12th January 2022
By Roland Sebestyén
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Researchers have reportedly found a new method to identify those whose performance has been impaired by THC, the main psychoactive compound in cannabis that causes a “high”.

Harvard researchers came up with a potentially highly effective technique called functional near-infrared spectroscopy (fNIRS), a form of noninvasive brain imaging procedure, to accurately detect individuals with THC impairment.

As more states in the US are opening up for legal cannabis use, the authors of the study found it important to distinguish between impairment and mild intoxication from THC, in order to make the roads safer.

Lead author Jodi Gilman, an investigator in the Centre for Addiction Medicine, MGH, and associate professor of psychiatry at Harvard Medical School said: “Our research represents a novel direction for impairment testing in the field.

“Our goal was to determine if cannabis impairment could be detected from the activity of the brain on an individual level.

“This is a critical issue because a ‘breathalyser’ type of approach will not work for detecting cannabis impairment, which makes it very difficult to objectively assess impairment from THC during a traffic stop.”

The study – and results

In the study, 169 people underwent the brain imaging procedure (fNIRS) after having been given oral THC or a placebo.

Those who reported intoxication after having taken THC showed an increased oxygenated haemoglobin concentration (HbO) in comparison to those who reported low or no intoxication.

Senior author and principal investigator A. Eden Evins, founding director of the Centre for Addiction Medicine, said: “Identification of acute impairment from THC intoxication through portable brain imaging could be a vital tool in the hands of police officers in the field.

“The accuracy of this method was confirmed by the fact impairment determined by machine learning models using only information from fNIRS matched self-report and clinical assessment of impairment 76 per cent of the time.”

“Companies are developing breathalyser devices that only measure exposure to cannabis but not impairment from cannabis.

“We need a method that won’t penalise medical marijuana users or others with insufficient amounts of cannabis in their system to impair their performance. While it requires further study, we believe brain-based testing could provide an objective, practical and much-needed solution.”

Gilman, J.M., Schmitt, W.A., Potter, K. et al. Identification of ∆9-tetrahydrocannabinol (THC) impairment using functional brain imaging. Neuropsychopharmacol. (2022). https://doi.org/10.1038/s41386-021-01259-0 

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