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Blood and Saliva Tests Don’t Indicate How Stoned You Are

A recent study has cast doubt over the reliability of roadside THC impairment testing. According to the data, blood and saliva testing for THC impairment are ineffective and inconsistent. In line with another study done by the US National Institute of Justice last year.

The study, published in Neuroscience & Biobehavioural Reviews, was carried out to accurately determine THC intoxication levels to measure impairment in motorists and how they drove. The findings may impact drug-driving laws worldwide.

The Purpose of The Study

The study was conducted by the University of Sydney’s Lambert Initiative for Cannabinoid Therapeutics. Their mission is “to conduct the high-quality research required to discover, develop and optimise safe and effective cannabinoid therapeutics in Australia and beyond”.

The team analysed all available studies involving the consumption of inhaled or ingested forms of cannabis (smoking, edibles, and more). They then characterised the relationship between driving performance (reaction times and attention span) and blood/saliva concentrations of THC.

Current detection methods used by law enforcement are similar to the tests that have been used for decades to determine alcohol intoxication.

blood saliva THC

Can THC Be Detected By Roadside Testing?

The Lambert Initiative’s Dr Danielle McCartney stated, “Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users, while no significant relationship was detected in regular cannabis users. This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis THC-induced impairment.”

The study also found that how ‘high’ or ‘stoned’ individuals felt within themselves was an equally poor indicator of genuine impairment. It should not be used to measure one’s ability to drive.

In addition, a window of impairment was identified as between three and ten hours, and that duration of the impairment is determined by not only the dose of THC but also means of consumption (smoked/vaporised /eaten), and whether the individual is a regular or occasional cannabis user.

blood saliva THC

The study also found that risk from impairment was markedly different from task to task – the same level of impairment could pose a greater or lesser risk depending on the complexity of the task.

Finally, the authors alluded to individual tolerance, noting that regular users may have to consume much greater quantities to achieve the same level of intoxication as an occasional user would, after consuming a much smaller quantity of cannabis.

Flaws in Current Testing Methods

The results of this new study cast reasonable doubt on the authority of current testing methods.

Right now, saliva-based testing is used in Australia, and blood concentration testing is employed in some U.S. states and Europe.

Expanding on the study’s findings, Dr McCartney added, “Our results indicate that unimpaired individuals could mistakenly be identified as cannabis-intoxicated when THC limits are imposed by the law. Likewise, drivers who are impaired immediately following cannabis use may not register as such.”

Ultimately, this means the current methods of testing motorists for cannabis intoxication are flawed, inaccurate and unreliable. This could potentially lead to cases being dismissed in court merely by citing the study’s findings. This news must surely send law enforcement scrambling back to the drawing board in search of more reliable means of detecting cannabis impairment in an age where medicinal use, in particular, is becoming far more widespread.

However, it also poses a potential roadblock in the fight to further decriminalise cannabis use worldwide. Without an accurate method of determining intoxication levels in motorists, lawmakers are hamstrung in setting parameters for safe use limits.

blood saliva THC

So What Happens Now?

The present system punishes for the presence of THC rather than intoxication levels. Professor Iain McGregor, Academic Director of The Lambert Initiative, added, “THC can be detected in the body weeks after cannabis consumption while it is clear that impairment lasts for a much shorter period of time. Our legal frameworks probably need to catch up with that and, as with alcohol, focus on the interval when users are more of a risk to themselves and others.

Prosecution solely based on the presence of THC in blood or saliva is manifestly unjust. Laws should be about safety on the roads, not arbitrary punishment. Given that cannabis is legal in an increasing number of jurisdictions, we need an evidence-based approach to drug-driving laws.”

All of that being said, use your head. Driving while stoned is never a good idea, and should anything happen you know it’ll be the weed that’s blamed. If you’re going to get high, catch a cab or the bus!


Danielle McCartney, Thomas R. Arkell, Christopher Irwin, Richard C. Kevin, Iain S. McGregor (2021). Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis,
Neuroscience & Biobehavioral Reviews, ISSN 0149-7634,

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French

Duncan Mathers