Just when we thought Canada had answered questions around road safety and the use of cannabis, saying it was not another problem to worry about, came a couple of United States studies warning that driving under the influence of cannabis is common and is indeed a threat to road safety, writes MedicalBrief.
Research from the University of California, San Diego, found “significantly diminished” driving ability among half of the people who smoked cannabis with THC – its psychoactive compound. Meanwhile, a study from Columbia University found that 40% of drivers who reported using alcohol and cannabis in the past year drove under the influence of one or both.
When Canada implemented cannabis legalisation in 2018, there was concern about increased traffic harms, especially among youth, as MedicalBrief reported last November: link to the archive story below. But research by the University of Northern British Columbia and others, which studied 250,000 reports of traffic-related injuries, found that legalisation has not resulted in increased injuries among all drivers or youth drivers
It is useful to have an increasing volume of research into cannabis and road safety. Cannabis usage does not seem to be growing much in countries where it is now legal, such as Canada – it was happening with similar frequency when outlawed. But legalisation obliges the authorities to investigate ways of identifying and reducing potential public harms flowing from the use of cannabis.
UCSD Health studies cannabis consumption on driving ability
On 6 February 2022, the University of California – San Diego (UCSD) publication The Guardian reported that a two-year study had found that at least 50% of subjects who smoked cannabis with THC in a randomised clinical trial had a significantly diminished ability to drive compared to subjects who took a placebo cigarette.
This study was conducted at the UC San Diego Center for Medical Cannabis Research (CMCR) and was published on 26 January 2022 in the online issue of JAMA Psychiatry. The following UCSD story was written by Kaitlin Lee and published on 6 February,
Before this study, there were a number of small studies that examined learning, attention, and speed of processing under the influence of cannabis. Several studies before this one that focused on driving ability looked at how drivers swerve or adapt to a car in front of them.
“This ground-breaking research indicates that cannabis use does impair driving ability, but factors differ from alcohol,” California State Assembly member Tom Lackey said. “It also underscores the need for further research on this topic.
“Policy-makers still need a better understanding of the effects of different ways of consuming higher concentration products to charter a path forward.”
However, CMCR Co-director and Professor Thomas Marcotte explained that the studies were limited in their ability to represent accurate real-world cannabis usage.
“Many of those studies were done with occasional users,” Marcotte noted. “So one of the goals of our study was to really approximate real-world circumstances where someone who is a regular user will smoke to a desired highness and we wanted to see how being acutely high will affect their performance.”
Furthermore, co-author Professor Robert Fitzgerald noted that the%age of cannabis in the cigarettes used was higher than the amount of cannabis used in previous studies.
“Most previous studies used marijuana that was typically 3% or 6% THC by content,” Fitzgerald explained. “We were using a [5.9] and a 13.4% cannabis, which is closer to what’s available in the dispensaries.”
Researchers recruited 191 regular cannabis users, ages 21 to 55 years old, to smoke a cannabis cigarette after abstaining for 48 hours. The% concentration of cannabis in the cigarette varied in three amounts: 13.4%, 5.9%, and a placebo of 0.02% THC.
After smoking, subjects went through a simulation presented on a Driving Simulator System (Systems Technology, Inc) for approximately 25 minutes. The simulation required participants to maintain their lane position and speed in a straight roadway while responding to a divided attention task on an iPad on the dashboard. Throughout the day, after different intervals of time, subjects would go through the simulation again, doing so multiple times.
Their performance was measured by several different variables, such as the measures of variation of lateral position and speed, and the number of correct divided attention stimuli identified while driving. All of these were calculated into a score to encompass the driving performance; the higher the score, the worse the performance.
Main takeaways
There were several main takeaways from this study. One was that impairment is present even in individuals who are regular users, yet not all of those who were high were significantly impaired: 57 out of the 125 individuals who smoked THC would be classified as impaired at 30 minutes after inhalation.
Additionally, many people at the beginning reported they were hesitant to drive, Marcotte noted. “Roughly 50% of the people would not go on the road after smoking and felt they were too impaired,” Marcotte said. “However, at 90 minutes after smoking, they started feeling the impairment was wearing off.”
However, the performance in the simulator did not change from the 30-minute and the one hour and 30-minute marks, despite what subjects claimed. Both groups of smokers with cannabis in the cigarette performed significantly worse than the placebo group at 30 minutes.
