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Accurately and consistently proving that a driver is impaired by cannabis use remains a daunting scientific challenge, toxicologists concluded during a recent conference.

Unlike alcohol, levels of impairment from cannabis can't be determined by a single measure. Even if intoxicating elements of cannabis are detected in the body, no precise amount signals a level of unsafe impairment. As a result, the lack of a legal standard hampers the courts and law enforcement.

Researchers and toxicologists at the University of California, Irvine's Cannabis and Driving workshop May 31, 2019,  acknowledged serious difficulties in being able to prove whether and to what degree some drivers might be impaired by recreational or medical marijuana.

“It's the No. 1 short-term consequence of cannabis legalization,” said Marilyn Huestis of Huestis & Smith Toxicology, the workshop's main speaker.

The scientific challenge is due in part to the need for more research focused on tracking impairment. Existing studies have shown that cannabinoids aren't like other potentially intoxicating substances.

With alcohol, for example, Huestis said, “There is a clear line between concentration and effect. Cannabis is just so much more complex.”

With alcohol, for example, there is a clear line between concentration and effect. Cannabis is just so much more complex. - Marilyn Huestis Click To Tweet

According to Huestis, researchers found no consistent pattern or correlation between detectable THC in the brain or blood and a person's impairment. They also found key differences in effects between regular and occasional users, and men and women.

However, studies have been able to prove some of the basics of impairment, Huestis said. Scientists know enough to conclude that cannabis use does have an impairing effect on driving. Cannabis affects multiple areas of the brain, including the neocortex, which is involved in motor commands and executive control.

“Executive control is taking in all the information from your environment and deciding if it's important or not,” Huestis said. “Once you decide it's important, you compare it to your memory, and your knowledge and you decide on a course of action. Then you have to implement it. Cannabis slows every one of those processes.”

But beyond that, toxicologists agreed current knowledge cannot yet deliver law enforcement a clear set of national standards that can be used in the vast majority of driving conditions. Regular users can show high levels of THC in certain field tests and not be critically impaired, while occasional users can show small amounts of THC and be extremely impaired for longer periods of time.

“The problem is, we want to match the window of detection with the window of impairment and we can't do that,” Huestis said. “What number are you going to pick that is going to work with chronic users and the casual users?”

Oral, urine, and blood tests all have limitations in proving levels of impairment. Crime lab toxicologist Jennifer Harmon of Orange County, California, said blood tests are popular in California, but have a very short window of time to reflect accurate results. The test also requires a technician to be at the scene to administer the test, which presents logistical challenges for police.

In the end, law enforcement officials are most interested in preventing unsafe driving, no matter the source of the intoxicant. According to Harmon, half of driving-under-the-influence (DUI) arrests in Orange County are due to the presence of at least two drugs, making it even harder to single out marijuana as the source of impairment. Scientific tests also aren't intended to determine the reason behind consumption.

”We can't tell the difference between someone using it recreationally and someone using it for medicine,” Harmon said. But she and other toxicologists are hopeful that oral liquid tests, currently used in most countries, will soon be sophisticated enough to provide law enforcement and courts with reliable and immediate testing.


Feature image: While chemicals in cannabis can be shown in tests, proving impairment is too complex, law enforcement and scientific experts concluded during a Cannabis and Driving workshop at the University of California, Irvine, on May 31, 2019. Linking the presence of cannabis to impairment at the time of driving has more variables than a comparable alcohol-impaired driving test, such as differences between sexes and frequency of use.   (Photo by nik radzi on Unsplash)

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