Driving under the influence of pot nearly doubles the risk of a serious or fatal car crash, a Canadian study finds.
In addition, "surveys of young drivers have also shown that rates of driving under the influence of cannabis have surpassed rates of drinking and driving in some jurisdictions," the Halifax team reports.
Not only is cannabis relatively easy to get, "many young people really don't believe that cannabis impairs," said lead author Mark Asbridge, an associate professor in the department of community health and epidemiology at Dalhousie.
"They know that alcohol impairs their driving, so if they're the designated driver at a party they'll switch alcohol for cannabis. I've heard stories: 'I'm the designated driver so I'm just going to smoke up.' "
"We just simply don't have the same messaging around drugs and driving" that exists around drinking and driving, he said.
Past studies into cannabis and crash risk have been mixed. Some have found an increased risk of being involved in a collision after using marijuana, while others have found either no association whatsoever, or even a lower risk — suggesting people were actually safer driving while intoxicated by pot than not.
All the crashes involved in the analysis took place on public roads and involved one or more moving vehicles such as cars, vans, trucks, buses and motorcycles.
The strongest association was with fatal crashes.
The study wasn't designed to answer the question: How much pot does it take before the crash risk increases?
Most studies in the analysis used any amount greater than zero as the cutoff for a positive test result. But, "for cannabis, there's not necessarily a cut off that we can identify where risk was most heightened," Asbridge said.
Earlier studies that suggested it might be safer driving under the influence of cannabis often relied on urine samples. The problem there, Asbridge says, is that markers for cannabis in urine "can stay in your body for weeks or even over a month so that's not a measure of recent use at all."