Additionally, performance didn’t differ between the two different levels of cannabis content, suggesting that the levels of cannabis intake resulted in comparable levels of highness.
The impairment lasted until about four-and-a-half hours for the entire group, with some individuals feeling less impaired after three and a half hours. However, the paper notes that at three and a half hours, the THC groups’ driving was no longer significantly different from the placebo group’s driving.
Generally, the paper noted there was no relationship between THC blood concentration and impairment. For instance, those who smoked the 5.9% concentration had a higher blood concentration than those who smoked the higher dose, showing that THC content in cigarettes does not yield an expected higher blood concentration.
Furthermore, the group with the highest cannabis usage in the past six months had significantly higher THC blood concentrations after smoking but didn’t perform worse than those with lower THC blood concentrations; this suggests behavioural tolerance. However, those in this group compensated by ingesting more THC, and thus were still as debilitated as other users.
“People who are regular users believe that because they use all of the time, they don’t get as stoned with the same amount of THC as someone who’s an infrequent user, and we actually found that is true,” Marcotte explained. “But then they ingested more THC to get that level of highness. So in the end, they’re just as impaired as the other group even though they have developed this tolerance.”
This finding is significant because this acts as evidence against ‘per se’ laws. Per se laws establish a statutory violation if a legal standard is breached, such as blood-alcohol concentration in driving under the influence laws.
This finding shows that when it comes to policing cannabis intake and driving, different laws may have to be made since impairment can’t be deduced by cannabis content in cigarettes, behavioural tolerance, or THC blood concentrations.
When asked how to police driving under the influence, Fitzgerald stated that he thinks that it comes down to both officer observations of driving performance as well as toxicology testing.
“Having a set cut point and you’re above this level you’re impaired, you’re below this level you’re not impaired – is not scientifically valid,” Fitzgerald stated. “That comes down to a combination of officer observations along with toxicology testing.”
Marcotte added that the study provides valuable information to users, warning them that they are not always the best judges of their ability to drive. He also noted users should know that a couple of hours after smoking may be the riskiest time to drive.
“An important thing for users to know is [that] because you’re an experienced user does not necessarily mean you’re going to be less impaired. If you smoke to a level of highness, you still can be a danger,” Marcotte said.
The paper notes there were several limitations with the study. Despite the sample’s range of how often subjects used cannabis, it didn’t include infrequent users, those who smoke medicinally, and nonusers.
Subjects were told to get to a level of highness as they would casually at home, meaning that the study didn’t address controlled dosing or very elevated levels of highness. Additionally, effects on vision and cognition were not measured, and no measurements were taken between an hour and 30 minutes to three hours and 30 minutes after smoking.
Marcotte stated that there will be an additional study looking into the combined effects of alcohol and cannabis on drivers.
“Quite often, you do not find cannabis alone in impaired drivers on the road. It’s usually a combination of alcohol or some other substance,” Marcotte explained. “We have a new study starting up to look at the combination of alcohol and cannabis, and seeing whether you can be an alcohol level below the legal limit and officially, you’re fine at least in terms of the blood levels. But what happens when you have some THC onboard as well?”
In the future, the paper notes that research should look into different factors relating to driving behaviour such as biological differences, personal experience with cannabis, and cannabis intake methods.
In terms of biological differences, Marcotte noted that there may be genetic, metabolic, or sex-based differences in how people process THC, but there have been few studies investigating this.
Fitzgerald elaborated that they’re interested in studying other methods of ingesting cannabis, such as oral routes like gummies and edibles in addition to highly concentrated forms like vapes and concentrated butane hash oil (also known as dabs). However, he and Marcotte noted that studying vapes and dabs would be difficult on a federal level.
“We cannot touch those products because they are federally illegal, but as you know vaping, dabbing gives you a concentrated THC, and there are many edibles that have yet to be investigated,” Marcotte clarified.
Over 40% of drivers with past-year alcohol and cannabis use report DUI
In a study from the Columbia University Mailman School of Public Health, researchers found that two in five drivers who reported using alcohol and cannabis in the past year drove under the influence of alcohol, cannabis or both.
The findings have just been published in the American Journal of Preventive Medicine. This following is the Columbia University material published on 8 February 2022.
Earlier research suggested that simultaneous alcohol and cannabis use increases driving impairment, leading to an uptick in the risk of traffic fatality more than either substance individually, particularly among young adolescents. Yet, until now, no nationally representative study has tested the relationships between simultaneous use and driving under the influence of these substances.
