Study: Half of Regular Cannabis Users Have Diminished Driving Abilities While High

A study conducted by researchers at the University of California San Diego Center for Medical Cannabis Research (CMCR) suggests that at least half of participants who smoke cannabis had a significantly diminished ability to drive compared to subjects who smoked a placebo cigarette, the university newspaper, The Guardian, reports. The study included 191 regular cannabis consumers, which CMCR Co-director and Professor Thomas Marcotte said was different than previous studies, of  which “many” were conducted with “occasional users.”

“So one of the goals of our study was to really approximate real-world circumstances where someone who is a regular user will smoke to a desired highness and we wanted to see how being acutely high will affect their performance.” — Marcotte to The Guardian

The consumers recruited for the study were aged 21 to 55-years-old and were directed to smoke a cannabis cigarette after abstaining for 48 hours. The concentration percentage of the consumed cannabis varied in three amounts: 13.4%, 5.9%, and a placebo of 0.02% THC. After smoking, participants completed a simulation presented on a Driving Simulator System (Systems Technology, Inc) for about 25 minutes, which required subjects to maintain lane position and speed in a straight roadway while responding to a divided attention task on an iPad on the dashboard. The study participants would complete the simulation multiple times throughout the day, after different intervals of time.

Performances were measured by different variables, including measures of variation of lateral position and speed, and the number of correct divided-attention stimuli identified while driving. These were calculated to encompass the driving performance — the higher the score, the worse the performance.

The study found that 57 out of the 125 individuals who smoked THC would be classified as impaired at 30 minutes after inhalation. The researchers also found that while impairment was present for the regular consumers, not all of those who were high were significantly impaired. About half of the participants indicated that they felt too impaired to drive prior to beginning the simulation, Marcotte said, but added that 90 minutes after smoking, the impairment was waning among that cohort; however, their performance in the simulator was unchanged from the 30-minute to 90-minute mark, despite that they thought they were less impaired.

The researchers also found that the group with the highest cannabis usage in the previous six months had significantly higher THC blood concentrations after smoking but didn’t perform worse than those with lower THC blood concentrations, suggesting behavioral tolerance.

“People who are regular users believe that because they use all of the time, they don’t get as stoned with the same amount of THC as someone who’s an infrequent user, and we actually found that is true,” Marcotte said in the report. “But then they ingested more THC to get that level of highness. So in the end, they’re just as impaired as the other group even though they have developed this tolerance.”

The paper, “Driving Performance and Cannabis Users’ Perception of Safety,” which was published in JAMA Psychiatry online on January 26, indicates that there were several limitations with the study in that it didn’t include infrequent users, those who smoke medicinally, and nonusers. Subjects were told to get to a level of highness as they would casually at home, meaning that the study didn’t address controlled dosing or very elevated levels of highness. Additionally, effects on vision and cognition were not measured, and no measurements were taken between an hour and 30 minutes to three hours and 30 minutes after smoking when participants claimed they were no longer feeling the cannabis’ effects.