A recent study has found that cannabis can impair a person’s ability to drive up to 4 hours after the drug is used.

In a study recently published in the JAMA Psychiatry, researchers from the Center for Medicinal Cannabis Research at the University of California San Diego said,

“Although performance was improving at 3.5 hours, recovery was not fully seen until 4.5 hours post smoking.”

The group of researchers looked at 191 regular cannabis users and found that smoking cannabis led to significantly declined simulated driving scores, according to the authors of the two-year randomized trial study.

As part of the study, the participants were given either a placebo cigarette or a cannabis cigarette containing either 5.9 % or 13.4 % levels of delta-9-tetrahydrocannabinol (THC), the psychoactive compound in cannabis.

The researchers found participants in the THC group showed a significant decline in Composite Drive Scores (CDS) related to a 25-minute simulated driving experience.

The CDS assessed driving variables such as following a lead car while varying speeds, noting swerving in their lane, and responding to divided attention tasks.

A sharp difference in driving scores between the placebo and the THC group was noted at 30 minutes and 1 hour and 30 minutes after the time of inhaling the cannabis cigarette, according to the published report.

According to the study, borderline differences in driving scores were found between the two groups at 3 hours 30 minutes and no differences were noted at 4 hours 30 minutes.

The researchers said in the report that the participants were hesitant to drive immediately after smoking but nearly 69% of the participants reported they were ready to drive at 1 hour 30 minutes post inhalation, setting up a potentially dangerous scenario.

“Although users in the THC group felt impaired and were hesitant to drive at 30 minutes, by 1 hour-30 minutes 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,” said Senior author Thomas Marcotte, PhD, co-director of CMCR and a professor of psychiatry at UC San Diego School of Medicine.
Marcotte added, “This may indicate a false sense of safety, and these first few hours may constitute a period of greatest risk since users are self-evaluating whether it is safe to drive.”

“The complete lack of correlation between blood concentrations and driving performance was somewhat surprising,” Co-author Robert Fitzgerald, PhD, professor of clinical pathology at UC San Diego School of Medicine stated.


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