NSW sets a THC driving limit — trauma doctors say the science isn't there

What just happened

NSW Premier Chris Minns has described incoming legislation as a “commonsense” approach to an unresolved tension in Australian road law: what to do with the approximately 300,000 Australians who hold medicinal cannabis prescriptions but cannot, under current rules, legally drive regardless of their actual state of impairment.

The proposed legislation introduces a legal saliva THC threshold of 50 ng/mL for medicinal cannabis holders — replacing a system where any detectable THC concentration resulted in a charge. Under the current framework, a person who used their prescribed cannabis product the night before, slept, woke fully unimpaired, and drove to work could still test positive and face prosecution. THC has a longer saliva detection window than its psychoactive effects, particularly in regular users who have established physiological tolerance.

In response, twenty doctors — including trauma surgeons, anaesthetists, and emergency medicine specialists — have signed a joint statement arguing that the 50 ng/mL figure has no established scientific basis as an impairment threshold, and that the law will “normalise impaired driving and increase the road death toll.” The statement was led by surgeon Dr John Crozier.

This is a dispute between two legitimate concerns — fairness to patients using lawfully prescribed medication, and the scientific validity of the specific number chosen to govern that fairness.

---

The both-and

The case for reform

The current law in most Australian states treats any detectable THC as legally equivalent to intoxication. For a patient prescribed medicinal cannabis for chronic pain, MS, PTSD, or treatment-resistant anxiety, this is not a minor inconvenience. It is a structural condition on their treatment: use the prescribed medication and stop driving, or stop the medication.

For Australians in regional and rural areas without reliable public transport, the driving question is not abstract. A person managing significant chronic pain with a medicinal cannabis prescription who relies on a car to attend work, medical appointments, or basic daily functions faces a real and ongoing dilemma under the current blunt prohibition.

The reform is responding to a documented gap between the law and the clinical reality of how THC behaves in the body over time. The TGA’s guidance on medicinal cannabis already acknowledges the complexity of individual THC pharmacokinetics. The argument from the NSW government is that a concentration-based threshold — imperfect as it is — is more calibrated to actual impairment risk than a zero-tolerance rule that catches everyone regardless of whether they are impaired.

The case against this specific threshold

The twenty doctors opposing the legislation are not arguing for the zero-tolerance status quo. They are arguing that 50 ng/mL is not a validated impairment threshold, and that setting one by legislation without the supporting science may make roads measurably less safe while appearing to solve a fairness problem.

The pharmacological concern is technically grounded. For alcohol, decades of epidemiological and experimental research have established that 0.05 g/dL blood alcohol concentration corresponds to a meaningful, measurable, population-wide increase in crash risk. The equivalent evidence base for THC driving impairment is substantially thinner — not because no research exists, but because the variability in individual response to a given THC concentration is far higher than for alcohol.

A regular medicinal cannabis user may function without meaningful impairment at a saliva concentration that would impair a first-time or occasional user significantly. Cannabis products vary widely in their cannabinoid ratios. Time since last dose, method of administration, and concurrent use of other medications all affect the impairment curve in ways that do not reduce to a single saliva number with reliable predictive validity.

The doctors’ statement frames this directly: the law creates a number that looks precise but is not clinically validated. If courts and roads authorities treat 50 ng/mL as equivalent to the alcohol standard — a known, well-validated threshold — they are working from a false equivalence.

---

2 cents

The tension here is genuine and not easily collapsed to one side. The current blanket prohibition treats all detectable THC as intoxication, which it is not — and the consequence falls most heavily on patients managing serious conditions who are using lawfully prescribed medication. That is a real problem that deserves a real solution.

But the doctors opposing this legislation are not being alarmist. The scientific problem with setting a THC impairment threshold is real. The concern that this law may be adopted by drivers and courts as a kind of validated clearance — equivalent in authority to the alcohol limit — before the evidence base supports it, is legitimate.

If you hold a medicinal cannabis prescription, this law — when it passes — changes your legal exposure in NSW. It does not change the pharmacology. Whether a specific product, dose, and the timing of when you last used it affects your driving capacity is a question for your prescribing doctor, ideally with TGA guidance on medicinal cannabis as one input. A legal threshold is not medical clearance.

Verdict: maybe — the reform addresses a genuine fairness gap, but the doctors’ scientific objection to the specific 50 ng/mL figure is substantive and not resolved by the legislation. This one is worth watching.

---

Sources cited

1. Australian Doctor — ‘Lives will be lost’: Trauma doctors warn medicinal cannabis driving threshold has no scientific basis. https://www.ausdoc.com.au/news/lives-will-be-lost-trauma-doctors-warn-medicinal-cannabis-driving-threshold-has-no-scientific-basis/
2. TGA — Guidance on medicinal cannabis products. https://www.tga.gov.au/resources/resource/guidance/guidance-medicinal-cannabis-products