Cannabis and tobacco are two of the most commonly used substances worldwide. A substantial body of evidence documents an association between cannabis use and increased risk of psychotic and affective disorders. Tobacco use has also been associated with increased risk of several psychiatric outcomes. Co-use of cannabis and tobacco is a common practice, comprising concurrent co-use (i.e. use of both products in a pre-defined time-period) and co-administration (i.e. used simultaneously within the same delivery method). The high degree of overlap between these substances, and insufficient measurement in existing research, introduces complexity in accurately assessing how these substances impact on risk of subsequent mental illness. Therefore, it is possible that unmeasured tobacco exposure has confounded observed associations between cannabis and multiple psychiatric outcomes. Furthermore, the potential impact of co-use for mental health is an underexplored area. Studying populations where cannabis and tobacco are typically not administered together (e.g. USA) compared to populations where co-administration is common practice (e.g. Europe), offers an avenue through which to explore this issue. This project will explore the individual and combined roles of cannabis and tobacco in mental illness, through triangulating data from countries with differing cannabis-tobacco co-administration profiles. To further strengthen basis for causal inference, this project will also triangulate evidence across statistical and design-based approaches for studying causal effects.