Blood levels of THC don’t correlate to level of impairment — new federally funded study proves what stoners knew for years

Blood levels of THC don’t correlate to level of impairment — new federally funded study proves what stoners knew for years

“THC levels in biofluid were not reliable indicators of marijuana intoxication.”

VicesJune 10, 2021 By Will Brendza

Testing for cannabis impairment is not as straight forward as testing for alcohol impairment, both scientists and Federal agencies are starting to realize.

It’s a conundrum that cannabis users have been puzzling over for many years. Because, anecdotally, anyone who smokes weed seriously, knows that tolerance levels between different people vary tremendously. Someone who smokes daily will be able to function pretty normally behind the wheel, compared to someone who only smokes once in a while. Bodyfat content and size can also affect how much of an effect cannabis has on a person. As will different modes of consumption — a regular flower user, hitting a concentrate or eating an edible might feel way higher than they normally get ripping their bowl (and visa-versa).

There are so many factors that go into how high a person gets. THC levels in the blood never seemed like a reliable way of actually determining how “impaired” someone gets from cannabis. It seemed completely subjective to whoever the blood belonged to. But beyond a bunch of stoners’ anecdotal claims, there was nothing to back that theory up.

Now, however, there’s science — from the National Institute of Justice (NIJ), of all places — that seems to prove it. In a federally funded study published in April of this year, the NIJ concluded that the level of THC in a person’s blood has no accurate correlation to impairment, whatsoever. Which, should raise questions for any state enforcing cannabis “per se” laws (that bar people from driving if their blood THC level is over a certain percentage of THC).

“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 says in their report.

The research examined 20 subjects, all of whom ingested cannabis either via edibles or vapor, and were tested with various field sobriety and cognitive tests. Groups were given doses ranging from 0 mg to 20 mgs.

All of the groups given THC showed signs of negatively impacted “cognitive and psychomotor” functions, according to the report. However, it goes on to explain that, “the one leg stand, walk and turn, and modified Romberg balance tests were not sensitive to cannabis intoxication for any of the study participants.”

Meaning that none of the standard sobriety tests used in road-side traffic stops, are useful in any regard to discern someone’s cannabis impairment (or lack thereof). Turns out, weed doesn’t affect your balance or ability to walk in a straight line like alcohol does — go figure.

Samples of the test subject’s blood, urine and spit were also collected before ingesting cannabis and every hour afterwards for eight hours. Those biofluids were sent to a forensic toxicology lab for testing. Much to the surprise of the scientists and chagrin of police and Federal agencies across the country, those results showed no observable correlation between the level of THC in someone's blood and their level of impairment.

“RTI concluded that, for their dosing study, THC levels in biofluid were not reliable indicators of marijuana intoxication.”

That conclusion should challenge the per say laws surrounding THC that are currently enforced in six states (Illinois, Montana, Nevada, Ohio, Pennsylvania and Washington). If the level of THC in a person’s blood doesn’t say anything about how high they actually are, there is no acceptable reason to use it to determine someone’s level of intoxication or sobriety.

It’s like trying to assess a person’s hearing with a color-blindness test. Sure, it can be done — but it doesn’t make any sense, and the results aren’t going to provide any clarity at all.