Developing the evidence base to address tobacco and cannabis co-use amongst young adults

  • Hannah Walsh

Student thesis: Doctoral ThesisDoctor of Philosophy


Background: Tobacco is the leading cause of preventable death, and cannabis the most frequently used illicit substance worldwide; the two substances are commonly co-used, but rarely co-treated. Co-use appears to be associated with a higher degree of mental health disorders, poor psychosocial outcomes, and lower cessation outcomes compared to single substance use. Tobacco and cannabis are each complex products which are intrinsically linked through co-administration and to a lesser extent concurrent use, but reveal key differences including legality, harms, and treatment responses, which present unique challenges when considering how to address problematic or dependent use of both.

Aim: This work aims to develop the evidence base for an intervention to address tobacco and cannabis co-use.

Methods: Three studies were conducted; first, a systematic review of co-use interventions, which included single substance use interventions which measured both substances pre and post intervention. Uncontrolled studies were reviewed using narrative description, and treatment effects on tobacco and cannabis cessation and reduction were meta-analysed using Bayesian methods. Second, an online questionnaire survey of young adults in three further education settings was conducted, using the Capability, Opportunity and Motivation behaviour change model to structure the content. Detailed co-use, demographic and personal factors were collected, and regression analysis used to identify factors predictive of quitting behaviours. Third, a qualitative interview study with young adults recruited via the survey explored experiences of and attitudes towards quitting either or both tobacco and cannabis. Data were analysed using Framework method. Finally, results from all three studies were synthesised and indicated intervention opportunities presented.

Results: The systematic review included 20 studies, 11 of which were included in the meta-analysis. Only six co-use interventions have been developed or tested and none could be included in the meta-analysis, although they do appear feasible. Bayesian meta-analysis showed that treatment effects were not seen on tobacco cessation (RR 1.10, CrI [0.68, 1.87]) nor on cannabis cessation overall (RR 1.48, CrI [0.92, 2.49]), although in subgroup analysis by intervention target, a small significant effect was seen on cannabis cessation in multi-substance interventions (RR 2.19, CrI [1.10, 4.36]) but not in cannabis targeted interventions. More studies required to test the efficacy of co-use interventions and measurement of co-use requires further development and should be included in single substance interventions.

The survey showed that amongst a sample of n=141 young adult co-users, over three quarters (77%) reported experiencing a mental health problem. Most (75%) consumed cannabis in a joint with tobacco, and a third (33%) usually used high potency cannabis. Motivation to quit tobacco was much higher than for cannabis, and 60% had no intention to quit cannabis. Multivariable models of quitting behaviours indicated that being LGBTQ+ decreased the likelihood of engaging in tobacco quitting behaviour (OR 2.60, CI 1.14-5.93, p=0.023), as did living in a deprived area (OR 1.31, CI 1.09-1.57, p=0.004). No individual factors were predictive of cannabis quitting behaviour.

The qualitative interview study identified a range of influences on both use and quitting for tobacco, cannabis and co-use; 18 young adults were interviewed. Cannabis was considered both a source and salve of mental health problems, and a rich, two-way relationship was described. Family, peers and relationships played an important role in initiation, maintenance and supporting quit attempts for both tobacco and cannabis. Young adults practised cannabis harm reduction, including co-administration as a form of this, although sometimes based on erroneous beliefs; they sought information to inform this but found this lacking, so developed their own knowledge and understanding based on personal and peers’ experiences, and online sources. By contrast they had a broad understanding of tobacco-related harm. Findings from all three studies were synthesised, and where indicated, potential intervention opportunities were described.

Conclusions: The findings from this thesis make a unique contribution to the expanding literature on co-use, reaching beyond existing evidence on co-use prevalence and harms to consider how to address problematic and dependent use employing mixed methods. The findings demonstrate the complexity of the relationship, explore harm reduction methods and identifying additional risk factors which require further investigation. Opportunities to intervene are required and identified in this thesis; further research is required to develop acceptable and feasible interventions amongst young adults in the UK.
Date of Award1 Jun 2023
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorAnn McNeill (Supervisor) & Maria Duaso (Supervisor)

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