Background and aims In Great Britain, cannabis and tobacco are commonly used substances, both independently and together. Use of either substance is associated with mental health problems, but prevalence of co‐use within these populations is unknown. We aimed to (1) estimate prevalence of cannabis use, frequency of use and routes of administration (ROA) among tobacco smokers and non‐smokers and (2) investigate mental health problems among non‐users, tobacco‐only, cannabis‐only and co‐users of both substances. Design Cross‐sectional national on‐line survey (Action on Smoking and Health) fielded in February–March 2020. Setting Great Britain. Participants Adults in Great Britain aged ≥ 18 years (n = 12 809) Measurements Tobacco use status [smoker (daily or non‐daily) or non‐smoker (never or ex‐smoker)], cannabis use frequency (never to daily), detailed ROAs of cannabis, self‐reported treatment for mental health disorders (depression, anxiety and any). Statistically weighted prevalence estimates were computed to ensure representativeness. Correlates were assessed using χ2 tests and logistic regression. Findings In Great Britain in 2020, 7.1% of the sample had used cannabis in the past year. Tobacco smokers had greater odds of using cannabis in the past year (21.9%) and using cannabis daily (8.7%) than non‐smokers [past‐year: 4.7%; adjusted odds ratio (aOR) = 10.07, 95% confidence interval (CI) = 8.4–12.0; daily: 0.7%; aOR = 24.6, 95% CI = 18.0–33.6)]. Co‐administration with tobacco was common (46.2% of non‐smokers, 80.8% of tobacco smokers). Co‐users reported the highest prevalence of any treatment for mental health problems (54.2%) in comparison to cannabis‐only (45.8%), tobacco‐only (33.2%) and non‐users (22.7%; all P ≤ 0.05). Conclusion Approximately one in 13 adults in Great Britain reports having used cannabis in the past year, approximately four times as many among cigarette smokers as non‐smokers. Co‐administration of cannabis and tobacco, via smoking, appears to be common, including among self‐identified non‐smokers. Mental health problems appear to be particularly common among dual users.

Background and aims Tobacco and cannabis are commonly co‐used, and evidence for the influence of co‐use on quit outcomes for either substance is mixed. We sought to determine the efficacy of tobacco and/or cannabis use interventions delivered to co‐users on cannabis and tobacco use outcomes. Method Systematic review with meta‐analysis and narrative review, using five databases and author requests for co‐use data. Controlled and uncontrolled intervention studies focusing on treatment of tobacco and/or cannabis use assessing use of both pre‐ and post‐intervention were included. Prevention interventions were excluded. Bayesian meta‐analysis was used across four outcome measures: risk ratio for tobacco and cannabis cessation post‐intervention separately; standardized mean change for tobacco and cannabis reduction post‐intervention separately. Narrative reporting of the same outcome measures in non‐randomized clinical trials (non‐RCTs) and quality assessment of all included studies were conducted. Results Twenty studies (12 RCTs and eight uncontrolled) were included. Bayesian meta‐analysis with informative priors based on existing data of 11 RCTs (six single‐substance, five multi‐substance interventions) delivered to co‐users (n = up to 1117) showed weak evidence for an effect on cannabis cessation [risk ratio (RR) = 1.48, credibility interval (CrI) = 0.92, 2.49, eight studies] and no clear effect on tobacco cessation (RR = 1.10, CrI = 0.68, 1.87, nine studies). Subgroup analysis suggested that multi‐substance interventions might be more effective than cannabis‐targeted interventions on cannabis cessation (RR = 2.19, CrI = 1.10, 4.36 versus RR = 1.39, CrI = 0.75, 2.74). A significant intervention effect was observed on cannabis reduction (RR = 0.25, CrI = 0.03, 0.45, nine studies) but not on tobacco reduction (RR = 0.06, CrI = −0.11, 0.23, nine studies). Quality of evidence was moderate, although measurement of co‐use and cannabis use requires standardization. Uncontrolled studies targeting both cannabis and tobacco use indicated feasibility and acceptability. Conclusions Single and multi‐substance interventions addressing tobacco and/or cannabis have not shown a clear effect on either tobacco or cannabis cessation and reduction among co‐users. However, dual substance interventions targeting tobacco and cannabis appear feasible.

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Introduction Co-use of tobacco and cannabis is highly prevalent among cannabis users and is associated with poorer tobacco cessation outcomes. This study explored the barriers and enablers influencing stop smoking practitioners’ ability to provide optimal support to co-users. Methods Online semi-structured interviews were audio recorded. Interviewees (n = 20) were UK-based certified stop smoking practitioners. An interview schedule informed by the ‘capability’, ‘opportunity’, ‘motivation’ (COM-B) model was designed to explore participants’ perceived barriers and enablers in better supporting co-users to achieve abstinence of both substances or tobacco harm reduction. The transcripts were analysed using framework analysis. Results Capability: practitioners’ lack of knowledge and skills undermines their delivery of smoking cessation interventions to co-users. Interestingly, when cannabis is used for medicinal reasons, practitioners feel unable to provide adequate support. Opportunity: service recording systems play an important role in screening for co-use and supporting co-users. When responding to clients’ specific needs and practitioners’ uncertainties, a positive therapeutic relationship and a support network of peers and other healthcare professionals are needed. Motivation: supporting co-users is generally perceived as part of practitioners’ roles but there are concerns that co-users are less likely to successfully stop smoking. Conclusions Practitioners are willing to support co-users, but their lack of knowledge and access to an appropriate recording system are barriers to doing so. Having a supportive team and a positive therapeutic relationship are perceived as important. Identified barriers can be mostly addressed with further training to improve tobacco cessation outcomes for co-users. Implications Supporting cannabis-related abstinence or harm reduction among co-users constitutes an essential part of stop smoking practitioners’ role. In order to offer adequate support, practitioners need appropriate recording, referral systems, as well as comprehensive training. By prioritizing these measures, practitioners shoud be able to better assist co-users and improve tobacco cessation outcome.