B3820 - Cannabis tobacco and mental illness isolating their relationship through triangulation of evidence - 23/06/2021

B number: 
Principal applicant name: 
Hannah Sallis | University of Bristol
Miss Chloe Burke, Dr Gemma Taylor, Dr Tom Freeman, Dr Robyn Wootton, Professor George Davey Smith
Title of project: 
Cannabis, tobacco and mental illness: isolating their relationship through triangulation of evidence
Proposal summary: 

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.

Impact of research: 
Cannabis is becoming increasingly liberalised as a medicinal and recreational drug. Accurately understanding the potential risks of cannabis use for mental health is crucial to developing evidence-based policies and preventative measures. The predicted impact of this research will be advancing our current understanding of the relationship between cannabis use, tobacco use and mental illness through employing innovative methods and robust approaches to causal inference. The findings from this research are expected to inform international cannabis and tobacco control policies. Integration of findings into policy and practice is being pursued through research group non-academic collaborations with influential panels.
Date proposal received: 
Tuesday, 22 June, 2021
Date proposal approved: 
Wednesday, 23 June, 2021
Mental health - Psychology, Psychiatry, Cognition, Addiction - e.g. alcohol, illicit drugs, smoking, gambling, etc., Mental health, Statistical methods, Mendelian randomisation, Mental health, Substance use