1. Elderly participants who consumed cannabis 30 minutes before driving in a simulator were found to have increased standard deviation in their lateral position (SDLP), which was used as a measure to determine the amount of swerving a driver experienced.
2. Blood tetrahydrocannabinol (THC) levels were not significantly correlated with SDLP, though the study was limited by its smaller sample size.
Evidence Rating Level: 2 (good)
Study Rundown: Although prior literature suggests that cannabis consumption increases risks of motor vehicle collisions while under the influence, specifics surrounding the role of tetrahydrocannabinol (THC) levels remains unclear. The study sought to determine the effects of cannabis on the driving behaviors and skills of older individuals who regularly consume cannabis. For the study, participants were instructed to bring their cannabis in the form of a rolled joint to the laboratory for their use, before participation in a driver simulator. Researchers then examined the standard deviation of lateral position (SDLP) drivers experienced while under the influence, in addition to measuring mean speed, standard deviation of speed, maximal speed, and break latency. Among 31 elderly (age >65) participants included in the study, SDLP was increased at 30 minutes post-consumption. Interestingly, blood THC levels were not significantly correlated with SDLP however. Although the study methodology and its small sample size restrict the generalizability of findings, they do corroborate prior literature that driving under the influence of cannabis will likely increase the risk of motor vehicle collisions, even in elderly individuals.
Click here to read the study in JAMA Network Open
Relevant reading: Driving performance and cannabis users’ perception of safety
In-Depth [retrospective cohort]: Participants in the study were recruited through public advertisements placed throughout the city on public transit and also on social media platforms. The participants that were enrolled fell between the ages of 65 to 79 years old. When contrasting the least square mean (LSM) after smoking cannabis, significant differences were observed between the cannabis versus no cannabis condition completing the single task for SDLP (ES=0.30; b=1.65; 95% confidence interval [CI], 0.37-2.93, t90.4=2.53; P=0.01) and the dual-task (ES=0.27; b=1.75, 95% CI, 0.21-3.28; t89.6=2.24; P=0.03). At the first 30-minute interval, SDLP was increased while MS was decreased, however, at the 180-minute interval there was no change in these levels. The levels of THC in participants’ blood were significantly elevated at 30 minutes but not 180 minutes. When comparing THC values and driving measures at 30 minutes, there was no significant relationship under both the single- and dual-task conditions for SDLP (r=0.147; t29 = 0.802; P = 0.43; SDLP dual-task: r=0.027; t29=0.145; P=0.89) or for MS (r=0.206; t29=1.135; P=0.27; MS dual-task: r=0.056; t29=0.305; P=0.76). The findings of the study indicate that cannabis affected older adults’ driving.
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