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A new study published in Psychopharmacology suggests that frequent cannabis users show no significant driving impairment in a simulator after a break of at least two days. The findings from the Center for Medicinal Cannabis Research at the University of California San Diego challenge some common assumptions about the lingering effects of cannabis and could have significant implications for public health and traffic safety laws.
Cannabis is a plant that contains chemical compounds called cannabinoids, with the most well-known being delta-9-tetrahydrocannabinol, or THC, which is responsible for the plant’s psychoactive effects. As cannabis use becomes more widespread, both for medicinal and recreational purposes, understanding its full range of effects on complex, real-world activities is a pressing concern for scientists and policymakers.
It is well-established that using cannabis right before driving can impair performance. Studies have consistently shown that acute intoxication from THC can lead to decrements in attention and memory, and in driving simulators, it can cause increased swerving and difficulty in maintaining a consistent speed or following another vehicle. A significant question that remains is whether these effects persist after the immediate “high” has worn off.
These potential lingering, or “residual,” effects have been a topic of debate. Some previous research has pointed to subtle deficits in learning and memory that can last for days or even weeks after stopping cannabis use, especially in long-term, frequent users. However, other studies have found these effects to be small or to disappear relatively quickly. What has been less clear is whether any of these potential lingering cognitive issues translate into observable problems with a practiced skill like driving. The researchers behind the new study sought to address this knowledge gap by examining if short-term residual effects from cannabis negatively impact driving performance in the absence of acute intoxication.
To investigate this question, the scientists designed a two-part study. The first part aimed to see if there was a relationship between a person’s history of cannabis use and their driving ability after a period of abstinence. The researchers recruited 191 regular cannabis users between the ages of 21 and 55. A key requirement for participation was that all individuals had to agree to abstain from using cannabis for at least 48 hours before their test sessions. This abstinence was confirmed using an oral fluid test on the day of the experiment.
On the testing day, before any other procedures, each participant completed a 25-minute session in a sophisticated driving simulator. This simulator featured a three-screen, wide-angle display, along with a steering wheel, accelerator, and brake pedals to mimic a real driving experience. The simulated drive included a mix of urban and rural settings and presented participants with common challenges like making left turns into oncoming traffic and avoiding potential crashes.
To get a comprehensive assessment of driving ability, the researchers combined multiple performance measures into a single Composite Drive Score. This score included variables like swerving within a lane, the ability to match the speed of a lead vehicle, and performance on a divided attention task that required participants to touch circles on a screen while continuing to drive. A higher score indicated poorer overall driving performance.
The researchers found no relationship between the participants’ overall driving performance and their history of cannabis use. They looked at several factors, including the total amount of cannabis a person had consumed over the last six months, the number of days they used cannabis in the last month, and the quantity they used per day. None of these measures of use intensity predicted how well a person performed in the driving simulator.
The scientists also divided the participants into three groups based on their consumption levels: low, medium, and high. Still, there was no difference in the average driving scores among these groups. The length of the abstinence period, which ranged from two to 21 days, also showed no connection to driving ability. Similarly, the age at which a person first started using cannabis did not appear to influence their performance.
The researchers also analyzed blood samples taken from the participants on the test day. They found no correlation between the concentration of THC or its main inactive byproduct, called THC-COOH, and the driving score. Some jurisdictions use legal blood THC limits to determine driving impairment. In this study, about five percent of participants had THC levels that would have classified them as impaired in some states, despite having abstained for at least 48 hours and showing no signs of poorer driving in the simulation.
The second part of the research was designed to make a direct comparison between very frequent cannabis users and people who do not use the substance. For this, the scientists identified the 18 participants from the first study with the highest intensity of use. These individuals had used cannabis on at least 28 of the previous 30 days and consumed, on average, more than two grams per day. Their driving performance was then compared to that of a separate group of 12 healthy adults who had not used cannabis in the past year. This non-using group completed the exact same training and 25-minute simulated drive as the cannabis users. The results of this comparison mirrored the findings from the first study.
There was no significant difference in the Composite Drive Score between the very frequent cannabis users and the non-using comparison group. The two groups performed similarly on the overall measure of driving ability as well as on all the individual sub-tasks, such as swerving and car following. The groups were well matched in terms of age, sex, and yearly miles driven, ensuring a fair comparison, although the non-using group did have more years of formal education on average.
The researchers acknowledged some limitations to their study. The sample size for the second study, which directly compared frequent users to non-users, was relatively small, which means it might not have had enough statistical power to detect very small differences between the groups. Another key point is that a driving simulator, while sophisticated, is not the same as driving on a real road. It is possible that subtle deficits might appear in real-world situations not captured by the simulation. The 25-minute simulated drive may also not have been long or complex enough to challenge participants in a way that would reveal lingering impairments, particularly in sustained attention.
Future research could build on these findings by using larger groups of participants, employing more complex or longer driving tasks, and controlling for potential confounding variables. Despite these limitations, the study provides a robust examination of residual cannabis effects on driving and suggests that after a two-day period of abstinence, frequent cannabis users do not exhibit impairment on this simulated task. These results contribute important data to ongoing discussions about cannabis, public safety, and the development of fair and evidence-based laws for driving under the influence.
The study, “Short-term residual effects of smoked cannabis on simulated driving performance,” was authored by Kyle F. Mastropietro, Jake A. Rattigan, Anya Umlauf, David J. Grelotti, Marilyn A. Huestis, Raymond T. Suhandynata, Igor Grant, Robert L. Fitzgerald, and Thomas D. Marcotte.
