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. Author manuscript; available in PMC: 2022 Sep 1.
Published in final edited form as: Psychol Addict Behav. 2021 Sep;35(6):621–627. doi: 10.1037/adb0000772

Combined Use of Alcohol and Cannabis: Introduction to the Special Issue

Ashley N Linden-Carmichael 1,*, Jeffrey D Wardell 2,3,4,*
PMCID: PMC8493786  NIHMSID: NIHMS1715907  PMID: 34591513

Abstract

Objective:

Combined use of alcohol and cannabis has increased in recent years among certain age groups, and it is well-established that individuals who use both alcohol and cannabis (especially if they use both at the same time) are at increased risk for substance-related harms relative to individuals who use only one substance. Far less attention has been placed on the patterns, predictors, and psychological processes associated with alcohol and cannabis co-use, which may inform prevention and intervention programming. Accordingly, this Special Issue was assembled to advance our understanding of the characteristics and consequences of combined use of alcohol and cannabis.

Method:

In this introductory article, the Guest Editors present the background for this work and provide an overview of the 14 articles that comprise this Special Issue.

Results:

Studies contained in this Special Issue capitalize on a variety of methodologies, with a particular focus on investigating typologies of alcohol and cannabis co-use, clarifying motivational and social contexts of co-use, and tracking co-use in daily life via daily diary and ecological momentary assessment designs. Experimental and neuroimaging examinations of co-use are also included. Collectively, the studies generally provide evidence that combined use of alcohol and cannabis is associated with unique characteristics, predictors, consequences and psychological processes relative to single substance use.

Conclusions:

The studies in this Special Issue provide new insight into combined use of alcohol and cannabis.They also highlight a number of promising avenues for future inquiry as the literature on alcohol and cannabis co-use continues to grow.

Keywords: alcohol, cannabis, marijuana, polysubstance use, simultaneous alcohol and marijuana use

Unprecedented changes to the legal status of cannabis are currently unfolding in the US and abroad. As alcohol and cannabis are widely used together (Subbaraman & Kerr, 2015), these changes have sparked interest among researchers in better understanding the combined use of alcohol and cannabis and the unique role of combined use in substance-related harms. A number of terms are used in this literature, including co-use, concurrent use, and simultaneous use. Some of these terms, such as co-use, are umbrella terms that are typically applied to describe any pattern of combined use of alcohol and cannabis. Others refer to a specific pattern of co-use. For example, concurrent use has been defined as the use of alcohol and cannabis on separate occasions, while simultaneous use typically refers to using both substances at the same time so that their effects overlap (see Sokolovky et al., 2020; Subbaraman & Kerr, 2015). To date, consistent cross-sectional evidence has demonstrated that individuals who use both alcohol and cannabis (i.e., co-use) are at increased risk for substance-related harms – including risk for driving under the influence, blacking out, and cognitive consequences – relative to individuals who use only one of these substances (e.g., Jackson et al., 2020; Linden-Carmichael et al., 2019; White et al., 2019). Moreover, among individuals who use both substances, individuals who engage in simultaneous use (often abbreviated SAM for simultaneous alcohol and marijuana use) report more negative consequences than those who use both substances but not at the same time (i.e., concurrent use). Among individuals who use alcohol, past-year SAM use was reported by an estimated 30% of 19–22-year-olds and 20–25% of 23–30-year-olds (Terry-McElrath & Patrick, 2018). As the rates of SAM use have increased historically among young adults who use alcohol (Terry-McElrath & Patrick, 2018), there is an increased need to understand the patterns, predictors, and consequences of combining alcohol and cannabis use to inform targeted prevention and intervention work.

