The impact of cannabis on driving.
BACKGROUND
Cannabis is known to have detrimental effects on human performance and may also affect driving adversely. However, studies designed to examine this issue have provided equivocal findings. We set up this study to further determine the effect of cannabis on driving.


METHODS
We used a cross-sectional, case-control design with drivers aged 20-49 who were involved in a fatal crash in the United States from 1993 to 2003; drivers were included if they had been tested for the presence of cannabis and had a confirmed blood alcohol concentration of zero. Cases were drivers who had at least one potentially unsafe driving action recorded in relation to the crash (e.g., speeding); controls were drivers who had no such driving action recorded. We calculated the crude and adjusted odds ratios (ORs) of any potentially unsafe driving action in drivers who tested positive for cannabis but negative for alcohol consumption. In computing for the adjusted OR, we controlled for age, sex, and prior driving record.


RESULTS
Five percent of drivers tested positive for cannabis. The crude OR of a potentially unsafe action was 1.39 (99% CI = 1.21-1.59) for drivers who tested positive for cannabis. Even after controlling for age, sex, and prior driving record, the presence of cannabis remained associated with a higher risk of a potentially unsafe driving action (1.29, 99% CI = 1.11-1.50).


CONCLUSIONS
Cannabis had a negative effect on driving, as would be predicted from human performance studies. This finding supports the need for interventions to decrease the prevalence of driving under the influence of cannabis, and indicates that further studies should be conducted to investigate the dose-response relationship between cannabis and safe driving.
Bédard
Michel
M
Public Health Program, Lakehead University, Thunder Bay, ON. michel.bedard@lake-headu.ca
Dubois
Sacha
S
Weaver
Bruce
B
D000063:Accidents, Traffic / Q000401:mortality* / Q000523:psychology; D000328:Adult; D001334:Automobile Driving / Q000523:psychology*; D001519:Behavior / Q000187:drug effects*; D002186:Cannabinoids / Q000009:adverse effects* / Q000097:blood; D016022:Case-Control Studies; D003430:Cross-Sectional Studies; D019992:Databases as Topic; D004305:Dose-Response Relationship, Drug; D000431:Ethanol / Q000097:blood; D005260:Female; D006801:Humans; D008297:Male; D002189:Marijuana Abuse / Q000453:epidemiology* / Q000523:psychology; D008875:Middle Aged; D018570:Risk Assessment; D012307:Risk Factors; D015813:Substance Abuse Detection; D014481:United States / Q000453:epidemiology
D002186:Cannabinoids; D000431:Ethanol