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Testing People For Marijuana Impairment Based On THC Levels Is ‘Not Reliable,’ Federally Funded Study Finds

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The amount of THC in a person’s system after consuming marijuana is not an accurate predictor of impairment, a federally funded study has determined.

The research, backed by a grant from the National Institute of Justice (NIJ), involved 20 people who either ate or vaporized cannabis with varying levels of THC. They were then tested with basic field sobriety and cognitive tests.

While the groups that received doses higher than 5mg of THC were all “negatively impacted” and experienced observable psychomotor impairment, the RTI International researchers found that “THC levels in biofluids were not reliable indicators of marijuana intoxication for their study participants.”

“Researchers investigated how marijuana affects skills required for safe driving and found that biofluid levels of THC did not correlate with field sobriety test performance or marijuana intoxication, regardless of how the cannabis was ingested.”

That raises questions about “per se” laws that are in place in several states, barring people from driving if they have more than a certain amount of THC in their blood.

“These important findings come as no surprise,” NORML Deputy Director Paul Armentano said. “Despite a handful of states imposing per se THC thresholds as part of their traffic safety laws, there exists no science demonstrating that these arbitrary limits are reliable predictors of either recent cannabis exposure or impairment.”

The study, which was published last year and promoted by NIJ in a tweet on Thursday, also found that various tests, including standing on one leg, balancing and walking and turning, “were not sensitive to cannabis intoxication for any of the study participants.”

Throughout the tests, participants’ blood, urine and oral fluid were collected and then sent to forensic laboratories.

“Results from the toxicology tests showed that the levels of all three targeted cannabis components (THC, cannabidiol, and cannabinol) in blood, urine, and oral fluid did not correlate with cognitive or psychomotor impairment measures for oral or vaporized cannabis administration,” NIJ said.

“Many of their study participants had significantly decreased cognitive and psychomotor functioning even when their blood, urine, and oral fluid contained low levels of THC,” the federal agency continued. “The researchers also observed that standardized field sobriety tests commonly used to detect driving under the influence of drugs or alcohol were not effective in detecting marijuana intoxication.”

In other words, THC does lead to impairment—but the concentration of that compound in bodily fluids does not accurately correlate with the extent to which a person is impaired. And in low doses, it seems some people are not negatively impacted, at least with respect to the standard tests that the researchers utilized.

Late last year, a different study published in the Journal of the American Medical Association found that low levels of CBD do not appear to have a significant impact on driving, and low-level THC consumption has an impact that is “modest in magnitude and similar to that seen in drivers with a 0.05%” blood alcohol concentration.

A a congressional research body also released a report in 2019 that found that evidence about cannabis’s ability to impair driving is currently inconclusive.

Researchers have found on several occasions that traffic fatalities do not increase after a state legalizes marijuana.

Of course, that doesn’t change the fact that both opponents and supporters of legalization generally caution against driving under the influence.

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Kyle Jaeger is Marijuana Moment's Sacramento-based senior editor. His work has also appeared in High Times, VICE and attn.

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