Here’s Why Testing for Cannabis Impairment is Not Straightforward

A federally backed study has cleared up some confusion regarding cannabis impairment. The research, which was financially funded by a grant from the National Institute of Justice, suggests that the level of THC (tetrahydrocannabinol) present in a person’s system after consuming the green plant is not a precise indicator of impairment.

In total, 20 cannabis consumers were selected for the study on cannabis impairment. Each study subject either ingested the plant in edible form or vaporized it; THC levels varied. Post-consumption, participants underwent some simple cognitive and field sobriety tests. Whereas the groups that consumed THC doses above 5 milligrams were all “negatively impacted” and demonstrated noticeable psychomotor impairment, the RTI International researchers discovered that “THC levels in biofluids were not reliable indicators of marijuana intoxication for their study participants”. A Tweet published by the NIJ (@OJPNIJ) on June 3, 2021 reads:

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. Read more: pic.twitter.com/1mWehGKCHQ

— Natl Inst of Justice (@OJPNIJ) June 3, 2021

According to the Deputy Director of  (NORML), Paul Armentano, the findings are unsurprising. His organization is responsible for fostering cannabis law reform in the United States for both medical and non-medical purposes.

“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”. said Armentano.

Nonetheless, the recent discovery stirs up debate regarding “per se” laws that have been enacted across various U.S. states that prohibit people from operating a motor vehicle if the psychotropic compound THC (up to a certain amount) is present in their blood.

Cannabis Impairment: Study Subjects Underwent Numerous Tests 

Despite its original publication in 2020, the study on cannabis impairment gained fresh attention on Thursday, June 4, when it was promoted by the NIJ on Twitter. In an attempt to properly understand the level of impairment experienced by cannabis consumers, a variety of tests were carried out. Throughout the duration of the tests, the blood, oral fluid, and urine of each participant were analyzed inside forensic laboratories.

The tests, which “were not sensitive to cannabis intoxication for any of the study participants,” included walking, turning, balancing, and standing on one leg. Signs of acute cannabis intoxication may include anxiety, confusion, delusions, high blood pressure, fast heart rate, nausea and/or vomiting, and hallucinations. 

“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,” the 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 simple terms, THC does cause impairment. However, the accumulation of THC in bodily fluids does not explicitly correspond with the degree of impairment. What’s more, researchers say that some consumers are not negatively impacted by THC in low doses.

Additional Studies Draw Interesting Conclusions

Numerous studies hypothesize that cannabis can cause subtle psychomotor impairment, whereas others go one step further. One such example was this meta-analysis of 60 studies, which suggested that cannabis causes impairment in areas associated with driving a vehicle safely; including motor coordination, visual functions, and tracking.

Separate research featured in the American Journal of Public Health in January 2017 assessed state rates of fatal motor accidents over a period of almost 30 years. Based on the results, “[medical marijuana laws] and dispensaries were associated with reductions in traffic fatalities, especially among those aged 25 to 44 years.”

In late 2020, a different study highlighted in the Journal of the American Medical Association showed that low levels of the non-psychoactive compound CBD (cannabidiol) do not have a major effect on driving safety and capability. What’s more, the impact of low levels of THC is “modest in magnitude and similar to that seen in drivers with a 0.05%” concentration of blood alcohol.

In spite of these findings, proponents and opponents alike often caution against driving—and engaging in other activities, such as operating heavy-duty machinery—when under the influence of cannabis.