People who use cannabis and alcohol at the same time have 2-3x higher odds of drunk driving and social consequences, while those who use both separately show much lower risk.
Anyone who uses both cannabis and alcohol, policy makers, harm reduction advocates.
2.30x drunk driving, 2.96x social consequences for simultaneous users vs alcohol-only drinkers
The Backstory
Cannabis is the most commonly used drug among people who drink. Everyone knows this. What nobody had measured — until Meenakshi Subbaraman asked the right question — was the difference between people who use cannabis and alcohol at the same time versus people who use both but keep them separate.
The distinction turned out to be enormous. And it reframed the entire debate about whether legal cannabis helps or hurts public health by changing how we think about alcohol.
The Question Nobody Had Asked
Researchers studying cannabis and alcohol had been lumping all co-users together. If you drank and you also used cannabis, you were a "co-user" — regardless of whether you lit a joint while drinking a beer or used cannabis on weeknights and drank on weekends.
Subbaraman, a researcher at the Alcohol Research Group (Public Health Institute) in Emeryville, California, recognized that this was like studying food by combining everyone who eats breakfast and dinner into one category. The pattern matters as much as the behavior.
How They Did It
Separating Simultaneous from Concurrent Co-Use
Data source
The 2005 and 2010 National Alcohol Survey — a telephone survey covering all 50 states and DC using random digit dialing. 8,626 current drinkers formed the analytic sample.
The NAS oversampled Black and Hispanic respondents, providing better representation of groups with different co-use patterns
The critical classification
One simple question changed everything: 'How often did you use alcohol and marijuana at the same time? Usually, sometimes, or never?' This split cannabis-using drinkers into three groups.
Simultaneous users: alcohol and cannabis together. Concurrent users: both substances, but separately. Alcohol-only: no cannabis use
Measure outcomes
Drunk driving (past 12 months), social consequences (15-item scale across five domains), and harms to self (6-item scale). All controlled for demographics and drinking patterns.
The controls are important — the question is whether simultaneous use adds risk beyond what heavier drinking alone would predict
Multivariable analysis
Logistic regression controlling for gender, age, ethnicity, education, employment, relationship status, drinking frequency, and quantity. Sampling weights adjusted for selection probability.
This is what separates the finding from simple correlation: after accounting for how much people drink, does adding simultaneous cannabis use independently increase risk?
Subbaraman & Kerr (2015), Alcohol Clin Exp Res 39(5):872-879
Three Populations, Three Risk Profiles
Population Analysis
The Three Types of Cannabis-Alcohol Users
| Group | Finding |
|---|---|
| Alcohol Only (88.6%) | |
| Concurrent Users (3.9%) | |
| Simultaneous Users (7.5%) |
Subbaraman & Kerr (2015)
65.7 vs. 13.9
days per year of heavy drinking (5+ drinks) among simultaneous cannabis-alcohol users versus alcohol-only users. Simultaneous users aren't just using cannabis — they're drinking dramatically more than everyone else. Nearly five times as many heavy drinking days per year.
This number challenges a common assumption: that people who combine cannabis with alcohol are simply doing two things at once. In reality, simultaneous users are a distinct, higher-risk population characterized by heavier overall substance use patterns.
Subbaraman & Kerr (2015)
The Risk That Doubles
After controlling for demographics and drinking patterns, the findings were unambiguous:
- Drunk driving — Simultaneous high
This is the single most dangerous behavior associated with co-use — two impairing substances compounding each other behind the wheel
OR 2.30 (p<0.001). Simultaneous users were more than twice as likely to report driving while impaired. The pharmacological mechanism is clear: alcohol increases THC absorption, and the combination produces greater cognitive and motor impairment than either alone.
- Social consequences — Simultaneous high
Social consequences cover five domains: legal/accidents, health, work, fighting, and relationships
OR 2.96 (p<0.001). Nearly three times the odds of experiencing legal trouble, work problems, relationship damage, health issues, or fighting. This was the largest effect size in the study.
