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THC Blood And Urine Tests Don’t Give Accurate Indication Of Impairment

A new study funded by the US National Institute of Justice (NIJ) has concluded that the level of tetrahydrocannabinol (THC) in a person’s blood, urine or saliva can’t be used to gauge their level of intoxication[i]. These findings raise some pretty big questions about the validity of roadside THC tests currently utilised by some police forces, suggesting that some drivers may be inaccurately deemed to be impaired while at the wheel.

THC Tests Go Under The Microscope

With recreational and medical cannabis legal in numerous countries and states around the world, the need to establish safe driving guidelines has rightly received a great deal of attention over recent years. Naturally, it’s critically important that no one drive while stoned or impaired in any way by weed, yet establishing a legal threshold for intoxication has proved to be tricky.

In certain locations, law enforcement relies on “per se” regulations that consider drivers to be over the limit if their bodily fluids are found to contain more than a certain concentration of THC. To analyse the suitability of such laws, the study authors recruited 20 people to take part in six dosing sessions over six weeks, during which participants ate weed brownies containing zero, ten and 25 milligrams of THC, and inhaled vapour consisting of zero, five and 20 milligrams of THC.

On each session day, the researchers collected blood, urine and oral fluids from each participant every hour for eight hours after consumption, while also conducting a range of tests in order to assess levels of cognitive and psychomotor function impairment.

“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,” explained in NIJ.

In other words, participants’ performance at the various tests could not be predicted by the level of THC in their bodily fluids, all of which suggests that this should not be used as a measure of intoxication.

The researchers also reported that THC levels did not match up with participants’ ability to stand on one leg or walk and turn – both of which are often used to determine whether drivers are intoxicated.

Does This Mean THC Tests Are Useless?

While these results provide strong evidence that “per se” laws regarding THC levels are not appropriate for determining legal intoxication thresholds, there is still some logic to using THC tests. After all, every dose used in the study generated some level of impairment – with the exception of five milligrams of vaped THC – which means that if a person tests positive for this cannabinoid then the chances are they are at least a little stoned, and therefore probably shouldn’t be driving.

Furthermore, THC is known to interfere with brain regions that control movement, balance, coordination and judgement, so the science still says that if a person is found to have THC in their blood, urine or saliva then they most likely aren’t in a fit state to be operating a vehicle.


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

Ben Taub