Key Takeaways
- DUI marijuana is a crime in Minnesota despite misconceptions about risks.
- Scientific studies cited in this article show lower accident risks for marijuana compared to alcohol.
- Presence of THC in the blood has no correlation with impairment or marijuana DUI.
- Carboxy-THC is a non-psychoactive metabolite and does not indicate recent use or DUI.
- Overall marijuana DUI poses a lower risk than common driving situations like driving with two passengers or while pregnant.
Driving under the influence of marijuana is a crime in Minnesota. And that did not change with legal marijuana use in Minnesota. But the failure of opposition to legalization, has also brought backlash channeled into exaggerated fears of marijuana DUI. But is it real?
Do marijuana users drive impaired?
What is the problem of impaired driving? Truly impaired drivers are at greater risk of causing a car accident due to bad driving. And some car accidents lead to injury or death. That’s the problem with DUI.
When it comes to driving after using a drug like alcohol or marijuana, we have four possible conditions:
- Driving with zero drug in blood. (Not DUI).
- Driving with less than psychoactive level of drug (Not DUI).
- Driving with psychoactive level of drug, but not causing impaired driving. (Not DUI).
- Driving with psychoactive level of drug in blood, causing impaired driving. (DUI).
Most people who use alcohol or other drugs avoid driving under the influence. Most people who use drugs (alcohol, marijuana, prescriptions, etc.) are responsible. They do not drive while impaired.
False Equivalence
Are the risks of a driver-caused injury or death accident for a THC-positive driver similar for to the risks for an alcohol-positive driver? No, the risks are far less for marijuana than for alcohol.
Says who? The scientific studies, and the National Highway Traffic Safety Administration (“NHTSA”), cited below.
Comparative risks of an accident; links to scientific studies
The psychoactive ingredient in marijuana is Delta-9-THC. THC-positive drivers typically possess a low — or even no — risk of motor vehicle accident compared to THC-negative drivers. Those drivers are not marijuana DUI violators.
Blood THC has little effect on traffic accidents
“The primary objective of this study was to analyse whether there is a significant association between driving under the influence of cannabis and unfavorable traffic events. … Our analysis suggests that the overall effect size for driving under the influence of cannabis on unfavorable traffic events is not statistically significant.”
Association of unfavorable traffic events & cannabis usage: meta-analysis, Frontiers in Pharmacology, 2018
“For both sober and drinking drivers, being positive for a drug was found to increase the risk of being fatally injured. When the drug-positive variable was separated into marijuana and other drugs, only the latter was found to contribute significantly to crash risk.”
Drugs & Alcohol: Their Relative Crash Risk, Journal of Studies on Alcohol & Drugs, 2014
Marijuana use can impair drivingF but does not necessarily lead to a marijuana DUI driver.
Compare Odds Ratios (OR): 1.05 – 1.4 for motor vehicle crash risk for acute cannabis intoxication vs. THC-positive
“Acute cannabis intoxication is associated with a statistically significant increase in motor vehicle crash risk. The increase is of low to medium magnitude (OR between 1.2 and 1.4).”
Effects of cannabis intoxication on motor vehicle collision, Addiction, 2016
“Adjusted odds ratios between drug class use and crash risk, adjusted for demographic variables: age, gender and race/ethnicity: THC = 1.05.”
US National Highway Traffic Safety Administration, Drug & Alcohol Crash Risk, 2015
When it comes to suspected marijuana DUI, acute intoxication is not the same as presence of THC.
Compare to Odds Ratios (OR) 2.2 for driving with multiple passengers
“Drivers with two or more passengers in the car possess a crash risk of more than two-fold (OR=2.2).”
Contribution of passengers versus mobile phone use to motor vehicle crashes resulting in hospital attendance by driver, ScienceDirect, 2007
Driving with two or more passenger is a greater risk of a crash than acute cannabis intoxication. So does acute cannabis intoxication equate with marijuana DUI, given the lower risk?
Compare to consuming small amounts of alcohol: marijuana DUI
“Driving with BAC levels .05 and .08 are more than six times more likely (OR=6.40) than of a sober driver to be responsible for a fatal motor vehicle accident.”
Cannabis, alcohol, & fatal road accidents, PLOS One, 2017
Compare to driving while pregnant: 42 percent relative increase in crash risk
“Driving while pregnant is equivalent to a 42 percent relative increase in crash risk.”
