Road safety is a top priority for road safety agencies, and there has been a great tussle over medical cannabis’ influence on driving. If traffic law enforcement officials find even prescribed THC in your body, you could face a penalty for the offense of Driving Under the Influence (DUI).

With the legalization of medical cannabis in Australia, there has been a debate on the influence of cannabis on driving. The debate has led to a rise in research on the subject, which could impact future laws.

What Is DUI?

When it comes to drug driving, Australia has a "zero tolerance" policy (1). As a result, any drug substance identified in the driver's system is considered a violation. This means that finding even a trace amount of alcohol or other illicit or licit drugs is deemed a crime even if it did not induce impairment.

So, what does a DUI offense entail? It occurs when a motorist is found to be unable to maintain reasonable vehicle control.

Opiates and antidepressants, for example, are medications designated under Poisons standards because they are suspected to impair psychomotor, cognitive, and driving skills (2). In general, they are thought to induce impairment or high crash risk.

Gauging Impairment

A DUI offense is assessed through a series of tests initiated by a police officer. The officer conducts a sobriety test, in which they may ask the motorist to do various tasks: they may assess their speech, response time, balance (by walking in a straight line), among other tests. Also, a urine or blood test may be conducted by the officer.

Some of the impairment effects tested on drugs that are legal or otherwise include:

Lowered reaction time
Poor coordination and reflexes
Poor concentration
Impaired eyesight

Therefore, if cannabis is to be evaluated on its impact on driving, these and other impairment effects have to be investigated.

Extensive and conclusive research has been done on CBD and THC effects on driving impairment, and there are some interesting findings.

Research On Medical Cannabis And Its Effect On Driving

The Lambert Initiative carried out a study at the University of Sydney in collaboration with Tilray and Alfred Hospital. The study, conducted on 14 volunteers with mild cannabis use, explored CBD and THC's effects on simulated driving (3).

The participants were intoxicated with high THC cannabis. Interestingly, while THC appeared to cause impairment, the participants were found to be 'safer' drivers.

There was a shift in driver behavior to reduce potential crash risk. For instance, the drivers would leave more space between themselves and the car in front of them, and they weren't prone to speeding or overtaking.

Interestingly, adding CBD didn’t alleviate feelings of intoxication or lower driving impairment. In fact, CBD seemed to heighten THC- induced impairment.

The same institutions conducted a follow-up study to assess the accuracy of mobile drug testing devices (4). The gadgets were to detect vaporized THC with varying levels of CBD content. Fluid tests were performed on a similar number of healthy volunteers at timed intervals for three hours. The study discovered noteworthy false-positive and false-negative results.

The DrugWipe 5s and DrugTest 5000 were the devices that were tested. None of the devices performed as expected in specificity, sensitivity, and accuracy, with none exceeding 80%.

The Lambert initiative undertook a road study at Maastricht University in the Netherlands. CBD has been discovered not to affect driving; however, THC can induce minor impairment for up to four hours (5).

This development gives those who use CBD as a medication peace of mind when driving.

On the other hand, when using THC or a combo of THC with CBD, impairment didn’t persist after 4 hours. Therefore, a window was established to assist patients in understanding the duration of impairment, particularly with THC use.

Regulation On The Use Of Medicinal CBD

The Australian Therapeutic Goods Administration TGA granted over 100,000 approvals for medicinal cannabis products (6).

Furthermore, in Australian jurisdictions such as Tasmania and Victoria, there has been advocacy in the Australian Senate to revisit the ‘presence-based’ drug-driving crimes to enhance patient access to medical cannabis.

Road safety agencies continue to emphasize THC's incapacitating effects on driving, and their influence can be seen in South Australia (7). When a measure to reform the medicinal cannabis situation was introduced in parliament, the Police Minister expressed alarm, and the bill was not passed.

On the other hand, politicians and advocacy groups are emphasizing the need for change (8). According to road officials, patients who are legally prescribed medical cannabis-infused with THC are being lumped together with illicit drug users. The lobbyists are pushing for a reform that ensures that medical cannabis patients are treated fairly.

In January 2020, a Magistrate in South Australia had a patient charged with DUI (9). The medical cannabis patient was found to have been driving with a prescribed drug in his system. The magistrate dropped the charge on a lack of proof of impairment. However, she noted that the conviction would stand if the patient was prosecuted again.


  1. ‘Zero Tolerance’ Drug Driving Laws in Australia: A Gap Between Rationale and Form?(2017)
    International Journal for Crime, Justice and Social Democracy

  2. The Role of Drugs in Road Safety (2008)
    Australian Prescriber

  3. Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition (2019)
    Springer Link

  4. An evaluation of two point-of-collection testing devices (2019)
    Wiley Analytical Science

  5. Cannabidiol (CBD) in cannabis does not impair driving, landmark study shows. (2020)
    The University of Sydney

  6. Medical Cannabis
    Australia Department of Health
    Medicinal cannabis driving plan inconsistent with road safety objectives, SA Police Minister says
    ABC news,-sa-police-minister-says/8686688

  7. Medicinal Cannabis Driving Laws Must Change Now (2020). Fiona Paten. Available from:

  8. Magistrate dismisses drug driving charge for medicinal cannabis user (2020)