Cannabis Effects on Driving Performance: Clinical Considerations

• Driving under the influence of cannabis has been identified as a public health concern as medical and recreational cannabis availability increases in some countries.

• A recent randomized clinical trial found similar levels of acute driving impairment with THC-dominant cannabis and with a combination of THC-CBD equivalent cannabis using on-road driving tests that provided real-world conditions; however, CBD-dominant cannabis did not produce significant cognitive or psychomotor impairment compared with placebo in this trial.

• Media coverage of this study conveyed the findings as CBD-dominant cannabis not causing driving impairment while THC-dominant cannabis does, with the latter lasting up to 4 h post-dose.

• It is recommended that clinicians counsel about the risks of driving impairment when patients disclose use of cannabis products containing THC.

Driving under the influence of cannabis (DUIC) is an important public safety concern as access to medical and recreational cannabis increases in North America and internationally [1]. Following alcohol, cannabis is the most commonly detected drug in both crash-involved drivers and the general driving population [2, 3]. Medical and recreational use may differ in the cannabinoids used, however, which may differentially impact driving ability. ∆⁹-tetrahydrocannabinol (THC) is the most psychoactive cannabinoid and responsible for the intoxicating “high” and impairing side effects associated with cannabis. In contrast, cannabidiol (CBD) may counteract some THC effects but can still produce effects of drowsiness [4] which may impair one’s driving ability. In addition to cannabinoid content, factors such as user tolerance, product potency, dosage, and route of administration affect the onset, intensity, and duration of impairment. It should also be considered that cannabis may affect driving performance acutely (in the short-term) or over the long-term if a person is using sufficient quantities of cannabis for a sustained period.

Although several studies have concluded there is moderate increased crash risk associated with acute cannabis use, few studies have explored the magnitude and duration of driving impairment with varying concentrations of THC and CBD including CBD alone [1, 5‒7]. A randomized clinical trial (RCT) assessing cannabis use on driving performance by Arkell et al. drew significant attention on this subject recently as the first study to examine four experimental drug conditions: (1) CBD-dominant cannabis (13.75 mg CBD), (2) THC-dominant cannabis (13.75 mg ∆⁹-THC), (3) THC/CBD-equivalent cannabis (13.75 mg CBD; 13.75 mg ∆⁹-THC), and (4) placebo (trace CBD, ∆⁹-THC) [8]. This commentary aims to elaborate on its findings relative to past research and to provide context on how the available evidence can guide conversations on cannabis use safety and ultimately inform policymakers on public health implications.

The Arkell trial used a randomized, double-blind, within-subject, placebo-controlled, crossover design. Participants (n = 26 enrolled; n = 22 completers) received a unique vaporized cannabis dose during four sessions (1 drug condition per session, 1 week apart) and completed two on-road driving tests during each session – one at 40–100 min post-dose and one at 240–300 min post-dose. Participants were accompanied by a trained driving instructor who had dual accelerator and brake control access. Cognitive tests, biological specimen collection, and subjective drug assessments were administered at baseline and at subsequent intervals post-dose. The primary endpoint was standard deviation of lateral position (SDLP, a measure of horizontal lane weaving) at 40 and 240 min after cannabis consumption. At 40–100 min post-dose, SDLP was significantly increased after THC-dominant (2.33 cm, 95% CI, 0.08–3.86; p < 0 .001) and THC/CBD-equivalent cannabis (2.83 cm, 95% CI, 1.28–4.39; p < 0.001) administration, but not after CBD-dominant cannabis (−0.05 cm, 95% CI, −1.49–1.39; p > 0.99), relative to placebo. At 240–300 min post-dose, all of the active doses were similar to placebo with no impairment detected.

The within-subjects design is a key strength as it minimizes bias that may occur with a between-subjects design and offers a statistical advantage of power. However, 15% of the randomized participants ultimately did not complete the study. Additionally, the sample in this study was comprised of a younger age group (mean 23.2 years, SD 2.6), which limits generalizability. While cannabis use is most prevalent among young adults (18–25 years) [1], adults ages 50 years and older represent the largest proportion of growth in the cannabis user population [9], yet generalizability to middle-aged and older adults is lacking, a particular concern with vision declines with age. Furthermore, older adults are more likely to use multiple prescription medications and may experience further impairment when combined with cannabis [10].

The study was conducted in the Netherlands and therefore, international differences in both driver education and the requirements to obtain a driver’s license should be considered. Similarly, differences in societal acceptance of cannabis vary by country, which may translate to differences in how law enforcement decides to adopt policies to assess or pursue suspected impairment, such as zero tolerance or effect-based “per se limit” approaches. The authors also acknowledge that the CBD dosage used was lower than that used in approved prescription products for epilepsy, but is more similar to CBD doses in recreational and medical cannabis products. While the CBD dosage in this study may have been on the low end, a recent RCT that tested a range of acute CBD doses (0–1,500 mg) found simulated driving was not significantly impairing or intoxicating, which supports findings in the Arkell et al. study [11]. Still, we cannot conclude that CBD does not cause any degree of impairment given strong sedation has been demonstrated in clinical trials of high-dose prescription products [4, 12].

In comparison to recent experimental studies in which cannabis was administered on-site (see Table 1 for a summary of relevant studies), there appears to be consistency in findings that THC, whether alone or in combination with CBD, adversely affects some conventional driving measures (i.e., SDLP [13] and mean speed [14]) but not all (i.e., simple processing-speed tasks [15]) [7]. Despite differences in strengths of cannabis, routes of administration, and use of a placebo group, some experimental studies appear to support a dose-dependent relationship between THC and driving ability impairment [14, 16]. Two trials demonstrated that peak plasma THC concentrations play a role in magnitude of impairment, in chronic users compared with occasional users [16], as well as with THC/CBD equivalent (11% THC, 11% CBD) cannabis suggesting a potential pharmacokinetic (or potentially pharmacodynamic) interaction [13].