- Harms to self — Simultaneous high
These are the harms users themselves recognize — not external judgments
OR 2.22 (p<0.001). More than double the odds of self-reported harm across friendships, outlook on life, home life, finances, work, and health.
- Drunk driving — Concurrent low
The distinction matters: it's not cannabis that increases driving risk, it's using cannabis while drinking
OR 1.18 (not significant). Concurrent users — who keep their cannabis and alcohol use separate — showed no elevated drunk driving risk compared to alcohol-only drinkers.
- Social consequences — Concurrent moderate
This may reflect the overall higher substance involvement of people who use two drugs rather than one
OR 2.29 (p<0.05). Concurrent users did show elevated social consequences, though not as high as simultaneous users.
- Harms to self — Concurrent low
People who use both substances but keep them separate don't report more personal harm than alcohol-only drinkers
OR 1.67 (not significant). No statistically significant increase in self-reported harms for concurrent users.
Subbaraman & Kerr (2015)
The Substitution Question
The individual-level data tells one story: mixing is dangerous. But Subbaraman's broader body of work — including a comprehensive review of 39 studies on alcohol-cannabis substitution and complementarity — reveals a more nuanced population-level picture.
Cannabis substitutes for alcohol (net positive)
When cannabis becomes more available (through legalization or medical access), some populations reduce alcohol consumption. Since alcohol causes substantially more harm than cannabis by every systematic assessment, this substitution could improve net public health.
Medical cannabis laws have been associated with reduced alcohol sales and alcohol-related traffic fatalities in some studies. Survey data shows many medical cannabis patients report using cannabis as a deliberate substitute for alcohol.
Cannabis complements alcohol (net negative)
Making cannabis more available doesn't replace drinking — it adds another substance to the mix. Simultaneous use is the dominant pattern among regular cannabis-using drinkers (77% of at-least-monthly users), and it produces worse outcomes than either substance alone.
Subbaraman's National Alcohol Survey data showed simultaneous users drink far more heavily than alcohol-only users. Among at-least-monthly cannabis users, 77% are simultaneous users — the riskiest pattern.
It's both — depends on the population
The relationship varies by age, gender, race, and policy context. Youth may substitute in more liberal cannabis environments. The general population shows modest substitution effects under legalization. But concurrent substance users are a distinct population whose behavior doesn't follow population-level trends.
Subbaraman's review of 39 studies found roughly equal numbers supporting substitution, complementarity, and independence — suggesting the answer depends on who you're studying and in what context.
Subbaraman (2016) review; Subbaraman & Kerr (2015)
Why This Matters for Policy
The substitution question has enormous implications for legalization debates. If legal cannabis replaces even a fraction of alcohol consumption, the net public health impact could be positive — because alcohol causes far more harm than cannabis on every systematic assessment.
But Subbaraman's data adds a critical caveat: the benefit depends on whether people substitute or stack. The policy goal should be to encourage substitution (cannabis instead of alcohol) while discouraging simultaneous use (cannabis plus alcohol). This argues for public health messaging that doesn't just warn about cannabis risks in isolation, but explicitly addresses the higher risks of combining the two.
For individuals, the message is clear: if you're going to use cannabis and you also drink, keeping them separate is dramatically safer than mixing. The pharmacological interaction — alcohol increases THC absorption, producing the "spins" and greater impairment — makes simultaneous use one of the riskiest patterns in recreational substance use.
Key Takeaways
Simultaneous versus concurrent use of alcohol and cannabis in the National Alcohol Survey
Subbaraman MS, Kerr WC (2015) · Alcoholism: Clinical and Experimental Research
Related Research
Key studies in this area
Drug harms in the UK: a multicriteria decision analysis
Nutt, King & Phillips (2010)
The comparative framework that makes the substitution question so important — alcohol scores 72/100, cannabis 20/100, meaning any substitution is a net harm reduction
Canadian legalization: five-year outcomes
Hall et al. (2023)
Real-world data on what happens to alcohol patterns after cannabis legalization at scale
Cannabis edible dosing and safety
Barrus et al. (2016)
Edible cannabis has different pharmacokinetics that complicate the simultaneous use risk profile — delayed onset can lead to combining while waiting for effects
Cannabis and car crash risk
Asbridge et al. (2012)
The driving risk that simultaneous use amplifies — cannabis alone roughly doubles crash risk, and alcohol adds multiplicatively
Cannabis as a substitute for prescription drugs
Corroon et al. (2017)
The substitution effect extends beyond alcohol — many patients report replacing prescription medications with cannabis
Is it safe to use cannabis and alcohol on the same day but not at the same time?
