Federal Study: THC Levels Not Reliable Indicator of Impairment for Arrest

Measuring one’s degree of marijuana impairment has long been an interest of not only scientists, but law enforcement prosecutors and some employers. Many thought there could be a parallel to alcohol testing; but instead of measuring one’s blood-alcohol concentration they could measure the amount of THC (the primary psychoactive component of cannabis) in one’s blood. The big problem with this, of course, is that THC doesn’t behave in the body the same way alcohol does. It isn’t processed as quickly. Thus, it’s not an accurate measure of one’s degree of impairment.

This is something our Los Angeles marijuana DUI attorneys have argued for years. Now, this same conclusion was backed by a federally-funded study. Backed by a grant from the National Institute of Justice, researchers tested the THC levels of 20 individuals who either vaporized or ate varying levels of THC. They were then subjected to numerous cognitive and field sobriety tests, similar to what are used by law enforcement.

The groups that received higher doses of THC (above 5 mg) were adversely impacted in terms of their sobriety – their psychomotor skills were visibly impaired – the level of THC in their blood and other biofluids didn’t reliably reflect that. Thus, the amount of THC in one’s blood was not a good indicator of marijuana intoxication.

There is little question that marijuana impairment does impact the skills needed for safe driving, including reaction time and spatial awareness. However, what the researchers determined was the amount of THC in the subjects’ blood, urine and saliva failed to correlate with how well they performed in field sobriety tests when they were clearly intoxicated – no matter how much THC they actually ingested.

What this does is raise substantial questions about per se marijuana DUI laws that are in place in six states, including nearby Nevada and Washington. These laws prohibit people from operating a vehicle if they more than a certain amount of THC in their blood. California does not have a per se limit when it comes to driving under the influence of marijuana.

The results of the study come as no surprise to our Los Angeles marijuana lawyers, nor are they a shock to longtime marijuana rights advocates. The fact is, the science was never there to support per se laws for marijuana impairment. Instead, these laws are based on arbitrary limits.

Beyond this, though, the researchers found that several, standard field sobriety tests – standing on one leg, balancing, walking and turning – weren’t affected by one’s cannabis impairment for any of the study participants, regardless of how much THC they ingested. In other words, many of the same tests used to determine if someone was driving under the influence of drugs or alcohol were not effective in singling out marijuana impairment.

Last year, the Journal of the American Medical Association (JAMA) published a study that indicated low levels of CBD don’t have any real impact on driving. Beyond that, a modest level of THC had a “modest in magnitude” impact that was similar to those who had a blood-alcohol concentration of about 0.05 percent. Even congressional researchers concluded three years ago in a published report that we don’t have conclusive evidence about the levels at which cannabis impairs one’s ability to drive. We also know that in states where marijuana has been legalized for recreation, traffic fatalities have not risen.

All this said, even most supporters of marijuana rights are careful to warn against driving under the influence of any substance, marijuana included. But it’s determining what “under the influence” means with regard to cannabis in a legal sense that has been throwing so many for a loop.

If you have been arrested for a marijuana DUI in Southern California, our dedicated marijuana DUI defense lawyers can help.

The Los Angeles CANNABIS LAW Group represents growers, dispensaries, ancillary companies, patients, doctors and those facing marijuana charges. Call us at 714-937-2050.

Additional Resources:

Testing People For Marijuana Impairment Based On THC Levels Is ‘Not Reliable,’ Federally Funded Study Finds, June 4, 2021, By Kyle Jaeger, Marijuana Minute