For most people who are investigated for impaired driving by drugs, the case goes like this: the individuals are pulled over for some sort of a traffic violation. The officer forms a suspicion that they have a drug in their body, or they smell burnt cannabis, or the driver admits smoking pot a few hours earlier. The officer engages in some sobriety tests, including a saliva test that is supposed to correlate the amount of THC in your saliva to a blood THC concentration.
But what if I told you that even if THC is found in your blood, it might not accurately reflect how impaired you are?
The connection between how much THC is in your system and how impaired you are doesn’t follow a predictable pattern.
The connection between THC levels and impairment isn’t as straightforward as it seems. Traditionally, we’ve been used to the idea that the more of a substance is in your system, the more impaired you’ll be. It’s like with alcohol – the higher your blood alcohol level, the drunker you are. But when it comes to cannabis, it’s not that simple.
Back in 2006, a study by Ramaekers and his team claimed they found a clear link between THC levels in the blood and impairment. This finding led to the introduction of the laws that prohibit having a certain THC concentration in your blood, similar to how we measure blood alcohol content (BAC) for drunk driving. However, recent research has cast doubt on the connection between the two.
Smith et al. conducted a study in 2023 using cognitive tests and a driving simulator to measure impairment from cannabis. They found that while there were some small changes in performance after using cannabis, these changes were overshadowed by the natural variations in people’s abilities. In other words, the differences in how well people performed on tests when sober were so significant that any impact from cannabis was hard to detect.
Other researchers, like Brooks-Russell and Pollard, have echoed these findings. They explain that unlike alcohol, where a higher BAC usually means more impairment, the relationship between THC levels and impairment is much more complicated. Even the NHTSA – the people who do the research to create impaired driving laws in the United States and train police in detecting impairment – acknowledges this. They say that just because THC is present in your system doesn’t necessarily mean you’re impaired.
So why is there a discrepancy between THC levels and impairment?
One reason could be individual differences in how people respond to cannabis. Between unique metabolisms, tolerance, and reactions to drugs, it is challenging to predict how someone will be affected by THC based solely on blood concentration.
Another factor to consider is the method of testing. Current tests for THC levels rely on blood, saliva, or urine samples. However, these methods may not accurately reflect how much THC is actively affecting your brain and impairing your ability to drive. THC can linger in the body for hours, days, and sometimes months after use, so a positive test doesn’t necessarily mean that someone is impaired by the THC.
Some researchers have tried to find a connection between THC that is released from its stores in the fat cells, but their results show only a tenuous connection that has not been well-repeated or respected by the scientific community.
Additionally, the effects of cannabis on driving can vary depending the dose, how often a person uses the drug, and the presence of other substances. That’s why alcohol remains the number one culprit in impaired driving, even when combined with THC. In fact, some studies have shown that in combination the impairment still really only comes from the alcohol and not the cannabis. Regular cannabis users may develop tolerance, further complicating the relationship between THC levels and impairment.
While science has been able to prove that alcohol impairs all drivers at the level of 80 mg%, the same is simply not true for cannabis and THC levels.
Relying solely on THC levels does not provide an accurate representation of someone’s level of impairment. Instead, behaviour and performance on standardized tests should be the metric by which these impairing factors are measured.
Except – get this! – that when it comes to the Drug Recognition Evaluation test, the performance that is expected from someone who is “impaired” by THC is said to be normal in most categories that are evaluated. That is, you are impaired because you are normal! This makes zero sense when the definition of impairment is literally deviation from the norm.
Unfortunately, despite the growing evidence challenging the link between THC levels and impairment, Canada has insisted on using THC concentration to ground a criminal offence that can lead to a lengthy license suspension and automatic criminal record. There is a clear disconnect between scientific research and the law in this country.
Ultimately, the goal should be to ensure public safety on the roads but those laws must also respect the rights and liberties guaranteed under the Charter. By addressing the limitations of assessing cannabis impairment, the government might be able to provide a more effective system for preventing impaired driving incidents related to cannabis use. But as of right now, they are failing on that.
If you’re facing cannabis impairment charges based on THC levels, it’s worth knowing that these levels might not accurately reflect your level of impairment. That is why it is important to retain a lawyer familiar with these issues so they can challenge the charges successfully.
Click the citations link below to see more information
Citations
References
Arkell, T. R., Spindle, T. R., Kevin, R. C., Vandrey, R., Irwin, C., & McGregor, I. S. (2021). The failings of per se limits to detect cannabis-induced driving impairment: Results from a simulated driving study. Traffic Injury Prevention, 22 (2), 102-107.
Blandino, A., Cotroneo, R., Tambuzzi, S., Di Candia, D., Genovese, U., & Zoja, R. (2022). Driving under the influence of drugs: Correlation between blood psychoactive drug concentrations and cognitive impairment. A narrative review taking into account forensic issues. Forensic Science International: Synergy, 4, Article 100224, pp. 1-13.
Brooks-Russell, A., Holdman, R., & Whitehill, J. M. (2023). Approaches to measuring cannabis use in injury research: Beyond drug detection. Current Epidemiology Reports, 10, 142-147.
Hill, A. B. (1965). The environment and disease: association or causation? Proceedings of the Royal Society of Medicine, 58, 295–300.
Logan, B. K., Kacinko, S. L., & Beirness, D. J. (2016). An evaluation of data from drivers arrested for driving under the influence in relation to per se limits for Cannabis. Washington, DC: American Automobile Association Foundation for Traffic Safety.
McCartney, D., Arkell, T. R., Irwin, C., Kevin, R. C., & McGregor, I. S. (2021). Are blood and oral fluid delta-9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis. Neuroscience and Biobehavioral Reviews, 134, Article 104433.
NHTSA. (5 January, 2024). Advanced impaired driving-prevention technology (Proposed rule). Federal Register, 89 (4), 830-857.
Pollard, J. K., Nadler, E. D., & Melnik, G. A. (2024; in preparation). Review of technology to prevent alcohol- and drug-impaired crashes: Update (Report number DOT-VNTSC-NHTSA-xx-xx). Washington, DC: National Highway Traffic Safety Administration (NHTSA).
Ramaekers, J. G., Moeller, M. R., van Ruitenbeck, P., Theunissen, E. L., Schneider, E., & Kauert, G. (2006). Cognition and motor control as a function of delta-9-THC concentration in serum and oral fluid: Limits of impairment. Drug and Alcohol Dependence, 85, 114-122.
Sevigny, E. L. (2021). Cannabis and driving ability. Current Opinion in Psychology, 38, 75-79.
Smith, S. J., Wrobel, J., Brooks-Russell, A., Kosnett, M. J., & Sammel, M. D. (2023). A latent variable analysis of psychomotor and neurocognitive performance after acute cannabis smoking. Cannabis, 6 (2), 123-132.
Spindle, T. R., Cone, E. J., Schlienz, N. J., Mitchell, J. M., Bigelow, G. E., Flegel, R., Hayes, E., & Vandrey, R. (2018). Acute effects of smoked and vaporized cannabis in healthy adults who infrequently use cannabis. JAMA Network Open, 1 (7), Article e184841.
White, M. A., & Burns, N. R. (2022). How to read a paper on the short-term impairing effects of cannabis: A selective and critical review of the literature. Drug Science, Policy and Law, 8, 1-25.
Wurz, G. T., & DeGregorio, M. W. (2022). Indeterminacy of cannabis impairment and delta-9-tetrahydrocannabinol (delta-9-THC) levels in blood and breath. Scientific Reports, 12, Article 8323.
