Proposed changes to NSW drug driving laws would mean that people prescribed medicinal cannabis who test positive for THC at the roadside could avoid charges if laboratory testing shows their saliva contains less than 50 nanograms per millilitre of THC.

The aim is to make things fairer. Yet, both the existing and proposed laws hinge on measuring THC in saliva – and the evidence that this is a reliable predictor of impairment is shaky at best.

What’s the current legal situation in Australia?

Currently, in all states and territories except for Tasmania it’s illegal for people to drive with any detectable amount of THC in their system, even if it was prescribed and is not causing impairment.

By contrast, if the NSW reforms pass, patients who register their prescriptions with Transport for NSW and complete online education on cannabis and driving would be exempt from the ‘drug presence offense.’ If any THC were present in their saliva at the roadside test, the sample would be sent to a lab to assess whether the levels fall above or below the cut-off.

In Tasmania*, a valid prescription for medicinal cannabis can be used as a medical defence for drivers who test positive for THC at the roadside, provided they are not impaired.

Since last year Victoria allows magistrates to decide on a case-by-case basis whether or not to cancel the licenses of drivers who test positive for THC while using prescribing medicinal cannabis – but it’s still illegal to have any THC in your system when driving, and a valid prescription can’t be used as part of a medical defence in that state.

Experts and advocates say there is no doubt that change is needed. The NSW government said the proposed reforms come in response to a recommendation from the NSW Drug Summit, after careful consideration. “Many people taking cannabis medicines currently face a choice between their health and being able to access transport for themselves, their work and their families,” it said, noting that key conditions would still need to be met.

But while many see it as a step in the right direction, there are concerns that the laws are still flawed.

Cannabis and driving risk

The Austroads guidelines explain that while THC “can cause cognitive and psychomotor impairments that degrade the ability to drive safely,” including at low doses, the deficits are “highly individualised,” and onset and duration of effects can vary widely between individuals.

The complex pharmacokinetics of cannabinoids make it hard to predict the severity of impairment, they add, noting that dose frequency, history of use and administration route can all play a role.

Associate Professor Vicki Kotsirilos, Australia’s first authorised prescriber of medicinal cannabis and the RACGP representative on the expert committee that developed resources to guide decisions about medicinal cannabis and driving also notes that the extent of impairment can vary from one individual to another.

“It’s not necessarily related to concentrations, although higher concentrations are more likely to cause impairment,” she says. “For example, if you’ve got a patient who’s never had cannabis, and they’re sensitive to THC, they still may get side effects such as drowsiness at lower doses.”

Certain people, such as the elderly and those with liver or kidney disease, may experience impairment at levels below the cutoff, Associate Professor Kotsirilos says.

Regular cannabis users “may experience less impairment on the road because they’ve adapted to higher levels,” she says, stressing this does not justify changing the regulations.

Is THC in saliva a reliable predictor of impairment?

A meta-analysis of 28 publications looking at the relationship between THC and driving outcomes found only a weak association between THC-related biomarkers in blood and oral fluid and impairment in occasional cannabis users – and no association at all in regular cannabis users.

“Research validating the relationships between THC-related biomarkers, subjective intoxication, and impairment of driving and driving-related cognitive skills is relatively limited,” the authors note.

They found that THC concentrations in blood and oral fluid – as well as peoples’ subjective ratings of intoxication—were “relatively poor indicators of cannabis-induced impairment. The use of per se limits as a means of identifying cannabis-impaired drivers should therefore be re-considered.”

A review published this year found that while there is clear evidence of cannabis-induced driving impairment, “there are no reliable or practical biochemical or behavioural methods used in real-time with drivers on the road to determine cannabis-induced impairment,” at present.

“In contrast to the valid inferences about impairment from alcohol concentration levels, there is markedly poor correspondence between levels of THC in biological specimens (e.g., blood, saliva) and driving-related cognitive impairment,” the authors said.

Ultimately, “there are no empirically-supported thresholds for blood or oral fluids that reliably indicate cannabis impairment,” they summed up.  

Kelly Rechtin, a clinical nurse educator with Australian medicinal cannabis company Humacology says the lack of scientific consensus over whether THC concentration alone can reliably determine driving impairment poses a challenge for the approach being proposed in NSW.

“Consequently, any specific THC threshold is, to some extent, an arbitrary line rather than a direct measure of a driver’s ability to operate a vehicle safely,” Ms Rechtin says.

Even with the reforms, patients could still experience consequences

If the reforms in NSW go ahead, registered medicinal cannabis users would still be banned from driving for 24 hours if THC is detected on roadside saliva testing.

This could have significant practical consequences, Ms Rechtin says.

“A patient could be on their way to work, attending a medical appointment, or collecting their children from school, only to face immediate disruption despite ultimately being found compliant with the law,” she says.

“For patients who use medicinal cannabis regularly and lawfully, the prospect of a 24-hour suspension following any positive roadside test may create considerable uncertainty and hardship.”

Still Ms Rectin says it would still be a step forward.

“Despite these concerns, the reforms remain a substantial improvement on the previous system. For years, medicinal cannabis patients in NSW faced automatic penalties simply for the presence of THC, regardless of impairment. While the new framework is far from perfect, it represents meaningful progress toward a more balanced and patient-focused approach.”

Associate Professor Kotsirilos agrees. “That is a big inconvenience for a patient who might’ve taken their medicinal cannabis just before they left for school or work or a doctor’s appointment, but at least they’re given a warning compared with zero tolerance as it currently stands.”

Tips to reduce the risks for patients taking medicinal cannabis

To avoid issues like these, Associate Professor Kotsirilos says it’s important to warn patients not to drive for six to eight hours after using cannabis products that contain THC. She advises people to consider timing when they use the products to allow plenty of time before they need to drive.

“For example, if they take it before bed, it’s likely not going to be detected in the saliva by the morning.”

Saliva THC levels do not accurately reflect blood levels, Associate Professor Kotsirilos adds, noting THC can be detected in saliva immediately and usually washes out within six to eight hours—while metabolites can be excreted in the urine for up to five days.

“Furthermore, patients should not drive if they feel drowsy or impaired, and they should avoid other drugs or alcohol that may impair driving,” she says.

Medicinal cannabis should only be prescribed “as a last resort when all other treatments have failed,” she adds, noting CBD dominant products are preferable.

“CBD is not tested in the saliva and we know that CBD does not usually cause impairment,” Associate Professor Kotsirilos says.

“If CBD alone fails, always start low, go slow, using the lowest-concentration THC products that are available.”

Make sure you get signed patient consent that contains clear warnings about possible risks, such as sedation with THC products, including impact on driving, she stresses.

It’s also important to monitor individual responses, she adds. “We never know which patient might be affected even at low levels, because they might be very sensitive to THC products.”

Austroads guidelines suggest considering a period without driving (or restricted driving) while patients adapt to their medication. They also advise discussing the implications of using THC-containing products for driving, and cautioning patients to avoid alcohol while using medicinal cannabis due to the heightened crash risk.

Associate Professor Kotsirilos and Dr Thomas Arkell also included more detailed suggestions for safeguards to manage the risks in this article published in MJA’s Insight Plus.