In moving toward a more comprehensive understanding of the antecedents and outcomes of alcohol and cannabis co-use, it is imperative that we extend our knowledge of co-use beyond broad between-person comparisons of individuals who use both substances relative to individuals who use only one substance. Indeed, there is a need to more fully contextualize co-use behavior within a biopsychosocial model that takes into consideration the pharmacological, psychological, and social/contextual factors at play when an individual combines cannabis with alcohol. A variety of designs and methods are needed to offer insight into different facets of co-use. For example, while cross-sectional findings are important for identifying specific populations at increased risk for harms, cross-sectional studies preclude us from examining how combined use unfolds in real-world or controlled settings, the long-term outcomes associated with combined use, or an in-depth understanding of contextual and psychological antecedents of combined use. Thus, longitudinal studies and ecological momentary assessment designs are important for overcoming the limitations of cross-sectional surveys. Further, experimental studies of the acute pharmacological effects of co-administration of cannabis and alcohol can shed light on the mechanisms underlying risk for acute harms associated with SAM use that cannot be readily gleaned from survey-based studies. Moreover, it is important to note the considerable heterogeneity in patterns of combined alcohol and cannabis use among individuals who co-use these substances. Thus, studies examining important subgroup characteristics, particularly among understudied populations, and offering more fine-grained resolution in the typology of co-use behavior are needed to provide a more nuanced understanding of co-use and to improve the generalizability of findings.

This Special Issue was assembled with the goal of advancing our understanding of the characteristics and consequences of combined use of alcohol and cannabis. The 14 articles included in this Special Issue capitalize on a variety of methodologies, particularly those that assess the patterns, predictors, and short- and long-term outcomes associated with combined use; behavioral, situational, and motivational antecedents of alcohol and cannabis co-use in daily life; and psychopharmacological and neurobiological investigations of cannabis and alcohol co-use or co-administration. Together, these articles provide deep insight into factors most salient to alcohol and cannabis co-use and shed light on critical future research directions.

Patterns, Predictors, and Short- and Long-Term Outcomes of Co-Use

One set of papers in this Special Issue focused on the typology of alcohol and cannabis co-use and developmental changes in the patterns of co-use behavior. In a large sample of college students, Looby et al. (this issue) examined differences in the typical context and patterns of cannabis use between students who engage in SAM use, students who engage in concurrent use, and students who use only cannabis. As many prior studies have focused on differences in alcohol outcomes as a function of co-use vs. alcohol-only use, this study helps to further contextualize cannabis outcomes as a function of co-use vs. cannabis-only use. The authors found that SAM use was not associated with greater cannabis consumption relative to cannabis-only use, and concurrent alcohol and cannabis use was actually associated with less cannabis consumption relative to cannabis-only use. Tucker et al. (this issue) further explored the typology of alcohol and cannabis co-use by characterizing use patterns at age 16 and modeling latent trajectory classes through age 22. Tucker and colleagues’ paper makes an important contribution for understanding the role of alcohol and cannabis co-use during adolescence on role transitions and functioning during the transition into emerging adulthood. Together these findings point to not only screening for alcohol and cannabis co-use but also targeting younger ages specifically.

Several papers in this Special Issue also focus on motivations for use and the role of social context in SAM use. Using qualitative interviews, Boyle et al. (this issue) identified common reasons for SAM use among 18 to 20 year-old college students. In addition to traditional motives for alcohol use (social, enhancement, coping, conformity; Cooper, 1994), other motivations for use emerged – including motivations specifically to cross-fade (to be drunk and high at the same time), to reduce harm (to drink less), and reactive/situational reasons (because it was available or offered to them). Given the potential for increased risk from SAM use noted above, further work exploring how individuals evaluate SAM use occasions is critically needed for prevention programming purposes. Meisel et al. (this issue) examined first- and second-year college students’ social network characteristics and found friends’ use generally paralleled their own use and that having more friends who use alcohol and cannabis was linked with greater odds of SAM use. Social context also played an important role in the link between co-use of alcohol and cannabis on young adult women’s risk for sexual assault (Read et al., this issue). Specifically, co-use conferred unique risk for sexual assault relative to use of either substance alone. Further, one’s social context – namely proximity to offenders – was found to mediate this risk.

