Executive summary: 

Background: Cannabis and tobacco use frequently co-occur. Adolescents who consume both substances experience more respiratory distress and psychosocial problems and are less likely to stop compared with those who use either tobacco or cannabis alone. This study examined time trends in tobacco and cannabis use among 15-year-olds in Europe and North America between 2002 and 2010. Methods:Twenty-eight countries were included and merged into six regions based on their welfare systems. Adolescents (n = 142 796) were divided in four ‘user groups’: ‘no-users’, ‘tobacco and cannabis users’, ‘tobacco-only users’ and ‘cannabis-only users’. Prevalence rates are reported by study-wave and region. Logistic regressions with study wave as independent variable were used to study trends in the user groups and regions. Interaction effects between time and gender were considered. Results: Overall, tobacco use, and concurrent tobacco and cannabis use decreased by 3 and 3.7%, respectively, but prevalence rates varied by region. Only in North America, an...