This aligns with findings from prospective observational studies of chronic cannabis users. A prospective cross-sectional study by Dahlgren et al. [17] found significant driving impairment among chronic, heavy, recreational cannabis users compared with healthy controls with the cannabis users abstaining at least 12 h prior to a simulated driving test. A similarly designed study by Doroudgar et al. [18] found significant differences between chronic cannabis users and nonusers in failed standardized field sobriety tests and slower visual reaction times, although they deviated less in speed. Limitations of these studies are differences in THC detection (Dahlgren et al. used plasma and oral samples; Doroudgar et al. used urinary samples). Additionally, Dahlgren et al. note that impulsivity personality traits may be an important confounder as an impulsive style of driving performance was observed among non-intoxicated cannabis users, characterized by increased accidents, speeding, lateral movement, and decreased rule-following [17].

What the totality of the research thus far lacks, as summarized well by the National Academy of Science, Engineering, and Medicine’s report, are important methodological limitations of “DUIC” not necessarily referring to acute intoxication but satisfied by recent use; and a lack of definitive causal evidence for the association of THC levels in blood with either acute intoxication or driving impairment, which has inhibited determining a clear dose at which driving becomes sufficiently unsafe as to increase motor vehicle accident (MVA) risk [1]. Epidemiological studies generally support that there is a moderate risk with cannabis use and being involved in or responsible for a MVA [7]. A meta-analysis by Rogeberg and Elvik of 21 case-control or culpability studies, found that DUIC (by self-reported cannabis use or the presence of THC metabolites in urine or blood) was associated with 20–30% higher odds of an MVA [6]. However, simulated driving studies were not included and the authors described the magnitude of the association as low to moderate [6].

It should be noted by clinicians that self-reported use and presence of a THC metabolite in screening tests (e.g., urine and blood tests) do not necessarily equate to acute impairment. Blood tests for THC are challenging to administer in a timely fashion immediately after an accident or concerning traffic event, which limits the utility of existing tests as enforcement tools. In a follow-up study in 2019, Rogeberg showed that culpability studies tend to misinterpret effect estimates and substantially exaggerate risk associated with cannabis [5], likely in part due to the lack of acuity in distinguishing impairment from recent exposure. A US National Highway Traffic Safety Administration report on marijuana-impaired driving concurs with exercising caution in the interpretation of risks obtained from culpability studies [19]. Another recent review concluded that there is evidence of significant correlation between high THC plasma concentration (≥3 ng/mL) and risk but not for lower levels of THC (1–2 ng/mL), though establishing a clear cut-off value remains elusive practically speaking [20]. Numerous factors affect THC blood concentrations including route of administration, regular cannabis use, and time-lapse between exposure and measurement, in addition to variability in collection, storage, and analytic methods. Yet, THC blood level cut-off are already recommended as per se limits by experts and policymakers in some USA, Canada, European countries, and other jurisdictions [20]. Overall, the breadth of the cannabis crash risk literature demonstrates that acute cannabis intoxication can impair driving-related skills and neurobehavioral skills (e.g., impaired executive function which increases reaction time, such as stopping at a red light) [7, 19, 21, 22]. Still, high-quality data sources for tracking cannabis use and monitoring DUIC are critically needed for public health research and policymaking [23].

The current evidence base begs two unanswered questions for DUIC: (1) how, or to what extent, do cannabis and its many constituents impair an individual’s ability to drive? and (2) is association between cannabinoid levels and the extent of impairing effects accurate and reliable? Scenarios not addressed in the Arkell study highlight questions that warrant more research including: tolerance by regular users (i.e., medical cannabis users); varying modes of administration (e.g., edibles have lower bioavailability but a delayed onset of action leading to longer lasting impairment [24]); impairment when cannabis is combined with other substances; and variability of cannabis products at the local, national, and global level. A conservative approach is to exercise caution following administration of, and up to at least 5 h after [7], THC-containing cannabis products before operating a vehicle. Canada’s “Lower Risk Cannabis Use Guidelines” may be considered as a starting point for clinical recommendations [25].

In the absence of standard and universal thresholds indicating impairment for DUIC, as well as lack of cannabis packaging warnings, it is recommended that clinicians counsel their patients on driving safety and risks, particularly if it is known that the patient is using THC-containing cannabis products. Counseling in a broad sense could communicate risks of impairment for at least 5 h after using a THC-containing cannabis product alone and longer if concomitant with alcohol or other substances; or complete abstention from driving after cannabis use for a significant period (e.g., at least 12 h), particularly if residing in a jurisdiction with zero-tolerance laws. However, the type, dosage, and frequency of cannabis product use should be considered by the clinician when tailoring communication for their patient’s needs.

The authors have no conflicts of interest to declare.

Brianna Costales, Amie J. Goodin, and Joshua D. Brown are supported by State of Florida appropriations to the Consortium for Medical Marijuana Clinical Outcomes Research (mmjoutcomes.org).

Brianna Costales and Amie J. Goodin conceptualized and drafted the work. Shanna L. Babalonis and Joshua D. Brown drafted and revised. All approved the final version.

Evidence in Context is part of the outreach effort of the Consortium for Medical Marijuana Clinical Outcomes Research to examine and discuss implications of research into cannabis and cannabinoids for clinical practice, thus providing a translational approach to these studies to make clear, concise, and actionable evidence available for clinicians and patients.