One critical limitation of the majority of work focusing on SAM use is the heavy reliance on data from primarily white college students. Racial-ethnic disparities in alcohol and cannabis co-use have grown in recent years, and recent reports indicate that African American adolescents are at high risk for alcohol and cannabis co-use relative to other substance use patterns, including single-substance use (Banks et al., 2020; Banks et al., 2017). An important contribution from Banks et al. (this issue) revealed that among a sample of African American youth, a complex relationship emerged when exploring the links between facets of racial identity and co-use of alcohol and cannabis. For example, greater public regard (i.e., adolescents’ perceptions of how others positively view their racial group) was (unexpectedly) linked with increased odds of co-use, whereas identity exploration (i.e., greater behavioral engagement with one’s own racial group) was linked with lower odds of co-use (as expected). It is imperative that the field continues to explore the role of race-related factors in co-use patterning to mitigate disparities in co-use.

Alcohol and Cannabis Co-Use in Daily Life

This Special Issue placed a large emphasis on studies examining alcohol and cannabis use in daily life via daily diary or ecological momentary assessment (EMA) studies. Such designs offer not only an understanding of how substance use behavior unfolds in real-world conditions (i.e., outside a controlled laboratory setting) but also the ability to conduct within-person comparisons of days when individuals report co-use relative to days when they report use of only one substance. While several diary studies have examined day-level consequences associated with alcohol and cannabis co-use (e.g., Lee et al., 2020; Linden-Carmichael et al., 2020a; Mallett et al., 2019; Sokolovsky et al., 2020), relatively less attention has been placed on antecedents to use. Toward this end, Gunn et al. (this issue) examined day-level physical, situational, and social characteristics associated with SAM use relative to days with only alcohol or only cannabis use among a sample of 18- to 24-year-olds currently attending college. Findings revealed that parties, friend’s homes, and being around more people were associated with SAM use occasions relative to single-substance use occasions. Adding to these findings, Jackson et al. (this issue) investigated the joint influence of situational motives and context on SAM use among college students and found that, in addition to the salience of social situations, SAM use was more likely than cannabis-only use when positive reinforcement motives were higher (i.e., using to increase sociability, using to increase positive affect). The examination of the complex interplay of motives and socio-environmental factors is critical as the field contributes to the limited but growing body of work focused on factors most salient to SAM use relative to alcohol- and cannabis-only use (e.g., Linden-Carmichael et al., 2020b; Lipperman-Kreda et al, 2017; Patrick et al., 2019; Stevens et al., 2021).

Identifying moment-, day-, and person-level factors associated with SAM use can provide critical information for developing ecological momentary interventions such as just-in-time-adaptive interventions (JITAIs) tailored to situations at higher risk of problematic SAM use, adapted to the individual. In addition to identifying situations most associated with SAM use, findings from Fairlie et al. (this issue) suggest that momentary interventions could target individuals’ plans as an indicator of SAM use. In their community sample of young adults, Fairlie and colleagues found that 42% of days individuals engaged in SAM use were unplanned SAM use days, but individuals reported more alcohol consumed, more hours high, and higher levels of subjective intoxication on planned SAM days relative to unplanned SAM days.

