Introduction Understanding the prevalence of cannabis use among tobacco smokers has important implications for research in terms of intervention effectiveness and measurement in smoking cessation trials. The co-use of these substances also has important implications for health service planning, specifically ensuring appropriate and adequate clinical treatment. To date, there have been no synthesis of the literature on the prevalence of tobacco and cannabis co-use in adult clinical populations. Improved understanding of the current prevalence, route of administration and specific subpopulations with the highest rates of tobacco and cannabis co-use will support future intervention development. We aim to provide a pooled estimate of the percentage of smokers who report using cannabis and to examine the prevalence of co-use by sociodemographic characteristics.
Methods and analysis We will conduct a systematic review using six scientific databases with published articles from 2000 to 2022 inclusive (CENTRAL, CINAHL, EMBASE, Medline, PsycINFO, Psychology and Behavioural Sciences Collection, Scopus). Peer-reviewed journal articles published in English that report on tobacco and cannabis use will be included. Rates of co-use (simultaneous or sequentially) and routes of administration will be assessed. Use in populations groups will be described. Quality assessments will be conducted for all included studies. Data will be synthesised using a narrative approach. This study will be conducted from June 2022 to the end of August 2022.
Ethics and dissemination This review is based on previously published data and, therefore, ethical approval or written informed consent will not be required. It is the intention of the research team to disseminate the results of the systematic review as a peer-reviewed publication and conference presentations.
PROSPERO registration number CRD42020194051.
- public health
- substance misuse
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Strengths and limitations of this study
Using the gold standard or meta-analysis, the proposed review will collate multiple studies to provide the first pooled prevalence of tobacco and cannabis co-use.
This review will also provide important information on the subgroups that require targeted intervention.
There are several limitations that need to be noted, the first, is publication bias and the second is that if the included studies measures are too heterogeneous, we will not be able to complete a meta-analysis.
Tobacco smoking continues to be a leading cause of preventable disease and death. An estimated 1.1 billion people continue to smoke tobacco. Over the past three decades, the prevalence of tobacco smoking has been increasing in low-income and middle-income countries (LMICs)1 where an inverse relationship is seen in high-income countries (HICs) where significant declines are noted. For example, in LMIC, the adult smoking prevalence rate has increased from 12.4% to 22.8% in Rwanda, from 33.8% to 39.5% in Indonesia, and from 16% to 40.4% in Zambia.2 While in HIC, the tobacco smoking prevalence has reduced from 30.1% to 13.7% in the USA,3 from 31% to 14.0% in Australia,4 30% to 14.2% in New Zealand5 and 33% to 14.1% in the UK.6 The decline in the prevalence of tobacco smoking appears to be less apparent among specific subpopulations in HIC.7 People who have a substance use disorder or are in treatment for substance misuse,8 people with a severe mental illness,9 people who are homeless or at risk of homelessness10 are found to have smoking rates 2–6 times higher than the general population.
Cannabis is another commonly used substance.11 Global estimates suggest that the number of cannabis users has increased in many countries, as has the treatment for cannabis use disorder.12 Changing regulatory environments and the legalisation of cannabis in some countries, and decreasing perceptions of risk associated with cannabis use,13 14 have been viewed to contribute to this recently increase in use.15–17 Other contributors to the increased use of cannabis include sociodemographic and environmental risk factors,18 including the COVID-19 pandemic.19 These factors may differ in between LMIC compared with HICs yet no review to date has examine this relationship.
Co-use can refer to concurrent use of both tobacco and cannabis, individually (sequentially), and to coadministration, or the use of the two substances at the same time, for example, cannabis and tobacco leaf mixed within roll-your-own cigarettes (concurrent).20 Patterns of tobacco and cannabis co-use appear to differ by country. A 2018 International Tobacco Control Survey identified that Canada, followed by the USA (29.1%), England (21.6%) and Australia (21.4%) had the highest rates of tobacco and cannabis co-use.21 There are limited studies examining tobacco and cannabis co-use in LMIC. Tobacco and cannabis co-use is often missed in population prevalence surveys.13
Previous reviews on tobacco and cannabis co-use have examined mechanisms of initiation22 23 toxicant exposure,24 and coadministered products (such as blunts or spliffs).25 While other reviews have focused specifically on certain populations such as adolescents and young adults.26 Lemrye et al27 in their review, detail the motivation and drivers of co-use including gateway theory and reverse gateway which may have implications in understanding co-use and interventions for cessation.
