Co-use of Alcohol and Cannabis: How Are They Related?

It is common for people who use alcohol also to use other drugs, increasing their risk for developing dependence on more than one substance. Heavy users of alcohol are likely to use and misuse other drugs such as nicotine, stimulants, and cannabis (1, 2), and cannabis use is a predictor of excessive alcohol use (3). It is important to understand the reasons for such co-use, which may occur at the same time or sequentially over days or years (4–6). It is important to understand the etiology and risks of co-use, to minimize harm and to prevent the development of dependence. Using more than one substance at a time increases the risk of harmful effects from either or both drugs and also complicates the treatment of problems associated with either substance. For these reasons, it is important to how use of one drug affects use of other drugs.

Metrik et al. (7) report an interesting, and somewhat surprising, finding from a controlled human laboratory study addressing this topic. They studied the effect of single doses of cannabis on the motivation to consume alcohol in young adults who were regular users of both cannabis and alcohol. Most of the participants met diagnostic criteria for Cannabis Use Disorder, and a smaller proportion also met criteria for Alcohol Use Disorder. The study tested the idea that use of one drug (in this case cannabis, or its active ingredient delta-9-tetrahydrocannabinol) directly increases desire for another (in this case alcohol). Participants first received placebo or different strengths of smoked cannabis under double-blind conditions, and then their motivation to use alcohol was assessed. The main dependent measures were urge to drink in response to alcohol cues, and choice of alcohol versus money. A strength of the study was the inclusion of placebo cannabis, and the use of two active doses of cannabis. Contrary to expectations, cannabis decreased craving for alcohol in response to alcohol-related cues, and decreased consumption of alcohol in the money vs alcohol choice task.

The findings are interesting in part because they were unexpected. The investigators postulated that cannabis would increase alcohol consumption, consistent with the idea that drugs facilitate further drug use. There is a common belief that ingestion of one drug increases the desire to use more of the same or other drugs. This is a central tenet of Alcoholics Anonymous and is supported by some experimental evidence (8). One drug may facilitate further use of the same or another drug for several reasons: It may increase the motivation to use (‘priming’ effects), or increase the rewarding effects of the second drug or the combined effects, or remove inhibitions that limit drug use. Yet, in this study the authors found that active doses of cannabis reduced alcohol consumption. Why might this be? The authors interpret this finding in behavioral economic terms, suggesting that the drugs serve as ‘substitutes’, i.e., that use of one drug can take the place of another. Another way to view this is that individuals seek an optimal level of intoxication, and subjective effects that exceed this level (resulting from the combination of cannabis and alcohol in this case) are unpleasant. Alternatively, cannabis may dampen the pleasurable effects of alcohol. Metrik et al discuss several of these possibilities. Their finding that cannabis reduced alcohol consumption is important because it is contrary to common belief that use of one drug facilitates use of other drugs, and raises questions about the mechanism underlying this interaction.

The question remains, however, why are cannabis and alcohol use related in epidemiological studies? Lifetime co-use of two drugs may occur for many reasons, apart from direct pharmacological interactions. For example, some individuals may be at risk for using drugs in general, regardless of their pharmacological effects. This may be because of personality traits such as risk-taking or sensation-seeking, or because of environmental factors such as ready access to drugs, or yet-to-be determined genetic factors. Or, some individuals might experience the effects of drugs as more pleasurable, regardless of class. Alternatively, some drugs might be co-used because one drug enhances the pleasurable effects, or dampens the negative effects, of the other. Studies such as this one by Metrik et al make an important contribution to our understanding of such questions. Their findings rule out one intuitively compelling explanation for the co-use of alcohol and cannabis, that cannabis directly increases the motivation to use other drugs, in this case alcohol. At these doses, and under these conditions of alcohol availability, active cannabis did not increase alcohol consumption, and indeed, it significantly decreased both the urge to consume alcohol and choice of an alcohol-containing drink versus money. Many questions remain. Would the same effects occur in a naturalistic non-laboratory setting, for example in the presence of peers. Would the converse hold true, that is, does ingestion of alcohol also reduces consumption of cannabis? Would the same effects occur at different doses of either drug, or in different samples of individuals? If use of cannabis truly does not facilitate alcohol consumption, then researchers can focus their efforts on identifying other factors that do account for the observed correlation in use of these two drugs.

From a public health perspective there is an urgent need to understand risk factors for using these two drugs, and interactions between them. Are certain populations at risk for use of either drug, or the combination, and if so, what is the source of this risk? For example, personality or genetic factors predispose certain people to use both drugs Does the availability of both drugs increase their use? That is, the simple fact that both drugs are available in certain settings may account for their co-use. Reports from states where cannabis has been legalized are mixed, suggesting that greater availability of cannabis can both increase and decrease use of other drugs (9, 10). We do not fully understand the risks of the combined effects of these drugs, and whether these risks related to concurrent use or sequential use (11, 12). We need answers to such questions to guide drug policies in the future to minimize costs to society. Studies like the one by Metrik et al. are an important step in understanding and preventing harms from the co-use of drugs.