“Alcohol and cannabis are two of the most common substances involved in impaired driving and motor vehicle crashes in the US,” said Priscila Dib Gonçalves in the Department of Epidemiology at Columbia Mailman School, and first author.
“Examining the effect of simultaneous alcohol and cannabis use on self-report driving under the influence of alcohol-only, cannabis-only, and both substances using a nationally representative sample could contribute to better understanding the impact in adolescents and adults.”
Drivers aged 16 years or older who reported any past-year alcohol and cannabis use in the National Survey on Drug Use and Health (2016-2019) were included with a final study sample of 34,514 individuals. The outcome was reporting any past-year driving under the influence of alcohol-only (DUI-A), cannabis-only (DUI-C), alcohol and cannabis (DUI-A+C), or no DUI.
Data were further collected via face-to-face household interviews using computer-assisted interviewing and audio computer-assisted survey instruments to increase the accuracy of responses to potentially sensitive questions.
Between 2016-2019, 42% of drivers with past-year alcohol and cannabis use reported any past-year DUI (8% DUI-A, 20% DUI-C, 14% DUI-A+C). Simultaneous use was associated with 2.88 times higher odds of driving under the influence of cannabis, and 3.51 times higher odds of driving under the influence of both alcohol and cannabis.
Most participants were male (57%), non-Hispanic white (67%) with a family income of $40,000 or less (63%), and living in a state with medical cannabis laws (68%).
Two-thirds of participants reported any tobacco use in the past year, and one-third used any drug other than cannabis. In addition, 8% reported daily alcohol use, 20% daily cannabis use, and 21% met criteria for alcohol use disorder, and 18% for cannabis use disorder. Over a quarter of the sample (28%) reported simultaneous alcohol/cannabis use.
Daily alcohol and cannabis use increased the likelihood of DUI-A and DUI-C, respectively, and both alcohol/cannabis daily use was associated with DUI-A/C.
“In the context of increasing daily cannabis use among adults, our findings connecting daily cannabis use and DUI raises public health concerns,” observed Dr Pia Mauro, assistant professor of epidemiology and senior author.
“Population changes in cannabis use frequency that may be associated with health hazards, including daily use, need to be continuously monitored.”
“Our study is unique in that it reports more recent nationally representative data (2016-2019) and compares different types of DUI categories,” noted Gonçalves. “From a harm reduction perspective, identifying which population subgroups are at high risk for DUIs could assist the development of more focused prevention strategies. Future research should also investigate the potential impact of low or “promotional” cannabis prices with higher levels of use, intoxication, and simultaneous use of other substances.”
Study details
Driving Performance and Cannabis Users’ Perception of Safety: A randomised clinical trial
Thomas D Marcotte, Anya Umlauf, David J Grelotti et al, Emily G Sones, Philip M Sobolesky, Breland E Smith, Melissa A Hoffman, Jacqueline A Hubbard, Joan Severson, Marilyn A Huestis, Igor Grant and Robert L Fitzgerald.
Author affiliations: University of California San Diego, Santa Clara Valley Medical Center in San Jose, California; LetsGetChecked Labs in Monrovia, California; Vividion Therapeutics in San Diego; Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire; Brainbaseline in Iowa City; and the Institute for Emerging Health Professions at Thomas Jefferson University in Philadelphia, United States.
Published online in JAMA Psychiatry on 26 January 2022.
Abstract
Importance
Expanding cannabis medicalisation and legalisation increases the urgency to understand the factors associated with acute driving impairment.
Objective
To determine, in a large sample of regular cannabis users, the magnitude and time course of driving impairment produced by smoked cannabis of different Δ9-tetrahydrocannabinol (THC) content, the effects of use history, and concordance between perceived impairment and observed performance.
Design, setting and participants
This double-blind, placebo-controlled parallel randomised clinical trial took place from February 2017 to June 2019 at the Center for Medicinal Cannabis Research, University of California San Diego. Cannabis users were recruited for this study, and analysis took place between April 2020 and September 2021.
Interventions
Placebo or 5.9% or 13.4% THC cannabis smoked ad libitum.
Main outcomes and measures
The primary end point was the Composite Drive Score (CDS), which comprised key driving simulator variables, assessed prior to smoking and at multiple time points postsmoking. Additional measures included self-perceptions of driving impairment and cannabis use history.
