It's probably safe to take medicines based on cannabis extract cannabidiol (CBD) and drive, new research has found.
The study also found the effects of cannabis containing psychoactive tetrahydrocannabinol (THC) wear off quickly, with drivers showing no impairment just four hours after getting high.
"These findings indicate for the first time that CBD, when given without THC, does not affect a subject's ability to drive," said the University of Sydney's Thomas Arkell, who led the research.
"That's great news for those using or considering treatment using CBD-based products."
In the study, 26 cannabis users vaped three different types of cannabis - mainly THC, mainly CBD, THC and CBD in combination - or a placebo, then hit the road.
The driving was done in the Netherlands on public roads - participants drove 100km at 95km/h on a highway with a sober person present, who had the ability to take control of the vehicle should the driver prove unable to drive safely (which happened 16 times over the course of the study). The first drive was done 40 minutes after inhaling, the second four hours later, and the amount of swerving measured.
They found CBD-dominant cannabis had no effect on driving ability whatsoever.
"The results should reassure people using CBD-only products that they are most likely safe to drive," said Iain McGregor of the University of Sydney's cannabis research unit the Lambert Initiative.
Inhaling THC-dominant cannabis or the THC/CBD mixture resulted in impairment in the first drive, but not in the second. Unlike alcohol, the level of THC detected in tests doesn't correlate to impairment, with the drug lingering in the system long after the effects have worn off.
Some users report CBD can ameliorate the psychoactive and psychomotor effects of THC. The Australian researchers found no evidence of this.
"Clinicians should caution their patients that cannabis products containing equal parts CBD and THC are no less impairing than products containing THC alone," the authors wrote in an accompanying editorial, published in the Journal of the American Medical Association.
Prof McGregor said the findings will help "patients using THC-dominant products to understand the duration of impairment".
"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.
"This is the first study to illustrate the lack of CBD effects on driving and to also provide a clear indication of the duration of THC impairment."
The scientists said more research is needed however, with the dose levels used in the study perhaps lower than what people ingest in the real world - but "enough to cause strong feelings of intoxication" nonetheless.