Moreover, of particular interest in the field is whether cannabis and alcohol are used as substitutes or complements (Risso et al., 2020; Subbaraman, 2016). For example, some populations of individuals who use both substances may use cannabis as a substitute for alcohol (e.g., those who use cannabis medicinally), such that greater cannabis use is associated with lower alcohol consumption (e.g., Gunn et al., 2019; Subbaraman & Kerr, 2018; Wardell et al., 2018). Daily diary and EMA designs are well suited to examine complementarity or substitution effects for alcohol and cannabis use (see Gunn et al., 2019; O’Hara et al., 2016). In this Special Issue, disparate findings emerged on this research question: Ito et al. (this issue) found evidence for complementarity in a sample of college students such that more alcohol was consumed on days when cannabis was used. In contrast, Waddell et al. (this issue) found greater support for substitution, such that for individuals with mean or low levels of impulsive traits (i.e., positive urgency and lack of perseverance), quantity of alcohol use was lower on days with co-use. Critically, there were several noteworthy differences between the samples – whereas Ito et al. examined associations among a sample of college students, Waddell et al. studied a slightly older sample of veterans, a group at increased risk for impulsivity and substance use (Bhalla et al., 2019; Hawn et al., 2019; Gunn et al., 2020; Meadows et al., 2015). These findings highlight the importance of examining person-level factors when considering one’s risk for increasing alcohol or cannabis use on co-use occasions.

Experimental and Neuroimaging Studies

While an increasing number of survey and EMA studies have provided new insight into alcohol and cannabis co-use, there is a need for more laboratory-based research on the psychopharmacological and neurobiological mechanisms that may play a role in co-use. Indeed, pharmacodynamic and pharmacokinetic interactions between cannabis and alcohol may give rise to a variety of subjective, cognitive, and behavioral outcomes associated with co-use (see Ballard & DeWit, 2011). For example, the effects of alcohol and cannabis on cognition and psychomotor functioning may combine in ways that increase risk for harms associated with acute impairment (e.g., risky behaviors and impaired driving abilities, Hartman & Huestis, 2013; Yurasek et al., 2017). Further, it has been argued that the rewarding subjective effects of cannabis and alcohol may combine in additive or synergistic ways to reinforce simultaneous use and increase motivation to consume both substances (Karoly et al., 2020; Lukas & Orozco, 2001; Patrick et al., 2018). This may partially explain higher levels of alcohol and cannabis consumption observed among individuals who use cannabis and alcohol simultaneously.

Experimental placebo-controlled studies that manipulate cannabis and alcohol administration are required to help isolate pharmacological effects from expectancy effects. While several such studies have been conducted (for review, see Yurasek et al., 2017), many of these studies are decades old and ignore important individual-level moderators. For example, there is a dearth of human laboratory research examining sex differences in the acute psychoactive effects of cannabis and alcohol co-administration. The Wright et al. study (this issue) helps to address this gap, providing a preliminary investigation of sex differences in subjective and cognitive effects of smoked cannabis, alcohol, and their combination among a sample of Canadian young adults. In this study, participants consumed a bolus dose of alcohol (target BrAC 0.08%) or placebo and then smoked a joint containing 12.5% ± 2% THC cannabis or placebo cannabis. Participants were instructed to smoke as much of the cannabis as needed to experience the same high they typically experience. Although the authors observed few sex differences in psychoactive effects of cannabis and alcohol, they did find that females chose to smoke less cannabis after consuming alcohol than males. This raises interesting questions about sex differences in the impact that alcohol’s pharmacological effects may have on cannabis self-administration behavior, which should be further investigated in future, larger studies. Future such studies could also examinine whether the order of alcohol vs. cannabis administration impacts the nature of the psychoactive effects experienced when the substances are co-administered.

Further, two primary constituents of cannabis, delta (9)-tetrahydrocannabinol (THC) and cannabidiol (CBD), produce different psychopharmacological effects and may interact with one another in dose-dependent ways (e.g., Solowij et al., 2020). Moreover, there is some limited evidence, primarily from animal studies, that CBD administration may reduce alcohol consumption and help to attenuate alcohol withdrawal and relapse (for review see Nona et al., 2019). However, human studies of the effects of CBD administration on alcohol consumption are lacking. The study by Karoly et al. (this issue) represents an important step toward addressing this gap using a creative quasi-experimental, naturalistic design. Adults from the community who used cannabis and reported light drinking (<4 days per week and no heavy drinking episodes) were randomly assigned to use a cannabis product with a low, medium, or high CBD:THC ratio for five days and reported how much alcohol they consumed over the same period. The study provides preliminary evidence that using high CBD cannabis products may be associated with reduced alcohol use in humans, highlighting the need for future well-controlled, double-blind studies of this effect.

