Tobacco and cannabis use are key public health concerns. Both tobacco and cannabis are consistently associated with the development of common mental health disorders such as anxiety and depression. Prevalence of mental illness in the UK is rapidly increasing, and has increased particularly among adolescents and young adults. Tobacco and cannabis use appear to be plausible targets for intervention to improve mental health. However, there are a number of challenges that have hindered understanding of this relationship, and which have implications for how interventions should be targeted. Firstly, substance use and common mental disorders both tend to have onset in adolescence, so it is difficult to establish temporality in this relationship. Secondly, substance use and mental health share common risk factors through genes and early-life events, so it is difficult to establish whether the relationship is causal or due to shared confounding. Thirdly, the effect of substance use on mental health may not be a direct effect. It is plausible that this relationship is mediated by the social and economic adult role transitions following adolescence, or that substance use itself may be a mediating factor between early life exposures and mental health. Finally, the effects of the aforementioned changes in tobacco and cannabis products on substance use patterns and mental health are currently unknown.

We propose to investigate key questions regarding causality and opportunities for intervention in the relationship between tobacco use, cannabis use (and their constituents) and mental health.