Pregnancy & risk of a traffic crash, CMAJ, 2014
Comparable to risk of driving while talking hands-free phone
“The maximum risk for cannabis intoxication alone, unmixed with alcohol or other drugs, appears to be more comparable to risks such as talking on a hands-free cellphone (legal in all states) than to driving with a BAC above 0.08, let alone the rapidly-rising risks at higher BACs.”
Driving while stoned: Issues & policy options, BOTEC Analysis/SSRN white paper, 2018
If cannabis intoxication is the same risk factor as a hands-free cell phone, does it amount to marijuana DUI?
Compare: texting & driving collision risk 23 times greater
“When the drivers texted, their collision risk was 23 times greater than when not texting.”
In Study, Texting Lifts Crash Risk by Large Margin, New York Times, 2009
Compared to alcohol, medicinal opioids, other drugs
“The highest risk of the driver being severely injured was associated with driving positive for high concentrations of alcohol (≥0.8 g/L), alone or in combination with other psychoactive substances. For alcohol, risk increased exponentially with blood alcohol concentration (BAC). The second most risky category contained various drug-drug combinations, amphetamines and medicinal opioids. Medium increased risk was associated with medium sized BACs (at or above 0.5 g/L, below 0.8 g/L) and benzoylecgonine. The least risky drug seemed to be cannabis and benzodiazepines and Z-drugs.”
Risk of severe driver injury by driving with psychoactive substances, Accident Analysis & Prevention, 2013
“The study concludes that drug use, especially alcohol, benzodiazepines and multiple drug use and drug–alcohol combinations, among vehicle drivers increases the risk for a road trauma accident requiring hospitalization. … No increased risk for road trauma was found for drivers exposed to cannabis.”
Psychoactive substance use & the risk of motor vehicle accidents, Accident Analysis & Prevention, 2004
Context deepens understanding of marijuana DUI
Conclusion? Marijuana could theoretically impair driving, for some, at times. But when it does, far less than alcohol impairs.
And it’s less impairing than other common legal practices like multiple passengers, and driving while pregnant. While it can sometimes cause impaired driving, we should view marijuana DUI fairly. We should view it along with other common risk factors for drivers.
Safer than alcohol
Marijuana DUI: When driving, marijuana is safer
And it’s safer than driving with two or more passengers. Here are the facts, with links to the science.
What does the science say about how marijuana intoxication affects driving? Dosage matters. But acute marijuana intoxication may influence psychomotor skills, such as reaction time, necessary to safe operation of a motor vehicle.
Effects short-lived
But these effects are quickly dissipate. And the effects are less dramatic than changes in psychomotor performance associated with drivers under the influence of alcohol.
“Experimental research on the effects of cannabis … indicate … that any effects dissipate quickly under one hour. Furthermore, while drivers feel high, they actually tend to compensate for their feelings.”
US National Highway Traffic Safety Administration, State of Knowledge of Drug-Impaired driving: 2003
Marijuana leads to conservative driving; alcohol to aggressive driving
In studies of on-road or simulated driving behavior, subjects under the influence of cannabis tend to drive cautiously. They compensate for perceived intoxication. They reduce speed and change lanes less. But subjects under the influence of alcohol tend to drive in a more reckless, aggressive manner.
“The compensatory behavior exhibited by cannabis-influenced drivers distinctly contrasts with an alcohol-induced higher risk behavior, evidenced by greater percent speed.”
Cannabis effects on driving longitudinal control with & without alcohol, Drug & Alcohol Dependence, 2016
“Subjects seemed to be aware of their impairment after THC intake and tried to compensate by driving slower, alcohol seemed to make them overly confident and caused them drive faster than in the control sessions.”
Effects of THC on driving performance, physiological state & subjective feelings relative to alcohol, Accident Analysis & Prevention, 2008
“THC’s effects differ qualitatively from many other drugs, especially alcohol. For example, subjects drive faster after drinking alcohol and slower after smoking marijuana. … Very importantly, our city driving study showed that drivers who drank alcohol over-estimated their performance quality whereas those who smoked marijuana under-estimated it. … “[S]ubjects in the marijuana group were not only aware of their intoxicated condition, but were … attempting to compensate for it. [D]rivers become overconfident after drinking alcohol and … become more cautious and self-critical after consuming low doses of THC, as smoked marijuana.”
