Cannabis, Tobacco and Mental Health: Isolating Their Relationship Through Triangulation of Evidence

: (Alternative Format Thesis)

Abstract

Cannabis and tobacco are two of the most commonly used psychoactive drugs worldwide, and observational studies suggest both are associated with higher risk of developing a mental health condition. It has been difficult to determine if these relationships are, in part, causal due to observational data biases such as reverse causation and confounding. Disentangling any independent causal effects is further complicated by the high prevalence of concurrent use of both substances and the simultaneous use or ‘co-administration’ of these substances, the latter of which is widespread in European contexts.

In this thesis, the causal effects of cannabis use, and tobacco use on risk of mental health outcomes are examined, with a focus on Major Depressive Disorder (MDD). This thesis adopts an aetiological triangulation of evidence approach to support more robust causal inference, examining these effects using published observational analyses (i.e., systematic review and meta-analysis; Chapter 2), a cross-contextual design using data from three longitudinal cohorts (i.e., Avon Longitudinal Study of Parents and Children [ALSPAC], Longitudinal Study of Australian Children [LSAC] and National Study of Adolescent and Adult Health [Add Health]; Chapter 3), and genetically informative methods (i.e., Mendelian randomization; Chapter 4 and Chapter 5).

Across the empirical studies conducted, there was stronger evidence for a causal effect of tobacco use on risk of MDD. Results suggest that the mechanism through which tobacco smoking affects MDD, does not appear to be caused via nicotine exposure; although future triangulation of evidence is required. There was weaker evidence to support a causal effect of cannabis use on risk of MDD. Importantly, differences in evidence strength across substances could be explained by various methodological considerations rather than true differences in causal effects.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Tom Freeman (Supervisor), Gemma Taylor (Supervisor), Robyn Wootton (Supervisor) & Hannah Sallis (Supervisor)