In addition to the acute pharmacological effects of cannabis and alcohol co-administration, there is a growing interest in understanding the neurobiological correlates of cannabis and alcohol co-use, which may provide insight into the neural bases for risky behaviors and harms associated with chronic co-use. However, there have been relatively few investigations of the neurobiological correlates unique to combined use of cannabis and alcohol (for review, see Karoly et al., 2020). The Grodin et al. (this issue) paper contributes to this emerging literature by examining differences in gray matter volume between individuals who use alcohol only vs. those who co-use alcohol and cannabis in a relatively large sample of adults (n=237). The study also helps to tease apart differences in gray matter volume that are specific to alcohol and cannabis co-use vs. tri-use of alcohol, cannabis, and tobacco. Interestingly, this study found little evidence for gray matter volume differences between the alcohol-only and co/tri-use groups, raising some questions about the specific impact of chronic co-use of cannabis and alcohol on gray matter volume in adults.

Future Directions

The articles in this Special Issue shed new light on the predictors, consequences, and mechanisms of alcohol and cannabis co-use. At the same time, they also underscore several limitations in the existing research on this topic and highlight a number of promising avenues for future inquiry. First, a large proportion of studies, especially existing EMA studies, have utilized young adult or college student samples. Although young adults are an important group to study, further research is needed in order to generalize findings on cannabis and alcohol co-use to additional populations. Importantly, a few studies in this Special Issue address questions related to the role of sex (Brands et al.; Grodin et al.) and race (Banks et al.) in cannabis and alcohol co-use. Future work should also attempt to examine co-use behavior among older adults, additional racial/ethnic groups, LGBTQ+ persons, individuals who use cannabis medicinally, and individuals in treatment for substance use, to name a few.

Second, there is considerable heterogeneity across studies with respect to the operational definitions of cannabis and alcohol co-use. As previously mentioned, a number of terms are used in this literature, including co-use, concurrent use, and simultaneous use. These terms are often not consistently applied across studies, and even the definition of simultaneous use varies considerably across studies (see Sokolovky et al., 2020). Further refinement of these operational definitions is needed. A recent EMA study by Sokolovsky et al. (2020) made an important step in addressing this issue by examining different operationalizations of simultaneous use based on incremental variations in the length of time between reported use of alcohol and cannabis. However, additional research is needed, including studies using complementary methods (e.g., laboratory administration designs), to further elucidate the operational definition of simultaneous use. Relatedly, it is also imperative that future work more comprehensively considers co-use of alcohol and cannabis within the context of the use of tobacco and other substances. For example, emerging work has highlighted the importance of assessing for the tri-use of alcohol, cannabis, and tobacco (Grodin et al., this issue; Roche et al., 2019). In one diary study, 40% of SAM days involved the use of another substance, primarily tobacco use and stimulant (e.g., Adderall) use (Linden-Carmichael et al., 2020). Consequently, it is imperative for future work to consider the broader context of polysubstance use.

Third, a major limitation that has challenged cannabis researchers for a long time is the difficulty inherent in obtaining reliable self-report data on quantities of cannabis consumed. This continues to pose a challenge for emerging research on combined use of cannabis and alcohol. Unlike alcohol consumption that can be assessed using standardized units with well-validated interviews and questionnaires, the heterogeneity of cannabis products in terms of their form (plant material, oils, edibles), administration methods (smoking, vaping, oral ingestion), and potency (e.g., percent THC, percent CBD), all make precise quantification and standardization of cannabis consumption very difficult to achieve via self-report. Accordingly, many studies rely on frequency of cannabis use (e.g., number of days used) as their metric of cannabis use. Although frequency may be useful for distinguishing infrequent from frequent use, this metric has limited utility for meaningfully estimating history of THC/CBD exposure or peak blood THC/CBD levels. Limitations in the measurement of cannabis consumption may be a contributor to mixed findings in the literature on alcohol and cannabis co-use. Importantly, there have been recent attempts to modernize self-report assessment of cannabis consumption (see Cuttler & Spradlin, 2017; Martin-Willett et al., 2020; Prince et al., 2018), but more work is needed to validate and implement comprehensive assessments of cannabis consumption in the context of cannabis and alcohol co-use research.

