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Cannabis Poses Less On-Road Risk Than Alcohol, US Crash Data Says

Thursday, 22 February 2007

Thunder Bay, Ontario:  US drivers involved in fatal crashes who had trace levels of cannabis in their blood or urine are less likely to have engaged in risky driving behavior than drivers who test positive for low levels of alcohol, according to case-control data published in the current issue of the Canadian Journal of Public Health.

Investigators at Ontario's Lakeland University, Public Health Program, reviewed drug and alcohol data from US drivers aged 20-49 who were involved in a fatal crash from 1993 to 2003. Researchers separated motorists into 'cases' (drivers who had at least one potentially unsafe driving action recorded in relation to the crash) and 'controls' (drivers who had no such driving action recorded).  Investigators sought to determine whether drivers who tested positive for cannabis but negative for alcohol were more likely to have engaged in risky driving behavior than drivers who tested negative for both pot and alcohol. Over the ten-year period, 1,647 drivers tested negative for alcohol, but tested positive for the presence of THC in their blood or urine.

Researchers concluded that alcohol-free drivers who tested positive for cannabis had a slightly elevated risk of engaging in a potentially unsafe action compared to sober drivers, even after controlling for age, sex, and prior driving record.  However, investigators also reported that drivers who tested positive for low levels of alcohol (.05 BAC) possessed a significantly greater risk of engaging in such risky driving behaviors compared to sober drivers.

Investigators wrote: "The findings point to cannabis as a potential risk factor in fatal crashes.  Individuals who tested positive for cannabis but negative for alcohol had a 29 percent excess risk of having driven in a fashion that may have contributed to the crash, compared to drivers who tested negative for cannabis. ... Yet these estimates appear small compared to alcohol and some prescription medications."

Motorists who had BAC of .05 percent, a threshold well below the legal limit for drunk driving in the US, had a 101 percent excessive risk of having driven in a risky manner compared to alcohol-free drivers, authors reported.  Drivers with a BAC of .10, just over the US legal limit for drunk driving, had a 200 percent excess risk.

Canadian researchers also determined that motorists who tested positive for cannabis were generally younger, male, and had a poorer driving record in the prior three years than drivers who tested negative for pot.

Responding to the study, California NORML Coordinator Dale Gieringer said: "This is the latest in a long line of studies to show that marijuana is a lesser on-road accident hazard than alcohol. Yet, ironically, federal drug policy has been more geared toward exaggerating the minor on-road risks posed by cannabis and downplaying the known risks posed by alcohol."

An analysis of French crash data published in the British Journal of Medicine in 2005 similarly reported that drivers who tested positive for cannabis in the blood were far less likely to have been involved in a fatal traffic fatality than drivers who possessed BAC levels of .05-.08.  

According to previous studies of on-road crash data, past use of cannabis (as indicated by the presence of marijuana metabolites in the urine) is not associated with an elevated risk of accident. By contrast, drivers who test positive for THC in the blood above 5 ng/ml (indicating recent use of pot) are typically associated with an elevated risk of accident compared to drivers who test negative for cannabis and/or test positive for very low levels of THC in the blood.

THC blood levels typically fall below 5 ng/ml in recreational (non-chronic) cannabis users within 60 to 120 minutes after inhalation.

For more information, please contact Paul Armentano, NORML Senior Policy Analyst, at (202) 483-5500 or Dale Gieringer, California NORML Coordinator, at (415) 563-5858.  Full text of the study, "The impact of cannabis on driving," appears in the January/February issue of the Canadian Journal of Public Health.