Researchers at the University of Sydney say drug driving laws are “out of date” and should reflect the results of a new study that found the impairment lasted around three to ten hours after using cannabis.

But the NSW government has rejected those calls and defended its zero tolerance approach to drug driving, with tests that can detect traces of the drugs weeks after use.

Drivers who use cannabis can face criminal prosecution for the presence of THC in their blood or saliva.Recognition:File image / Queensland Police Service

Danielle McCartney, a clinical fellow at the University of Sydney’s Lambert Cannabinoid Therapeutics Initiative, said a person could have the intoxicating component of cannabis, tetrahydrocannabinol (THC), in their body and still be able to drive.

Dr. McCartney-led study, published in an upcoming issue of the journal Neuroscience & Biobehavioral Reviews, found that cannabis users had a “window of impairment” between three and ten hours from THC.

“Our study shows that cannabis is unlikely to interfere with driving for more than five hours if inhaled, or for more than eight hours if ingested,” she said.

Factors that influenced impairment included how regularly a person consumed cannabis, how it was consumed (inhalation, ingestion), and how much they consumed. Dr. However, McCartney said there was no straightforward association between THC levels and impairment, unlike alcohol, where 0.05 / 0.08 BACs were widely accepted limits.

“High levels of THC in blood and oral fluids are possible without harm, just as low levels are not always ‘safe’,” she said. “This makes it difficult to pinpoint the point at which the depreciation usually subsides.”

Dr. McCartney said it is becoming “increasingly clear that our drug use laws are not only out of date but also utterly unreasonable with tens of thousands of Australians using legal, prescribed medical cannabis.”

“Our drug use laws should be about improving road safety and minimizing injuries, not criminalizing drivers who just have the very presence of a drug in their system,” she said.