A recent review did explore treatment implications for those that co-use tobacco and cannabis,28 highlighting the challenges of single substance cessation and noting that interventions to address co-use cessation are lacking. Another, important gap in the growing literature is a comprehensive pooled prevalence estimate of tobacco and cannabis co-use. Synthesising existing data including correlates of use29 is important to understand the extent to which tobacco and cannabis co-use is occurring.
There appears to be an increased physiological effect of both substance when used together. The mechanisms underlying the co-use of tobacco and cannabis have been identified as shared genetic factors, environmental factors (peer influences; availability; younger age), and economic factors (lower socio-economic factors).22 More recently, another possible factor influencing co-use of tobacco and cannabis is the common route of administration, for example, smoking/inhalation of both substances.20 To date, only one review has examined route of administration and only focused on combustible forms of tobacco and cannabis co-use.25 A synthesis of the literature that includes all possible routes of administration would fill this gap in the literature.
Compared with other substances, cannabis use among tobacco smokers appears to be more common than the co-use of other substances that occur at a remarkably reduced rate such as alcohol (33.3%–45.7%), cocaine (37.5%–42.9%), stimulants (30%–51.7%) and hallucinogens (35.6%–41.7%).30–32 The relationship between tobacco and cannabis use is synergistic in that tobacco use increases cannabis dependence symptoms20 33 and precipitates cannabis relapse34 and similarly cannabis use increases the likelihood of nicotine dependence35 and decreases tobacco cessation.36 Given this, people who co-use both tobacco and cannabis are at greater risk for serious for serious health and psychosocial problems.23 Co-use is associated with increased risks of toxicant exposure, poorer physical and mental functioning.25
Understanding and accounting for current cannabis use among participants involved in smoking cessation research is important as it may have an impact on the intervention effect but also the measurement effect. If smokers are recruited into a smoking cessation intervention but their cannabis use is not addressed, then relapse to tobacco smoking is likely if they regularly mixed their tobacco with cannabis. Similarly, since cannabis use is detected in carbon monoxide breath analysis, this method of biochemical verification of tobacco smoking status may be inaccurate. Therefore, an alternate biochemical method such as serum or salivary cotinine may be preferable for self-reported tobacco smokers who use cannabis.37
Only limited research has described the demographic and clinical characteristics of people who use both tobacco and cannabis. These studies predominately focused on young people who use these substances. Demographic characteristics such as older adolescents38 39 another found that people who co-use tobacco and cannabis were younger than tobacco-only users,26 of male gender and ethnicity. A synthesis of the current literature is critical for the development of an evidence base foundation for developing future randomised controlled trials. This review will provide the first synthesis of the literature and provide pivotal information for specific countries to influence the development of socially and culturally appropriate interventions to further the evidence base.
There is a clear need for an improved understanding as to the co-use of tobacco and cannabis use as this will have practical implications in the design of smoking cessation studies. The tiered approach of assessing co-use as explained by Hindocha and McClure40 suggests that consideration of individual-use data as well as co-use data be explored as often there is a lack of nuance in the current methods of collection and analysis. Our review will ask the following questions. (1) What is the percentage of smokers who report also using cannabis; (2) What is the nature of co-use of tobacco and cannabis (eg, mixing together or smoking at different times, including routes of administration) and (3) What is the prevalence of co-use tobacco and cannabis by sociodemographic characteristics (age; gender; country—low and high income; clinical characteristics such as mental health diagnosis and substance use disorder).
