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).