New research shows cannabidiol (CBD) is safe for driving and the effects of THC fade in just four hours in a huge boost for drug-driving reform campaigners.

The study, led by the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney and conducted at Maastricht University in the Netherlands, was published today in the Journal of the American Medical Association

Lead author Dr Thomas Arkell said: “These findings indicate for the first time that CBD, when given without THC, does not affect a subject’s ability to drive. That’s great news for those using or considering treatment using CBD-based products.” 

Lambert Initiative
Professor Iain McGregor from the Lambert Initiative

The Academic Director of the Lambert Initiative Professor Iain McGregor said the results prove the current laws in Australia are not fit for purpose: “With rapidly changing attitudes towards medical and non-medical use of cannabis, driving under the influence of cannabis is emerging as an important and somewhat controversial public health issue. 

“While some previous studies have looked at the effects of cannabis on driving, most have focused on smoked cannabis containing only THC, not CBD, and have not precisely quantified the duration of impairment.” 

Professor McGregor said the study was the first to demonstrate CBD does not impair driving and to provide a clear indication of the duration of THC impairment.

“These results suggest there should be no prohibition around driving after use of CBD products,” he added.

“With respect to THC, laws need to be revised to capture the nuance and subtlety of THC-induced impairment. Prohibition of THC at any concentration in the blood or oral fluid of drivers is problematic since the mere presence of THC does not indicate impairment.

“A more enlightened approach would be to advise users not to drive within four hours of cannabis use, and longer if the driver still feels intoxicated.”

“A more enlightened approach would be to advise users not to drive within four hours of cannabis use, and longer if the driver still feels intoxicated.”

Professor McGregor acknowledged the published study only uses a single dose of vaporised CBD and said the research team are currently finishing a follow-up study with very high oral doses of CBD “that should further confirm the safety of CBD with driving”.

The research involved 26 healthy participants inhaling vaporised cannabis containing different mixes of THC and CBD or a placebo, then going for a 100-kilometre drive under controlled conditions on public highways both 40 minutes and four hours later. The amount of THC vaporised by participants was enough to cause strong feelings of intoxication. 

Cannabis containing mainly CBD did not impair driving while cannabis containing THC, or a THC/CBD mixture, caused mild impairment when measured 40 minutes later but not after four hours. 

The tests were done at Maastricht University in the Netherlands using a scientific test that measures standard deviation of vehicle position (SDLP), an index of lane weaving, swerving and overcorrecting. SDLP increases under the influence of alcohol and drugs such as Valium and Stilnox. 

Dr Arkell said: “With cannabis laws changing globally, jurisdictions are grappling with the issue of cannabis-impaired driving. These results provide much needed insights into the magnitude and duration of impairment caused by different types of cannabis and can help to guide road-safety policy not just in Australia but around the world.” 

“Road safety is a primary concern. These results should allow for evidence-based laws and regulation for people receiving medical cannabis,” added Dr Arkell. 

Drug-driving laws have become a hot topic in Australia with campaigners including former New South Wales magistrate David Heilpern arguing the current rules do not make sense.

In Victoria, the state government said recently it will establish an implementation group to explore new legislation which would treat medicinal cannabis like any other medicine. However, In New South Wales, a move by the Greens to make medicinal cannabis use a defence against a drug-driving conviction was voted down in October.

Professor McGregor said the study was done with occasional cannabis users and that impairment may actually be far less in regular users, including medicinal cannabis patients, due to higher levels of tolerance.

“We therefore plan to repeat the driving study, but with medical cannabis users and determine whether their driving is impaired at short intervals after use, as we saw with occasional users.

“It is conceivable that THC might even improve driving in some patients if it relieves a medical condition such as chronic pain. These studies are urgently needed,” he added.

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