US National Highway Traffic Safety Administration, Marijuana & Actual Driving Performance, 1993
Attempting to equate marijuana DUI, with alcohol DUI is a mistake. But many, not yet knowing any better, presume this false equivalency.
Compensatory Behaviors: Unlike alcohol, which often leads to increased risk-taking and speed, cannabis users typically overestimate their impairment and attempt to compensate. Slower speeds are a common result. Many drivers under the influence of cannabis decrease their speed and increase following distances, compared to their usual practice. Reduced risk-taking is a another common consequence. They are less likely to attempt to overtake or pass other vehicles compared to sober or drunk drivers. And cannabis can increase awareness in some respects. Users are often acutely aware they are impaired and may exhibit a cautious driving style, though this does not fully eliminate the risk of a crash. And these behaviors may account for the low comparative risk, compared to drivers with alcohol, multiple passengers, or driving while pregnant.
Studies: THC adverse effects small; improved driving
Compared to alcohol, subjects in on-road driving performance assessments typically demonstrate modest changes in psychomotor performance after administering THC.
While THC can reduce driving performance, it has sometimes improved driving performance; compared to control groups with no THC or alcohol.
“Most marijuana-intoxicated drivers show only modest impairments on actual road tests. … Although cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.”
Effect of cannabis compared with alcohol on driving, Am. Journal on Addictions, 2009
“THC’s adverse effects on driving performance appear relatively small.”
US National Highway Traffic Safety Administration, Marijuana & Actual Driving Performance, 1993
THC in the blood alone does not mean the driver is marijuana DUI.
D-9-THC levels not correlated with impairment
“One of the program’s objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC and/or its metabolite, THC-COOH, in single samples. The answer is very clear: it is not.
US National Highway Traffic Safety Administration, Marijuana & Actual Driving Performance, 1993
Plasma of drivers showing substantial impairment in these studies contained both high and low THC concentrations; and, drivers with high plasma concentrations showed substantial, but also no impairment, and even some improvement.”
NHTSA and the scientific studies say, there is no correlation between blood plasma THC levels and impaired driving.
“The psychoactive ingredient in marijuana, delta-9-tetrahydrocannabinal (THC), does not correlate well with impairment.”
US National Highway Traffic Safety Administration, Marijuana Impaired Driving: Report to Congress, 2017, p 13
Marijuana DUI: Driving with blood plasma THC in context
The main, psychoactive ingredient in marijuana is THC. It could cause driving impairment in some drivers, though nowhere close to the effect of alcohol.
Odds of motor vehicle crash risk, compared to normal driver:
- Alcohol (BAC levels .05 and .08): more than 600% increase
- Drivers with two or more passengers: 220% increase
- Driving while pregnant: 42% increase
- Acute cannabis intoxication: up to 40% increase (similar to driving while talking on hands-free cell phone)
Difference: THC vs. its metabolites
To understand the issue of marijuana DUI, we need to know about metabolites. The human body’s metabolism breaks down food and drug chemicals into other, different chemicals. We call the products of this natural process “metabolites.”
Metabolites of alcohol
Take alcohol, for example. After a person drinks alcohol, their body works to metabolize it; breaking it down.
The body metabolizes alcohol in several steps. Enzymes help break up the alcohol molecule, for better elimination. An enzyme metabolizes alcohol to acetaldehyde. Then, in the next step, the body metabolizes acetaldehyde down to another, less active byproduct called acetate. Then the body breaks down acetate into water and carbon dioxide.
So, acetaldehyde, acetate, water and carbon dioxide are all metabolites of alcohol.
Metabolites of THC
In the case of THC (delta 9-tetrahydrocannabinol, or▵9_THC), labs test for two of its metabolites: hydroxy-THC and carboxy-THC. These common names for the metabolites can be confusing, because:
- Metabolites, hydroxyl-THC and carboxy-THC, are not THC, but;
- both have “THC” in their names.
This confuses many into thinking that the metabolites are THC. But they are not. They are chemicals other than THC, which result from the body metabolizing and breaking down THC into different chemicals.
Carboxy-THC is not psychoactive
A psychoactive substance changes functioning of the nervous system and causes in changes in perception, mood, cognition, and behavior.
A blood test can reveal whether a substance is detectable (present), and a level. But it cannot alone reveal whether if present, that substance is at a psychoactive level. And if a substance is at a psychoactive level (the person can feel some effect), that does not mean that the person is impaired. Among other implications, this means that a person with no potentially psychoactive substance in their blood, cannot possibly be impaired by a psychoactive substance.
