Cannabis use doubles chances of vehicle crash

New research has found that drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a vehicle collision as those who are not under the influence of drugs or alcohol.

In a new study published in today in the journal BMJ, Canadian researchers reviewed nine studies with a total sample of 49,411 people to determine whether the consumption of cannabis increases the risk of a motor vehicle collision.

The authors’ analysis of the data found that if cannabis is consumed before driving a motor vehicle, the risk of collision is nearly doubled.

The results come at a time when the NZ Drug Foundation is actively campaigning against ‘drugged driving’ here in New Zealand, a nation notorious for its high cannabis consumption.

In an accompanying editorial, Prof Wayne Hall from the University of Queensland Centre for Clinical Research, questions the benefits of roadside drug testing on public health, noting “…there is no evidence that roadside drug testing has reduced deaths related to cannabis or other drugs or deterred drug users from driving while impaired.” He called for further studies to analyse the impact of road-side drug testing.

Our colleagues at the Australian Science Media Centre collected the following expert commentary. If you would like to talk to a New Zealand expert please contact the NZ SMC (; 04 499 5476)

Prof Mathew Martin-Iverson is Professor of psycho-pharmacology at the School of Medicine & Pharmacology, University of Western Australia

“There are some issues with this research as the authors only state the relative risk of an accident given the presence of tetrahydrocannabinol (THC; the active ingredient of marijuana) in people that have had serious accidents – they do not state the absolute risk. They describe the risk as ‘doubling’ but that is not particularly helpful – does it mean that the risk of an accident went from 50% to 100% chance or from 0.0001% to a 0.0002% chance? The former would be very significant but the latter would not be.

“Random road-side testing in Victoria Australia in 2007 found 0.66% of the drivers were positive for THC (87/13176 tests). The risk of Victorians suffering an injury or fatal motor vehicle accident in a year is about 0.11%. So, I suppose that 0.66% of the drivers of Victoria have a risk of a serious accident going up from 0.11% to 0.22% if they use cannabis frequently when driving during the year. Is this a significant issue?

“Of course, one supposes that it depends on the level of intoxication rather than a positive THC screen (which may remain positive well after intoxication is no longer an issue). I wouldn’t recommend anyone to drive while intoxicated on any substance.”