Subbaraman's data suggests that concurrent use (both substances, but separately) is substantially less risky than simultaneous use. Concurrent users showed no significant increase in drunk driving or self-reported harms compared to alcohol-only drinkers. However, concurrent users did show elevated social consequences, and the pharmacological effects of cannabis can persist for hours, so "separate" use still requires genuine time separation — not just different drinks.
Why does mixing cannabis and alcohol make things worse?
Alcohol increases THC absorption from cannabis, meaning the same amount of cannabis produces a stronger high when combined with drinking. This creates greater cognitive impairment, motor coordination problems, and the characteristic nausea and disorientation known as "the spins." The combination impairs judgment more than either substance alone, which is why drunk driving risk specifically doubles with simultaneous use.
Could legal cannabis reduce alcohol problems?
Possibly, through substitution — if people choose cannabis instead of alcohol for some occasions, and if cannabis causes less harm per use (which evidence suggests it does). But the benefit depends on whether legalization promotes substitution or stacking. If people add cannabis to their drinking instead of replacing it, the net effect could be negative. Policy and public health messaging play a critical role in which pattern dominates.
Who is most at risk from simultaneous use?
Young adults (18-29), males, and unmarried individuals were most likely to be simultaneous users. But the risk profile extends beyond demographics: 77% of people who use cannabis at least monthly and also drink are simultaneous users. If you use cannabis regularly and also drink, you are statistically in the highest-risk co-use category.
What the researchers found
Simultaneous cannabis-alcohol users had 2.30x odds of drunk driving, 2.96x social consequences, and 2.22x harms to self vs alcohol-only users. Concurrent users showed no significant increase in drunk driving or self-harm.
Why it matters
First study to distinguish simultaneous from concurrent alcohol-cannabis co-use in the general population, revealing that mixing is dramatically riskier than using both but separately.
The numbers in context
N=8,626. 7.5% simultaneous users, 3.9% concurrent. Simultaneous: 2.3 avg daily drinks, 65.7 heavy drinking days/yr. Alcohol-only: 0.71 drinks, 13.9 days. OR drunk driving 2.30, social consequences 2.96, harms 2.22.
How the study worked
Secondary analysis of 2005 and 2010 National Alcohol Survey. Multivariable logistic regression controlling for demographics and drinking patterns.
Who was studied
8,626 current drinkers from US general population (NAS 2005/2010), oversampled Black and Hispanic respondents.
What this study cannot tell us
Cross-sectional, self-report, no cannabis dose data, 52% cooperation rate.
How to read the evidence
Strong: large nationally representative sample, rigorous multivariable analysis. Limited by cross-sectional design.
When this study was published
Published 2015 using 2005/2010 data. Cannabis landscape has changed substantially since.
The bigger picture
The substitution-vs-complementarity debate determines whether legal cannabis is a net positive for public health. If people switch from alcohol to cannabis, harm decreases. If they stack, it increases.
Questions still open
- Can messaging shift people from simultaneous to concurrent patterns? Does legalization promote substitution or stacking?
Common questions
Is it safe to use both but separately?
Why does mixing make things worse?
Read the original research
Simultaneous versus concurrent use of alcohol and cannabis in the National Alcohol Survey.
Alcoholism: Clinical and Experimental Research, 39(5), 872-879
Citation
Subbaraman, Meenakshi S; Kerr, William C. (2015). Simultaneous versus concurrent use of alcohol and cannabis in the National Alcohol Survey.. Alcoholism: Clinical and Experimental Research, 39(5), 872-879. https://doi.org/10.1111/acer.12698