Why there is no defence to driving stoned (including "medicinal" cannabis use)

As we begin our Easter Holidays in Australia and the Greens/Cannabis parties seem hell bent on making our roads more dangerous by allowing you to drive stoned on the roads (let's face it, if you've got a pulse you've got a "prescription" for "medicinal" cannabis), I thought bringing some common sense and science to a debate on buzzwords may be in order. For some people reading this, you’re probably still on the well-worn line that roadside drug testing should “measure impairment like alcohol.”

There’s a fundamental problem with that idea. No biological test on earth proves impairment. Not blood alcohol. Not breath alcohol. Not THC.

Impairment is ultimately a behavioural assessment. Once you move into “prove impairment,” you’re no longer measuring chemistry, you’re asking police to enforce opinions. That’s not a road we want to go down.

The Reality of THC Testing in NSW

In New South Wales, prosecution doesn’t occur from the roadside device alone. It requires laboratory confirmation, and in practical terms: You are not getting prosecuted unless your oral fluid THC exceeds 10 ng/mL at confirmation. (in my experience it actually needs to be an absolute minimum of 16 ng/mL to survive the trip to the lab, and in summer it would need to be really high)

That is not a trivial threshold.

By the time a sample:

• sits in a collection device

• is transported

• and is analysed days or weeks later

THC will have degraded, often substantially (anything from 35% - 90% loss of THC evidence).

So the number that matters is not what was measured. It’s what was there at the roadside.

How Did We End Up With These Numbers?

This is where science actually does come in, just not in the way people think.

With alcohol, limits like 0.050 BAC were derived from controlled studies where participants were dosed and their performance assessed(1). The conclusion was simple:

Most people show measurable impairment around that level.

So naturally the question became have we done the same for cannabis?

Yes.

What the Science Actually Says

In 2013, Rebecca Hartman and Marilyn A. Huestis published one of the most comprehensive reviews on cannabis and driving:

Cannabis Effects on Driving Skills(2)

Their conclusion was clear:

Blood THC concentrations of 2–5 ng/mL are associated with substantial driving impairment. 

They also showed:

• Crash risk increases after cannabis use

• Driving performance (reaction time, lane control, divided attention) deteriorates

• Higher THC concentrations increase culpability risk

And importantly:

There is no reliable way to predict impairment from THC concentration in an individual. 

Blood vs Oral Fluid (Where People Get Lost)

Here’s where the confusion creeps in.

• Blood THC reflects systemic concentration

• Oral fluid THC reflects recent use, especially after smoking

Once the mouth is no longer contaminated (i.e. not immediately after smoking), a rough relationship emerges: ~2 ng/mL blood THC ≈ ~5 ng/mL oral fluid THC

And that is not a coincidence.

5 ng/mL is the confirmatory cut-off in AS/NZS 4760:2019.

This is not an “impairment limit.” It is a risk threshold, aligned with where impairment becomes statistically likely across a population.

Why This Isn’t a “Gotcha” System

A common argument is:

“I wasn’t impaired.”

That may even be true for a given individual. Just like an alcoholic may be functional at a BAC of 0.080.

But the law isn’t written for individuals. It’s written for public safety.

Because the same body of evidence shows:

• At ≥5 ng/mL blood THC, crash culpability rises sharply

• At higher concentrations, impairment can be comparable to very high alcohol levels in terms of driving performance degradation

So the system takes a position of If you are in that range, the risk is unacceptable.

The Key Point Most People Miss

If you fail a roadside drug test for THC, one of two things is true:

1. You used very recently

Your oral fluid levels will be extremely high. We cannot meaningfully interpret impairment, only confirm recent use

2. You used hours earlier and still test positive

Your levels are still high enough to sit within a range associated with impairment risk

Either way, you should not be driving.

“Medicinal Cannabis” Doesn’t Change the Physics

This is where the defence tends to fall apart.

THC does not care why you took it.

• It affects reaction time

• It affects attention

• It affects motor control

Whether it was "prescribed" or not does not change:

Its effect on driving ability, or the risk it creates on the road

The Bottom Line

• There is no reliable way to tie THC concentration to individual impairment

• There is strong evidence that certain ranges are associated with increased crash risk

• Oral fluid testing is designed to detect recent use within that risk window

And that leads to the uncomfortable but unavoidable conclusion:

If you are detected at prosecution levels, there is no scientific or logical basis to defend driving.

Not because we can prove exactly how impaired you were. But because we know enough to say the risk you posed to yourself and other road users was not acceptable.

(1) Ferrara SD, Zancaner S, Georgetti R. Low blood alcohol levels and driving impairment. A review of experimental studies and international legislation. Int J Legal Med. 1994;106:169–177

(2) Hartman RL, Huestis MA. Cannabis effects on driving skills. Clin Chem. 2013;59(3):478-92.