly (ORESPAD assessment  = 1.17, 95% CI = 1.03-1.34, P = 0.02) since 2007. After controlling for a range of other risk factors, abstinence from alcohol intoxication and cigarette use, as well as the perceptions of even minimal cannabis use as risky remained the factors most robustly associated with lower likelihood of cannabis use among youth with realistic use opportunities. CONCLUSIONS Approaches accounting for realistic use opportunities proved critical in our understanding of underage cannabis use, both at the national and individual level, and may be informative for development of prevention strategies in the era of cannabis liberalization. In addition, delineation of realistic opportunities from behaviours conditioned upon such opportunities is generalizable to a range of public health issues.