But the long-lived metabolite Carboxy-THC is not psychoactive, at all, ever.
According to information provided by the National Highway Traffic Safety Administration (“NHTSA”), Carboxy-THC is “not psychoactive.” Drugs and Human Performance Fact Sheets, Cannabis / Marijuana (Δ 9 -Tetrahydrocannabinol, THC), NHTSA:
[Blood] Plasma THC concentrations generally fall below 5 ng/mL less than 3 hours after smoking. THC is highly lipid soluble, and plasma and urinary elimination half-lives are best estimated at 3-4 days, where the rate-limiting step is the slow redistribution to plasma of THC sequestered in the tissues. … Plasma THC concentrations in occasional users rapidly fall below limits of quantitation within 8 to 12 h. THC is rapidly and extensively metabolized with very little THC being excreted unchanged from the body. THC is primarily metabolized to 11-hydroxy-THC which has equipotent psychoactivity. The 11-hydroxy-THC is then rapidly metabolized to the 11-nor-9-carboxy-THC (THC-COOH) which is not psychoactive.”
NHTSA, Drugs and Human Performance Fact Sheets, 2014
While THC in the blood might impair some drivers, Carboxy THC cannot. Carboxy THC is not evidence of marijuana DUI, ever.
Marijuana DUI: Blood D-9 THC vs. Carboxy-THC
Unlike urine, blood tests combined with other evidence, can support allegations of being under the influence of marijuana. Studies have shown that high THC blood levels can sometimes coincide with impaired driving. But low THC blood levels have almost no relation to bad driving. And sometimes THC positive drivers have shown improved driving.
Carboxy-THC has zero psychoactive effect. It cannot affect driving one way or another. It’s a metabolite of THC. But it’s not THC. So then why test for it?
A positive lab test for Carboxy THC shows past use of marijuana. But it does not show recent use. In fact, the first time you smoke marijuana you’ll immediately have THC in your blood, but no Carboxy-THC. Your body will need time to break down the THC, first into Hydroxy-THC, then into Carboxy THC.
So Carboxy-THC can indicate lack of recent use, in this brief situation. In any event, Carboxy THC cannot and does not indicate recent use, or possible impairment. It’s not evidence of marijuana DUI, as a result.
Metabolites confusing, not probative: Testing & Marijuana DUI
Labs can and do commonly test for D-9-THC and the metabolites Hydroxy-THC and Carboxy-THC. Lab reports usually show levels for all three.
In non-driving contexts, you may want to know about marijuana use in the past month or so. For example a probation officer might want to know about past use, where a condition of probation is “no use of non-prescribed marijuana.”
But in a marijuana DUI case a positive lab result for Carboxy-THC has no value. Because it does not indicate recent use. On the other hand, a blood THC level can be evidence of use within the past 12 hours or so.
Is marijuana DUI real?
The truth? We know that driving with 0.08 BAC alcohol or with two or more passengers is far riskier. And we know that the risk is comparable to driving while pregnant or while talking on your hands-free cell phone. But neither of those are a crime.
All of those create slightly elevated risks. But only one is currently a crime. And this makes no sense, because the fear that driving after marijuana is the same as driving after alcohol, is just not based on evidence. It’s not real. Because the evidence shows the risk of marijuana DUI is extremely low.
To equate the problem of marijuana DUI with the problem of DUI-alcohol is a false equivalence. They are not equal risks. Not even close. See:
Marijuana Legalization in Minnesota: Doing It the Right Way
About the author & marijuana DUI
Thomas C. Gallagher is a Minnesota criminal defense attorney, with decades experience helping people fight DUI alcohol charges. And Attorney Thomas Gallagher defends clients from charges of marijuana DUI.
Police stops for possible driving offenses or equipment violations, sometimes lead to a car search.
And when they do, if the police officer finds marijuana in a motor vehicle, she may check out the driver with suspicion of possible marijuana DUI. “When did you last use marijuana today?” is the leading question. And after an arrest, police will want a blood sample for testing; and seek cooperation with extended field exercises. More about that on our page, defending DUI marijuana cases (link below).
Question? Call Attorney Thomas Gallagher, 612 333-1500
Attorney Thomas Gallagher teaches Continuing Legal Education courses on Marijuana DUI law to prosecutors, defense attorneys, and judges.
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