We will complete a systematic review examining the prevalence of cannabis use among tobacco smokers generally and by specific sociodemographic and clinical characteristics as well as the nature of the use of these substances (whether concurrently or individually). This review will be conducted in line with the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines with data managed using Stata 15 Statistical Software.41
We will include the following study designs: cohort, cross-sectional, observational, baseline studies, national reports. We will exclude published editorials, letters or conference proceedings (including abstracts), qualitative studies, thesis dissertations and studies quoting the incidence rate rather than prevalence. Further, we will not use genetic epidemiological studies that report prevalence estimates in family members of individuals who use tobacco and cannabis. This study will be conducted from June 2022 to the end of August 2022.
Study populations and participants
Person-level data will be included from any adult population (aged >18 years and older) including but not limited to educational populations (eg, university populations), forensic or correctional populations, treatment seeking populations (people receiving treatment for substance use disorders or mental health conditions, oncological conditions and other chronic diseases). We will include all routes of administration of tobacco use including alternate nicotine devices such as vaporisers.
We will include studies that report primary data on the prevalence of tobacco and cannabis use (such as total number of participants and percentages or proportions). Given that there is no standard assessment tool of tobacco and cannabis co-use we will include all measures as documented in the literature. For example, separate items that measure cannabis and tobacco use: ‘Do you currently smoke any tobacco products?’ with response options (1) yes, at least once a week; (2) yes, less often than once week; (3) no, not at all; ‘During the past 30 days, on how many days did you use cannabis?’; ‘How many times is cannabis used per day on using days’. Co-use has been defined in several recent US studies as the use of both substances within the past 30 days.
Additional eligibility criteria
Only articles that are published in English and that have been published in peer reviewed journals will be included.9
Ethics and dissemination
This review is based on previously published data and therefore ethical approval or written informed consent will not be required. It is the intention of the research team to disseminate the results of the systematic review as a peer reviewed publication and conference presentations.
The search will be conducted in six databases with published articles from 2000 to 2022 inclusive: CENTRAL, CINAHL, EMBASE, Medline, PsycINFO, Psychology and Behavioural Sciences Collection, Scopus, using keywords consistent across all databases and Medical Subject Headings (MeSH) applicable to specific databases. The Medline search terms are outlined in table 1. Each search term is related to an overarching theme that map largely to the review aims (tobacco use and cannabis use). It should be noted that the search terms have been developed with guidance from the Cochrane Public Health and Tobacco Addiction Group search terms.
Two research assistants (AD, AL) will screen all titles and abstracts using the previously described inclusion criteria overseen by the research academics (ES, JR, TH). Any disagreements will be discussed. If a resolution cannot be found, the senior review author (BB) will hold a discussion until a resolution is found. The two RAs (AD, AL) completing 50% each. Again, any disagreements will be discussed between the authors and the research assistants. All title, abstract, and full-text data screening will be completed using the Covidence online software V.1919 73d6c782.
The team will develop a data extraction form based on the aims of the study. The data extraction form will include: country, year of publication, author, sample size, gender, tobacco use prevalence (lifetime, last 12 months, current), cannabis use prevalence (lifetime, last 12 months, current), co-use prevalence and routes of administration will be extracted for each paper. The co-use prevalence and calculated 95% CIs for each estimate will be extracted. For papers reporting measures over time, we will use the prevalence data from the baseline assessment. Two post-doctoral research academics (ES and JR) will extract data from all included publications. Any disagreements will be discussed between all authors. If a resolution cannot be found, the senior review author (BB) will be consulted in order to determine a resolution. Data will be extracted using the Covidence software.
All cohort, cross-sectional and case–control studies will be appraised according to Newcastle-Ottawa Quality Assessment Scale for cohort and case–control studies (adapted for cross-sectional studies). Using the tool each study is judged on eight items, that can be categorised into three further groups: (1) The selection of the study groups, (2) The comparability of the groups and (3) The ascertainment of either the exposure or outcome of interest for case–control or cross-sectional studies respectively. Articles will not be excluded based on their score on the quality appraisal tool. Quality scores will guide judgement of the methodological quality of the trial and reliability of the findings.
