This briefing was based on a roundtable meeting hosted by the Institute of Alcohol Studies on 29th June 2023. At the roundtable, Dr Elle Wadsworth presented research on alcohol and cannabis co-use in Canada and the United States immediately before and one year after legalisation of non-medical cannabis in Canada in 2018, and Dr Sadie Boniface facilitated an informal discussion on research gaps and priorities in this area. Attendees included academics and civil society organisations.

Background
Cannabis use is increasingly liberalised across the world. At the time of writing, non-medical cannabis is legalised in 23 US states and the District of Columbia, Canada, Uruguay, among others. In Europe, Luxembourg and Malta have moved to legalisation of non-medical cannabis use. More recently, Germany’s government has published a draft bill to legalise the use of cannabis for personal use, which could trigger other countries in Europe to pursue cannabis legalisation (Sabaghi, 2023).

People who use cannabis are likely to use alcohol too but measuring and understanding cannabis and alcohol co-use is complex. Previous literature has focused on understanding whether cannabis and alcohol are ‘substitute’ or ‘complementary’ substances for each other. In addition, cannabis and alcohol co-use can also be defined as simultaneous co-use or use of both substances but not necessarily at the same time, for example, over the course of a typical month.

Developing an understanding of what has happened in jurisdictions where non-medical cannabis has been legalised, such as Canada and the United States, can provide lessons on the impacts and outcomes of legalisation for alcohol use and harm, and provide lessons for alcohol policy.

Case study: Co-use of cannabis and alcohol before and after Canada legalised non- medical cannabis in 2018 (Hobin et al., 2023)

Cannabis and alcohol are two of the most commonly used psychoactive substances in Canada and the United States, with similar patterns observed in the UK. An earlier study reported an increase in co-use of alcohol and cannabis in US states where non-medical cannabis was legal compared to US states where it was not legalised (Kim et al., 2021). Another study reported a small increase in co-use after the legal cannabis market opened in Washington State (Subbaraman & Kerr, 2020).

In this study, co-use was defined as monthly or more frequent use of both cannabis and alcohol. The study used repeated cross-sectional survey data from the International Cannabis Policy Study (ICPS) to examine whether non-medical cannabis legalisation in Canada was associated with changes in regular co-use of cannabis and alcohol relative to US states that had and had not legalised non-medical cannabis. The ICPS survey provides detailed population-based data using policy-specific measures across multiple countries.

The results showed an increased in cannabis use and co-use of alcohol and cannabis between 2018 and 2019 in Canada and the US. There was no evidence to suggest differences in patterns of co-use in this period between Canada and comparator US states where cannabis was legal or illegal. The observed increases in co-use between 2018 and 2019 were largely due to increases in cannabis use across the population, including in people who used alcohol.

Changes in Public Opinion, Patterns of Use, and Cannabis and Alcohol Co-Administration
In the US, with changes towards the liberalisation of cannabis policies, studies suggest people are becoming more supportive of non-medical cannabis legalisation (Chiu et al., 2021). There has been a decline in perceived risk of cannabis use, and a reduction in the price of products, accompanied by an increase in the potency of products available (Chiu et al., 2021). In Canada, the objective of the of legalisation were “to prevent young persons from accessing cannabis, to protect public health and public safety by establishing strict product safety and product quality requirements and to deter criminal activity by imposing serious criminal penalties for those operating outside the legal framework” (Cannabis Act, 2018). Following this, cannabis legalisation has been associated with a reduction in cannabis-related arrests, as well as increases in prevalence of cannabis use in young adults but not in high school students (Hall et al., 2023). Legalisation has also resulted in increased access to a variety of higher potency products at lower prices (Hall et al., 2023). In the US non-medical cannabis was first legalised in Colorado and Washington in 2012, and in Canada non-medical cannabis was legalised in 2018. However, it is still too soon to fully understand the effects on public health of different regulatory approaches.

The impact on alcohol use and harm should form part of any evaluation following changes to the legal status of cannabis. This could include ensuring surveys, e-health records, and analytical capacity are sufficient to monitor emerging trends, as well as measuring the impact on public health and inequalities.

Psychopharmacological studies suggest simultaneous consumption of alcohol and cannabis might result in increased levels of Delta-9-Tetrahydrocannabinol (THC) in the blood, the main psychoactive component in cannabis (Hartman et al., 2015). Simultaneous consumption has also been associated with negative additive effects, such as poorer driving performances when using both substances (Downey et al., 2013).

The public health impact of potential changes in simultaneous co-use is an unexplored area of research by population-based studies, and further research could add to conversations around policy considerations for alcohol and cannabis use.

Impact of Cannabis Legalisation on Drug and Alcohol Services
Most treatment services in the UK are combined alcohol and drug services. Cannabis regulation could potentially have an impact on treatment services by increasing access to and use of cannabis and, as a result, increasing the prevalence of cannabis use disorder (CUD). CUD is a clinical diagnosis broadly defined as the continued use of cannabis despite clinically significant impairment (American Psychiatry Association, 2013). A higher prevalence of CUD and more people seeking treatment could put extra pressure on treatment providers, in an already stretched system.

The evidence so far from other countries does not seem to suggest an increase in CUD following legalisation. For example, US states that have legalised non-medical cannabis has not seen a significant rise in people seeking treatment for CUD following legalisation of cannabis (Aletraris et al., 2023; Mennis et al., 2023). However, it is worth noting that changes in rates of CUD may not come immediately after legalisation. In the UK where cannabis is illegal, rates of cannabis use has remained fairly stable while rates of treatment seeking for cannabis use problems have increased significantly over the last decade (Manthey, 2019). This shows that the prevalence of CUD and treatment seeking behaviour is impacted by a wider range of factors, such as the strength of the cannabis (Freeman, 2019), prompting proposals to cap THC content in cannabis or tax cannabis products based on THC content (Hall et al., 2023).

Further research is needed to understand the impact that different cannabis use patterns and legalisation models can have on health outcomes and substance use treatment systems. These are important considerations when informing public health policies seeking to regulate cannabis products......