Many Regular Cannabis Users Unaware of Detrimental Effects to Their Driving Ability After Smoking

Man with marijuana in car
With cannabis becoming increasingly legalized in the US, researchers examined the factors associated with driving impairment while smoking cannabis.

Ad libitum smoking of cannabis was associated with simulated driving decrements; however, many individuals were unaware of their level of impairment. These findings, from a double-blind, placebo-controlled, parallel, randomized clinical trial, were published in JAMA Psychiatry.

Between 2017 and 2019, cannabis users were recruited by the University of California San Diego. Participants (N=191) were aged 21 to 55 years, they had used cannabis ≥4 times during the past month, drove ≥1000 miles in the past year, and were willing to abstain from cannabis use for 2 days prior to experimental days. Participants were stratified by frequency of cannabis use and randomized to smoke a cigarette containing 13.4% (n=62), 5.9% (n=66), or 0.02% (control; n=63) Δ9-tetrahydrocannabinol (THC) and underwent repeated ~25-minute driving simulations.

The study population comprised 61.8% men, aged mean 29.9 (standard deviation [SD], 8.3) years, and who had used cannabis 16.7 (SD, 9.8) days in the past month.

At 15 minutes, blood THC differed significantly between treatment groups (P <.001). Nearly half (48.3%) of placebo recipients believed they had received THC. Among the THC recipients, stratified by regular use, individuals who had the lowest use had the lowest blood concentrations (median, 11.8; IQR, 3.8-31.4 ng/mL) and the highest users had the highest level (median, 60.5; IQR, 20.4-93.5 ng/mL).

The number of simulated vehicular crashes did not differ between cohorts (odds ratio [OR] range, 0.78-1.57; P >.75).

The Composite Driving Score (CDS) was not significantly different from baseline among the control group (all P ≥.07). Among the combined THC recipient cohort, CDS was significantly affected at 30 minutes and 1.5 hours (both P <.001). These findings indicated that the effects on driving began to wane by 3.5 hours and recovered at 4.5 hours after smoking.

Despite adverse effects on the CDS, 47.5% of the THC recipients indicated they would drive in their current state at 30 minutes and at 1.5 hours, 68.6% would drive.

There was no relationship between blood THC levels and CDS at 30 minutes (r, 0.25; P =.78) or at any time point thereafter.

For ethical reasons, this study allowed individuals to smoke their cannabis cigarette until they reached the level of highness they desired, which may have biased the differing dosing levels.

“In a placebo-controlled parallel study of regular cannabis users smoking cannabis with different THC cohort ad libitum, there was statistically significant worsening on driving simulator performance in the THC group compared with the placebo group. […] A lack of insight regarding driving impairments, particularly at 90 minutes, is of concern, given that users will likely self-evaluate when they feel safe to drive,” the study authors concluded.

Disclosure: Multiple authors declared affiliations with industry. Please refer to the original article for a full list of disclosures.


Marcotte TD, Umlauf A, Grelotti DJ, et al. Driving performance and cannabis users’ perception of safety: a randomized clinical trial. JAMA Psychiatry. Published online January 26, 2022. doi:10.1001/jamapsychiatry.2021.4037