Tags: Cannabis; alcohol; substitution; complementarity

 

Thumbnail: This report summarizes the available literature regarding the effects of alcohol and cannabis; whether alcohol and cannabis complement or substitute for each other. Overall, the study finds stronger evidence for substitution, rather than complementarity, though effects may be different in different populations.

 

The problem: The question of whether or not the effects of alcohol complement or substitute for the effects of cannabis, has been a matter of debate for some time. As cannabis policy changes, it is important to consider the potential for spill-over effects on other substances of interest. Some previous reviews have found that legalization of cannabis leads to increased use of cannabis in combination with other substances. Alcohol has long been recognized as a substance with the potential for significant harm, therefore it is important to understand the potential effects of cannabis use in combination with alcohol.

 

The study: This review replicates previous literature compilations and includes new data from animal studies. 65 articles were included in this review; 64 of which were studies in humans and one reported a study in animals. The analysis of these articles evaluated findings in several categories, including patterns of use, substitution practices, substance use disorders, policy evaluation and animal studies. This review assessed studies cannabis and alcohol use with the primary goal of evaluating evidence of people substituting one substance for the other or complementary (combined) use of the two.

 

Main Points:

• 30 studies found evidence for substituting one substance for the other

• 17 studies found evidence of complementary use of the two

• 14 did not find evidence for either substitution or complementarity

• Four studies found evidence for both substitution and complementary use

• Studies that looked at the impact of legal policy found that increased legal drinking age was associated with decreased alcohol use but increased cannabis use. Separately, turning 21 (and being able to legally purchase alcohol) was linked to decreased cannabis use but increased alcohol use

• Differences between complementary and substitution use appear to vary by age, as studies in different age cohorts report different findings

• Among former alcoholics or light drinkers, studies found increased alcohol consumption when these people abstained from cannabis use. One study found that recovering alcoholics were more likely to relapse when using cannabis

• Studies looking at the impact of decriminalization on substitution found different results, depending on the population being studied

• Some studies show that increasing the price of alcohol can reduce both alcohol and cannabis use in adolescents

• One animal study suggested that cannabinoids might have potential use in the treatment of alcohol use disorders

 

Conclusions: This analysis finds mixed support for substitution and complementary use of alcohol and cannabis, however it finds stronger support for substitution (however this may be a result of bias in the studies, as most were conducted with the goal of researching substitution in the context of policy changes).

 

Why this is a good study:

• This study included a large number of papers, forming a broad view of the topic and allowing us to see what areas are understudied

• Unlike previous reviews, this study included a controlled animal study, which ads a new prospective to the field

 

Why this isn’t a perfect study:

• As a retrospective review of existing studies, the overall results were at the mercy of any flaws in how those studies were conducted

• Most of these studies looked at usage in the context of policies (either alcohol or cannabis), leaving gaps in other areas

• Many of the available studies were poor quality (based on population size, sampling bias, etc), making it difficult to draw strong conclusions

• Most studies were based on self-reported usage, which is often biased and less reliable

• Most studies measured outcomes differently, making direct comparisons less reliable

• Evidence from animal studies is scarce, so the conclusions here are not as strongly supported as the human data

 

What this study adds:

• This study highlights areas where were need more research, particularly in the areas surrounding the more recent, and rapid, changes in cannabis policy

• This is the first review to include animal studies, and suggests that cannabinoids might have a potential role in the treatment of alcohol-related disorders

 

What it doesn’t:

• This study does not tell us much about the preference of users for substitution or complementary use, in the absence of a policy change to drive their decision.

 

Funder: None stated

 

Author conflicts: Author SB works at the Institute of Alcohol Studies which receives funding from the Alliance House Foundation. The other authors declare no conflicts of interest.