Fourth, the majority of recent studies examining alcohol and cannabis co-use rely exclusively on self-report, which is prone to several biases. As mentioned, there is a need for more experimental and laboratory-based research to isolate important pharmacological and neuropsychological mechanisms in co-use. In addition, studies using biometric methods for collecting data on alcohol and cannabis co-use in daily life are needed. While some biometric methods are being increasingly applied (e.g., transdermal alcohol sensors; see Teymourian et al., 2020; Wang et al., 2019), further development and validation of these methods, particularly for remote monitoring of cannabis consumption, are necessary to move EMA research on co-use forward. Translational research programs that help to bridge the gap between pre-clinical and human research paradigms, as well as laboratory and field research, will also represent a significant step in the attempt to better understand alcohol and cannabis co-use.

Fifth, the landscape of state-by-state policies in the US regarding the legalization of cannabis use for medicinal and/or recreational purposes is rapidly changing. While some studies derived their sample from multiple states with different legislation (e.g., Jackson et al., this issue), it is important to note that some studies focused exclusively on use patterns in a state where recreational and/or medicinal cannabis use was or was not legalized, which may impact the generalizability of findings across studies. For example, college students residing in a state where recreational use was decriminalized reported higher SAM use (White et al., 2019). Moreover, the legal age for purchasing cannabis and alcohol varies across jurisdictions where recreational cannabis is legal, which might impact findings of co-use studies among young adults. Indeed, the Wright et al. study (this issue), which involved co-administration of alcohol and cannabis, included participants as young as age 19 (the legal age for purchasing both alcohol and cannabis in Ontario, Canada). As policies continue to change, more research is needed to examine the impacts of legalization in the US and internationally.

Finally, the emerging research on cannabis and alcohol co-use and associated outcomes has the potential to inform intervention efforts. As research on the combined use of cannabis and alcohol continues to evolve, an important next step is to develop and evaluate the utility of tailored interventions that target co-use as a specific high-risk behavior. The rich EMA data that is emerging (several such studies appearing in this issue) may be particularly valuable for informing mobile health interventions.

Conclusion

This Special Issue reflects a burgeoning interest among addiction researchers in better understanding the combined use of cannabis and alcohol. This interest has coincided with the recent trend toward increasingly liberal cannabis policies in the US and globally, along with predictions that co-use of alcohol and cannabis may increase as a result. While this Special Issue provides a showcase of the significant advancements that have recently been made in this research area, it also highlights the need for more work to address several remaining limitations and unanswered questions. Given that the trend toward legalization of cannabis appears likely to continue, the study of alcohol and cannabis co-use will remain a timely and important topic of inquiry.

Public Health Significance.

The studies in this Special Issue suggest that combined use of alcohol and cannabis is associated with unique characteristics, predictors, outcomes, and psychological processes that distinguish this behavior from use of cannabis or alcohol alone. Findings from these studies provide new insight into the combined use of alcohol and cannabis that may inform interventions and suggest several avenues for future research.

Acknowledgments

This work is supported by award K01 AA026854 from the National Institute on Alcohol Abuse and Alcoholism (NIAAA). The NIAAA did not have any role in study design, collection, analysis, and interpretation of the data; writing the report; and the decision to submit the report for publication.

Footnotes

The authors have no conflicts of interest to report.

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