US Researchers Discloses Impact of Cannabis on Driving

The largest study of its kind has revealed the impact of THC on driving impairment, with potential implications for current drug laws.

A two-year randomised trial, conducted at the Center for Medicinal Cannabis Research (CMCR) at University of California San Diego School of Medicine, has found no relationship between blood THC concentrations and driving performance. 

Researchers recruited almost 200 regular cannabis users to consume different levels of delta-9-tetrahydrocannabinol (THC), the intoxicating compound in cannabis, or a placebo, immediately before a series of driving simulation tests over several hours.

Eighteen states in the US have now legalised cannabis for recreational use, 13 have decriminalised its use and 36 have medical cannabis laws, provoking questions about how cannabis may affect the abilities of drivers under the influence.

Compared to participants who took the placebo, the THC group (who had smoked a cannabis cigarette with either 5.9 percent or 13.4 percent THC) displayed significantly diminished ability on a Composite Drive Score (CDS) that assessed key simulated driving variables, such as swerving in lane, responding to divided attention tasks and following a lead car. 

However, not all individuals displayed significantly diminished driving skills compared to the placebo group, with researchers concluding that approximately 50 percent could be described as “impaired.”

The comparative decline was sharpest at the 30-minute and one hour-30 minute marks, after inhaling cannabis, then levelled to borderline differences with the placebo at three hours-30 minute mark.

There were no differences found at four hours-30 minutes.

The authors said driving scores did not differ based on THC content of the cigarette, both the 5.9 percent and 13.4 percent groups performed similarly.

The group with the highest use-intensity cannabis in the past six months attained significantly higher blood THC concentrations after smoking, but performed no worse than those with lower THC concentrations, indicating behavioural tolerance.

However, they appeared to compensate by ingesting more THC and performed no better than less frequent users.

The study found no relationship between post-smoking blood THC concentrations and simulator performance. 

Co-author Robert Fitzgerald, PhD, professor of clinical pathology at UC San Diego School of Medicine and director of the Toxicology Laboratory and associate director of Clinical Chemistry Laboratory at UC San Diego Health said: “The complete lack of correlation between blood concentrations and driving performance was somewhat surprising. It’s strong evidence against developing ‘per se’ driving under the influence statutes.”

“These laws establish a statutory violation if a legal standard is breached, such as blood-alcohol concentration in driving under the influence laws.

The findings, say the authors, indicate that cannabis use resulted in diminished driving ability, but when experienced users controlled their intake, impairment could not be inferred based on THC content of the cigarette, behavioral tolerance or THC blood concentrations.

 California State Assembly member Tom Lackey, commented: “This groundbreaking research indicates that cannabis use does impair driving ability, but factors differ from alcohol.

“For example, these data show that per se laws for THC levels are not supported scientifically. It also underscores the need for further research on this topic. Policymakers still need a better understanding of the effects of different ways of consuming higher concentration products to charter a path forward.”

The authors wrote that future research should address factors such as individual biologic differences, personal experience with cannabis and cannabis administration methods in relation to driving impairment.

Other research

These findings are in keeping with other research, published last year, which found that THC in blood and saliva are poor measures of cannabis impairment.

Researchers at the University of Sydney’s Lambert Initiative analysed all available studies on the relationship between driving performance and concentrations in blood and saliva of THC.

For infrequent, or occasional cannabis users, some significant correlations between blood and oral fluid THC concentrations and impairment were observed. However, the researchers note that most of these relationships were “weak” in strength.

No significant relationship between blood THC concentration and driving performance was observed for ‘regular’ (weekly or more often) cannabis users.





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