The risk of bias (quality) of all studies meeting the inclusion criteria will be formally assessed by ES, JR and TH. Disagreements will be resolved by discussion with the senior review author (BB). Ratings will be presented in a table and will be used to inform the narrative synthesis.
Assessment of study heterogeneity
Heterogeneity will be examined using visual inspection of box plots, forest plots and using the I2 statistic, Egger’s test will be applied.42 Where there is evidence of high heterogeneity (I2 >75%), heterogeneity will be explored via subgroup analyses according to population characteristics. If the heterogeneity among the studies is large, a random-effects model will be used to calculate the pooled OR. Otherwise, a fixed-effects model will be applied. We will use a generalised linear model approach, due to methodological concerns with the Freeman-Tukey double-arcsine transformation method.43 44 Funnel plots will be generated by statistical software to enable the assessment of publication bias. If publication bias is detected the approximate number of missing studies using the trim-and-fill method45 will be conducted to provide insight into degree of bias.
Grading the strength of evidence
As recommended by the Cochrane Handbook for Systematic Reviews of Interventions, the overall quality of evidence of the outcomes (tobacco and cannabis co-use prevalence) will be presented using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. This involves a within-study risk of bias (methodological quality), directness of evidence, heterogeneity, precision of effect estimates and risk of publication bias. The overall quality of evidence will be rated at four levels: high, moderate, low and very low. If feasible, the analysis could include individual participant-level meta-analysis.
ES and JR will assess the quality of evidence using the GRADE tool and will present the findings to the fellow authors. The team will discuss the assessment and modify the reported strength of recommendations.
Depending on the outcome measure employed by studies, pooled estimates by lifetime use, 12 months and current use will be calculated. If studies have used similar definitions and measures, cannabis prevalence plots including the associated confidence intervals for each study will be presented. If there are national prevalence estimates for cannabis use, we will examine the difference between the study and the national prevalence.
In order to obtain the national smoking prevalence rates, official country statistics websites and the WHO Health Observatory Data Repository46 will be consulted and the prevalence data extracted. If the national cannabis prevalence estimates are not available for all included countries for all years in which included studies were completed, we will report this information descriptively in the results section.
Included studies will be examined by sociodemographic and clinical characteristics to provide individual prevalence estimates and distributions. These will include sociodemographic characteristics such as age, gender, country, nationality and indigeneity. These will also include clinical characteristics such as mental health diagnosis as per the clinical diagnostic handbooks and tools (such as the Diagnostic and Statistical Manual of mental disorders47 or the International Classification of Disease (ICD)48) including substance use disorders and clinical populations such as individuals receiving treatment from addiction treatment or mental health services, as well as those with cardiovascular disease, diabetes or cancer.
Patient and public involvement
No patients involved.
At the time of submission of this protocol, the authors had completed screening of titles, abstracts and full texts, and were beginning data extraction and quality assessment.
The co-use of tobacco and cannabis and the subpopulation who are most likely to co-use these substances are important considerations for intervention planning. This systematic review will provide pooled prevalence estimates of cannabis use among tobacco smokers. This study will report on tobacco and cannabis co-use by specific sociodemographics and clinical characteristics to provide a more in-depth examination and synthesis of the available data. This study will also provide the routes of administration of tobacco and cannabis co-use as this is an important aspect when developing behavioural interventions for cessation. As with all research, naturally there are limitations to be considered. First, is publication bias, it is likely that given the rigorous nature of predetermined search strategies some publications may be missed from this review. Second, is that if the included studies measures are too heterogeneous, a complete a meta-analysis may not be feasible. However, in using the gold standard of meta-analysis, the proposed review will collate multiple studies to provide the first pooled prevalence of tobacco and cannabis co-use. This review will also provide important information on the subgroups that require targeted intervention. As such, the benefits of this review will outweigh any such methodological limitations.
Patient consent for publication
Research Assistance thanks to Alexandra Denham and Alexie Lye for their database assistance and screening abstract reviews.
Contributors ES, JR and TH developed the protocol and drafted the manuscript. BB reviewed drafts and provided methodological feedback and guidance. All authors approved the final manuscript.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review Not commissioned; externally peer reviewed.