Cannabis 101

Cannabis DUIDs: How Much Is Too Much?

Published on March 25, 2014 · Last updated July 28, 2020

Saturday night is coming to a close, and after a relaxing evening full of Trainwreck and camaraderie with your friends, it’s time to go home. As a veteran cannabis consumer, you know the short drive home will be a simple task. Besides, it’d been at least an hour since you inhaled anything other than oxygen and a sleeve of Oreos.

It would be easy to shorthand this article by saying, “Don’t smoke and drive,” but there will always be people who won’t accept that their confidence is not the only thing separating them from a DUID (Driving Under the Influence of Drugs).

As technological as we humans claim to be, roadside impairment devices have not kept up with the sprinting pace of cannabis policy reform. Lawmakers in Washington and Colorado established per se limits on THC blood levels with the passage of recreational laws, but some scientists and critics believe these limits to be unscientific and arbitrarily derived.

If you are a regular cannabis consumer, it would behoove you to familiarize yourself with the current state of cannabis DUID measurement and understand that there are more factors at play than you might realize.

How Long Is THC in My System?

Two hours have now gone by since your last breath of cannabis and you feel completely sober, being the seasoned, high-tolerance consumer you are. You may feel unimpaired, but the blood running through you is still riddled with THC. This is because your body stores the chemical for long periods of time — especially if you’re a daily consumer.

One 2009 study published in the journal Addiction examined THC blood concentrations in frequent cannabis consumers, both before and after a one-week abstinence period. The results demonstrated perplexing variability among the subjects, some of which had no detectable THC in their system while others had high concentrations even after seven days of no cannabis. Other studies have also noted significant differences in long-term THC retention between occasional partakers and heavy consumers.

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Franjo Grotenhermen, a researcher of cannabinoids and toxicology in Germany, explains why these results are problematic for zero-tolerance laws and per se limits.

“This approach effectively sets the legal limit at the technically achievable limit of detection (LOD), making the legal limit a function of the detectability of a drug, rather than the impairment caused by it,” Grotenherman writes. “Zero-tolerance laws will classify many cannabis users as impaired drivers even if they separate drug use and driving by many hours. The same applies to the increasing number of individuals who legally use cannabis for medicinal purposes and, while not acutely impaired, may present with measurable THC concentrations at all times.”

Science exploring the impact of cannabis on motor skills comes in at a slow trickle, so it could be a while before we have enough research to inspire a change in policy. Until then, frequent cannabis consumers should be aware that everything from body weight to metabolism affect the body’s storage of THC.

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Measuring Intoxication

The difficulty of establishing THC blood concentrations that accurately reflect impairment is just one facet of the problem. Another is the technology police use to measure that content, adding another murky layer to the issue.

With one blow of a breathalyzer, a police officer will get a pretty good idea of whether or not a driver has had too much to drink. Because of its complex storage and circulation, no comparable test exists for cannabis intoxication. Nevertheless, police continue to rely on underdeveloped testing to determine a DUID case.

How does alcohol affect THC blood concentration?

A study published earlier this year assessed the accuracy of roadside testing methods, including oral swabs and observational sobriety evaluations also known as the Drug Evaluation Classification (DEC) Program. Accuracy was measured by comparing the results with off-site blood tests.

The researchers first evaluate Drug Recognition Experts (DREs) who perform DEC evaluations in Canada, Australia, Europe, and the United States. Data was collected across studies beginning in 1985, both from on-site police fieldwork and controlled lab tests. Field evaluation accuracy ranged from about 74% to 90%. Not bad, until you consider that those accuracy rates cannot possibly include any cases of impairment that went undetected. Controlled lab environments reflect much lower success rates: DREs identified only 30% to 53% of subjects who had consumed cannabis.

Oral swabs performed considerably better, with accuracy rates falling between 52% and 100%, depending on the brand. “If on-site oral fluid tests had higher levels of device reliability,” researchers noted, “they could be seen as the best technique, given that they have the ability to detect the psychoactive cannabis compounds indicating recent use or impairment and are less subjective than the DEC as performed by police officers. Unfortunately, these devices have not yet achieved an acceptable level of reliability.”

Urine tests are a whole different beast as they measure for both active THC and its inactive metabolites, which are detectable for days (or weeks for regulars) after use. These tests are becoming less popular for DUID evaluations because, a) they can’t pinpoint recent use, b) the test can be adulterated, and c) most police don’t like touching urine.

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Researchers believe blood tests are better indicators of intoxication, but given their invasive nature and complexity, they are impractical for on-site use. The authors conclude that current testing methods are not up to snuff, and that more research is necessary to improve measurement, per se limits, and our understandings of cannabis motor impairment.

Know the Law and Your Limits

Shoddy testing and unreasonable THC limits might be a frustrating reality, but it’s our reality nonetheless. For now, they are the cost of forward movement toward legalization, and eyes are turned toward us to act them out responsibly. Knowing the law and, more importantly, your cannabis limits will go a long way while research plays catch-up.

Laws and procedures can vary widely across state and country borders. Some areas like California have vague “effect-based” DUID laws, where a person is only tested if detectably impaired. Other states like Washington and Colorado have an objective cut-off THC blood concentration of 5 ng/mL. And then there are states like Arizona and Georgia who have even less tolerance for drugged drivers, giving DUIDs for any trace of cannabis or its metabolites in the body.

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This puts many of us, patients and adult consumers alike, in an unsettling position where the extent of our personal control goes no further than our actions and decisions behind the wheel. We have just one job: to recognize our impairment honestly and resist driving after recent use. After all, you would hope that an unimpaired driver would never make it as far as that off-site blood test.

Image Sources: Nicholas A. Tonelli, MorganFranjo Groterherman, Jeffrey Smith, Adrianna Broussard

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Bailey Rahn
Bailey Rahn
Bailey is a senior content manager at Leafly, specializing in strains and health. She's spent 7+ years researching cannabis products, spreading patients’ stories, and exploring healthy ways of integrating cannabis into daily life.
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