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Decoding Cannabis: Over the limit, but not impaired? What the science shows
Decoding Cannabis: Over the limit, but not impaired? What the science shows
8 min read
Sam North
Welcome back to Decoding Cannabis, our ongoing series where we grab one piece of recent scientific research focused on cannabis and break down the complicated wording, graphs, stats, and overall findings into clear, direct insights.
This week: Can drivers be over the legal blood driving limit for cannabis, while still being unimpaired? A 2026 driving simulator study from the Centre for Addiction and Mental Health in Toronto suggests exactly that, and it backs up the findings from similar research from 2022.
Contents
How did the study test cannabis and driving?
The participants were split into two groups:
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The control group, who had not consumed any cannabis for at least a month
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And the “frequent cannabis users”, meaning most of them consumed cannabis daily.
The two groups were matched by age and sex, with the only major difference being that the “frequent cannabis users” were asked to consume cannabis the night before testing, which worked out to be 12 to 15 hours before they jumped in the driving simulator.
An important caveat to point out here: the amount of cannabis the “frequent cannabis users” consumed was not regulated. Participants were asked to buy their own pre-rolled cannabis joint from a legal retailer (in Canada, recreational cannabis is legal) and smoke it unsupervised at home, the night before the test.
The researchers then worked out how much each participant had smoked by weighing what was left of the joint and subtracting it from the starting weight, then multiplied that by the THC percentage on the packaging to estimate the THC consumed.
The result was a rough estimate rather than a precise/controlled dose, but for this type of test, it was enough to grab good data. On average, the participants in the “frequent cannabis users” group consumed around 159 mg of THC, from flower averaging roughly 30% THC. That is quite a substantial amount of what is classed as ‘high potency cannabis’, though the gap between the lightest and heaviest users was wide.
Another key point to bring up here is that this type of spread reflects how people actually use recreational cannabis in the real world. They choose their own products, and how much of that product they use, which somewhat bolsters the results of this observational study.
Does cannabis affect your driving the morning after?
Not measurably, at least not for the people who took part in the study.
The study participants were asked to answer a few important questions before beginning the simulated driving test. These included:
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How much will the study drug (the cannabis) affect your driving?
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How well will you drive?
The cannabis group were honest and cautious in their answers.
For the “How much will the study drug (the cannabis) affect your driving?” question, they were open with their feeling that the cannabis may still affect their driving, and that they may still be impaired. This was scored on a 6-point scale, from not at all too extremely. The average score was only 1.5, sitting right down at the slightly end - the effects they were still feeling were mild, but still higher than the control group.
The second question, "How well will you drive?", told a similar story. The cannabis group did not overrate themselves before the test, and when asked after the test how they thought they went, they actually rated their own driving slightly lower than the control group did.
But here’s the really fascinating part: none of that showed in how they actually drove…
Across every single metric, including lane control, speed, following distance, and general reaction times, the cannabis group were bang on par with the control group. There was no meaningful difference between how the two groups scored whatsoever.
Is blood THC a poor measure of driving ability?
If the cannabis group drove just as well as the control group, you might expect their blood to show barely any THC at all. It did not, and this is where the study starts to correlate with previous studies looking at similar data.
Tested at the time of driving, again 12 to 15 hours after they last consumed any cannabis, their blood THC averaged 2.8 ng/mL (the typical participant was lower, at 1.2, with a wide spread). Plenty of the group still had THC in their blood; some had a lot. Yet, when the researchers crunched the numbers, there was no meaningful correlation between blood THC levels and any measure of driving. The same was true for THC level in saliva. This study, and a previous study from 2022, show that we cannot look at someone's blood THC and reliably predict how well they will drive.
Both studies indicate that THC simply does not behave in the same way as alcohol when it comes to driving impairment. With alcohol, blood concentration maps fairly neatly onto impairment; the more you have in your system, the worse you drive, which is why breathalyser limits work. THC does not follow that logic.
The problem with the UK's 2µg/L limit: presence vs impairment
The UK’s current legal driving limit for THC is 2 µg/L of blood, which is the same unit as ng/mL, just written differently. The cannabis group's average of 2.8 ng/mL was comfortably above that legal limit.
Yet 12 to 15 hours after smoking their pre-rolled joints, their driving scores were the same as someone with zero cannabis in their system. Had a number of these participants been pulled over and blood-tested in the UK, they would have been over the limit and likely faced some level of prosecution, despite proving in the simulator test that they were driving safely.
And this is exactly the debate now playing out right now in New South Wales, Australia.
On the 3rd June 2026, the NSW government introduced a bill to Parliament that would stop legally prescribed medical cannabis patients from being automatically penalised under the state's drug presence laws. It's not law yet, but we will keep a close eye on any movements and keep you updated.
What this means for UK medical cannabis patients
Right now? Unfortunately, not a lot.
Two observational studies from Canada and the USA do not change UK law, and neither does a bill being debated in sunny Sydney. . The 2 µg/L limit still stands, and police can and do still test for it.
But there is one thing that all UK medical cannabis patients need to understand: The UK has what is called a statutory medical defence. As long as you:
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Have a legal prescription, and can prove as such
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Are taking your medical cannabis as directed by your prescribing specialist
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Have judged that you are not impaired
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And have a current and valid driving licence
You are legally entitled to drive, even if you would test above the THC limit.
That doesn't mean you will never be stopped, tested, and potentially arrested roadside. The statutory medical defence is something that would be raised in court, it does not act as a shield when the tip of the spear is coming straight for you.
That said, by the letter of the law, a patient who meets those conditions has not committed an offence, and the defence exists precisely to protect you.
Which brings us nicely into two of the more underrated advantages of being a Releaf patient: easily being able to prove your patient status, and having access to a legal hotline should you ever find yourself in an anxiety-spiking situation involving the authorities…
The Releaf difference: support that goes beyond your consultation
Proving your legal UK medical cannabis patient status is not something we believe any patient should have to worry about, but we are also pragmatic. Until the law hopefully catches up to the emerging data, these situations happen, and being ready for them can make a world of difference to how you feel in the moment, and potentially the outcome.
That is why every Releaf patient gets a medical cannabis card free when you pay for your first prescription. Designed with police support, it lets you prove your status in seconds: a quick QR code scan confirms to the authorities that you are a registered patient with a valid, legal prescription. No fumbling for paperwork, no lengthy explanations.
And for moments when you may need some advice or support, there's Releaf Protect, a dedicated legal helpline built with law firm Irwin Mitchell. Whether it's a roadside stop, a dispute at work, or any number of other issues, you can speak to someone who genuinely understands medical cannabis law, rather than facing it alone.
Neither makes you immune from being stopped. But together, they mean you are never left to handle it unsupported.
Ready to learn more? Head to our fast and free medical cannabis eligibility checker. It takes just 30 seconds to complete, and may be your first step to better health and quality of life.
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Decoding Cannabis: Over the limit, but not impaired? What the science shows
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