How Do Cannabis Edibles Affect Driving? New Research Insights

Everyone is aware of the dangers of drunk driving, but what about cannabis-impaired driving? A per se threshold of 2 or 5 ng/mL of THC in blood is the fixed legal limit used by some states to prosecute cannabis-impaired driving¹; however, unlike alcohol, these numbers do not reliably prove active impairment.

A recent randomized, double-blind, placebo-controlled, crossover clinical trial sought to understand whether driving simulator performance is dose-dependently impaired by commercially available edible cannabis. Investigators found that cannabis edibles dose-dependently impaired lateral vehicular control in a driving simulator at blood THC concentrations below the aforementioned common per se thresholds.²

Study Details

The study was completed at a single-site tertiary care academic hospital in Toronto, Ontario, Canada, between September 2024 and April 2025. Participants were healthy adults aged 19 to 45 years endorsing at least weekly cannabis use and monthly use of cannabis edibles. Participants were given edible, oral delta-9-tetrahydrocannabinol (THC) soft-chew gummies, 0 or 2 mg each, to achieve doses of 0, 2, 10, and 20 mg. They were then asked to participate in driving simulations 2 and 5 hours after THC exposure. This was the primary outcome (reported as difference in means [95% CIs]) assessed in the intention-to-treat population. Secondary outcomes included driving simulator standard deviation of speed (SDS) and reaction time (RT), self-reported driving willingness and perceived ability, and whole-blood THC and metabolite levels.

Each drive included 3 scenarios: the first 2 scenarios were nearly identical drives on a rural road, but 1 had no distractor task (single task), whereas the other scenario (dual task) had an added secondary task (such as counting backward by 3s from a large, randomly generated number). The third scenario captured reaction time, and participants were instructed to stop when they saw a stop sign directly facing them, but not to stop when they saw a stop sign at an angle.

A total of 52 participants were deemed eligible at enrollment, 10 withdrew before receiving a dose of THC, and 2 were removed from the analysis. Thus, a total of 40 participants (20 [50.0%] female and 20 [50.0%] male) were included in the analysis (mean [SD] age, 30.4 [6.9] years).

There was a significant main effect of THC dose on standard deviation of lateral position (SDLP) during the single task of driving (F_3,214 = 19.24; P < .001), but no significant dose by time interaction. Relative to placebo, SDLP was significantly increased at 20 mg (3.3 [95% CI, 2.4 to 4.3] cm; P < .001) and 10 mg (2.0 [95% CI, 1.0 to 2.9] cm; P < .001), but not 2 mg (0.7 [95% CI, −0.3 to 1.6] cm; P = .16). The secondary driving measures reaction time and standard deviation of speed were also negatively influenced by THC exposure.

Investigators found a significant effect of THC dose on maximum speed (F_3,108 = 2.94; P = .04), which was higher at 20 mg (difference in means, 2.019 [95% CI, 0.528-3.510] km/h; P = .008) and 10 mg (difference in means, 1.640 [95% CI, 0.119-3.161] km/h; P = .04) compared with placebo.

Driving impairments included:

• Decreased lane control
• Increased speed variability
• Increased reaction time, especially at the highest dose (20 mg)
• Participants felt less willing to drive and perceived lower driving ability in a dose-dependent manner, even at the low dose

Do We Need a New Standard for Cannabis-Impaired Driving?

Previous research has suggested that a difference of 2.4 cm in SDLP indicates impairment, equivalent to a blood alcohol concentration of 0.05%.³ However, all observed peak THC concentrations in this study, including in the high and medium dose conditions (3.4 and 1.6 ng/mL, respectively), were near or below common per se thresholds, such as the 5 ng/mL hybrid offense threshold in Canada or the lower 2 ng/mL threshold, yet the participants demonstrated clear evidence of driving impairment.

“Increasing use of edible THC products may present a unique challenge to enforcement of per se legislation,” said the study authors.²

References

1. Fitzgerald RL, Umlauf A, Suhandynata RT, et al. Per se driving under the influence of cannabis statutes and blood delta-9-tetrahydrocannabinol concentrations following short-term cannabis abstinence. Clin Chem. 2025;71(12):1225-1233.

2. Le Foll B, Matheson J, Antwi P, et al. Dose-dependent effects of cannabis edibles on simulated driving performance: a randomized clinical trial. JAMA Netw Open. 2026;9;(8):e2631306.

3. Verster JC, Roth T. Standard operation procedures for conducting the on-the-road driving test, and measurement of the standard deviation of lateral position (SDLP). Int J Gen Med. 2011;4:359-371.