A new study has looked at how cannabis affects driving ability.

The University of Sydney research found that cannabidiol (CBD) - a cannabis component now widely used for medical purposes - does not impair driving.

It also found that moderate amounts of the main intoxicating component tetrahydrocannabinol (THC) produce mild driving impairment lasting up to four hours.

“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,” says lead author Dr Thomas Arkell.

The study involved giving 26 healthy participants four different types of cannabis in a random order to vaporise on four separate occasions. Each participant’s driving performance was then assessed on the road in real-world conditions along a 100-kilometre stretch of public highway in a dual control car with a driving instructor present.

The tests were done at Maastricht University in the Netherlands using a well-established 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.

There has been substantial growth in medical treatment using cannabis-related products in Australia and overseas. This includes increasing use of CBD-containing products for conditions such as epilepsy, anxiety, chronic pain and addictions. Many currently available products also contain a mixture of THC and CBD.

The research involved people inhaling vaporised cannabis containing different mixes of THC and CBD, then going for a 100-kilometre drive under controlled conditions on public highways both 40 minutes and four hours later.

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

“With cannabis laws changing globally, jurisdictions are grappling with the issue of cannabis-impaired driving,” Dr Arkell says.

“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.”

The study is accessible here.