How long cannabis affects driving ability following consumption, revealed

31st January 2022

A new study has found that, while cannabis users may present an increased willingness to drive at 90 minutes following consumption, this may represent a false sense of security regarding driving safety.

In fact, it likely takes much longer to ensure users are safe to drive. While the most intense effects of cannabis may wear off during this time, the drug can continue to have effects on the mind and body for hours after consumption.

In the study, published in JAMA Psychiatry, researchers reported that the perception of driving impairment decreased at 1 hour 30 minutes, despite no objective improvement in driving.

Participants were given either placebo, 5.9% or 13.4% THC cigarettes before sitting in a driving simulator.

The researchers collected Composite Drive Scores, assessed prior to smoking (baseline) and at several time points following consumption. The participants’ self-perception of driving impairment and cannabis use history were also recorded.

The results indicated that THC stays in the system for much longer than users anticipated. Furthermore, THC was found to impair driving abilities for up to four and half hours following consumption.

Of the 191 regular cannabis users that participated in the study, the Composite Drive Score of those in the THC group significantly declined at 30 minutes and 1 hour 30 minutes following consumption, in comparison to baseline.

Senior author Thomas Marcotte, PhD, co-director of CMCR and a professor of psychiatry at UC San Diego School of Medicine, said: “Although users in the THC group felt impaired and were hesitant to drive at 30 minutes, by 1 hour-30 minute they believed the impairment was wearing off and were more willing to drive. This was despite their performance not significantly improving from the 30-minute point.”

Researchers also reported “borderline” differences at 3 hours and 30 minutes. Finally, no differences in scores were identified at  4 hours and 30 minutes.

Despite scoring lower on key driving simulator variables, it is important to note that there were no significant differences between the three groups on the number of crashes at any time point in the Driving simulator.

The researchers from the Centre for Medicinal Cannabis Research at the University of California San Diego concluded: “Smoking cannabis ad libitum by regular users resulted in simulated driving decrements. However, when experienced users control their own intake, driving impairment cannot be inferred based on the THC content of the cigarette, behavioural tolerance, or THC blood concentrations.

“Participants’ increasing willingness to drive at 1 hour 30 minutes may indicate a false sense of driving safety. Worse driving performance is evident for several hours post smoking in many users but appears to resolve by 4 hours 30 minutes in most individuals.

“Further research is needed on the impact of individual biologic differences, cannabis use history, and administration methods on driving performance.”

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