Results
Of 191 cannabis users, 118 (61.8%) were male, the mean (SD) age was 29.9 (8.3) years, and the mean (SD) days of use in the past month was 16.7 (9.8). Participants were randomised to the placebo group (63 [33.0%]), 5.9% THC (66 [34.6%]), and 13.4% THC (62 [32.5%]).
Compared with placebo, the THC group significantly declined on the Composite Drive Score at 30 minutes (Cohen d = 0.59 [95% CI, 0.28-0.90]; P < .001) and 1 hour 30 minutes (Cohen d = 0.55 [95% CI, 0.24-0.86]; P < .001), with borderline differences at 3 hours 30 minutes (Cohen d = 0.29 [95% CI, –0.02 to 0.60]; P = .07) and no differences at 4 hours 30 minutes (Cohen d = –0.03 [95% CI, –0.33 to 0.28]; P = .87).
The Composite Drive Score did not differ based on THC content (likelihood ratio χ24 = 3.83; P = .43) or use intensity (quantity × frequency) in the past 6 months (likelihood ratio χ24 = 1.41; P = .49), despite postsmoking blood THC concentrations being higher in those with the highest use intensity.
Although there was hesitancy to drive immediately postsmoking, increasing numbers (81 [68.6%]) of participants reported readiness to drive at 1 hour 30 minutes despite performance not improving from initial postsmoking levels.
Conclusions and relevance
Smoking cannabis ad libitum by regular users resulted in simulated driving decrements. However, when experienced users control their own intake, driving impairment cannot be inferred based on THC content of the cigarette, behavioural tolerance, or THC blood concentrations.
Participants’ increasing willingness to drive at 1 hour 30 minutes may indicate a false sense of driving safety. Worse driving performance is evident for several hours postsmoking in many users but appears to resolve by 4 hours 30 minutes in most individuals.
Further research is needed on the impact of individual biologic differences, cannabis use history, and administration methods on driving performance.
Study details
Simultaneous Alcohol/Cannabis Use and Driving Under the Influence in the US
Priscila D Goncalves, Sarah Gutkind, Luis E Segura, Joao M Castaldelli-Maia, Silvia S Martins and Pia M Mauro.
Author affiliations: University of Columbia.
Published online by the American Journal of Preventive Medicine on 7 February 2022.
Abstract
Alcohol and cannabis are commonly involved in motor vehicle crashes and fatalities. This study examines whether simultaneous use of alcohol/cannabis is associated with higher odds of reporting driving under the influence of alcohol and cannabis in the US.
Methods
Drivers aged ≥16 years with any past-year alcohol and cannabis use in the 2016–2019 National Survey on Drug Use and Health (N=34,514) reported any past-year driving under the influence of alcohol-only, cannabis-only, both alcohol/cannabis, or not driving under the influence.
Survey-weighted associations between simultaneous alcohol/cannabis use and each of the driving under the influence outcomes were computed adjusting for sociodemographics and daily alcohol/cannabis use. Analyses were conducted from November 2020 to September 2021.
Results
In 2016-2019, 42% of drivers with past-year alcohol and cannabis use reported driving under the influence (8% alcohol-only, 20% cannabis-only, 14% alcohol/cannabis).
Simultaneous alcohol/cannabis use was associated with 2.88-times higher adjusted odds of driving under the influence of cannabis-only (95% CI=2.59, 3.19) and 3.51-times higher adjusted odds of driving under the influence of both alcohol/cannabis (95% CI=3.05, 4.05), compared to not driving under the influence.
Associations with driving under the influence of alcohol-only were unexpectedly in the opposite direction (adjusted conditional odds ratio=0.59, 95% CI=0.45, 0.79).
Conclusions
Overall, 2 in 5 drivers who used alcohol and cannabis reported driving under the influence of alcohol and/or cannabis. People reporting simultaneous alcohol/cannabis use were more likely to report cannabis-related driving under the influence.
Prevention strategies should target individuals reporting simultaneous alcohol/cannabis use to reduce the occurrence of driving under the influence.
See also from the MedicalBrief archives
Cannabis legalisation has not led to increased traffic injuries in Canada
Canada’s legalisation of cannabis is a success story, despite a shaky first act
Safe driving: Cannabis intoxication not reliably measured by field tests, THC levels
Luxembourg first in Europe to legalise cannabis; Canada sees mostly good results