Both drugs, alcohol and cannabis, cause impairment of driving ability which can result in motor vehicle collisions, But the drugs can act in different ways and what we know about alcohol-related accidents may not apply to those caused by marijuana.

Injured Drivers in BC

A recent study provides a fascinating comparison of injured drivers who tested positive for alcohol, cannabis and other drugs. The blood drug concentrations of 1,097 drivers attending one of seven trauma centres in British Columbia between 2010 and 2012 were determined by GC or LC-MS.

Alcohol was the most frequent drug detected, as expected, and was found in 17.8% of the drivers. A BAC > .08 g/100mL was found in 15.4% of the drivers. Cannabis was the second most common drug detected in injured drivers. It was present in 12.6% of the drivers (including the metabolites of THC). A positive THC (the active drug) concentration was detected in 7.3% of the drivers. A blood THC concentration in excess of 3 ng/mL was found in 3.0%.

Characteristics of Alcohol and Cannabis Injured Drivers

The following Table compares the characteristics and odds ratio of MVCs involving drivers with a BAC > .08 g/100mL and those with w blood THC concentration in excess of 3 ng/mL.

In contrast to what we know about alcohol, injured drivers with a blood THC concentration of > 3 ng/mL were less likely to be male, were younger, more involved in multi-vehicle crashes, that occurred throughout the day (rather than at night for alcohol) and to have MVCs on weekdays rather than weekends.

Yet we still conduct studies on the prevalence of drugs in drivers mainly on weekend evenings, which is the most effective time for measuring alcohol-impaired drivers. But not for cannabis-impaired drivers.

We still have a lot to learn about this new drug and its effect on driving and how to develop effective countermeasures for cannabis.

Conclusion

Alcohol and, to a lesser extent, cannabis are known to cause crashes. Our findings suggest that alcohol continues to be the most important cause of impaired driving followed by cannabis. The number of drivers using cannabis may increase with the growing numbers of medical marijuana dispensaries in BC and with legalization of marijuana in some US states.





YouTube video

For more on the effects of THC and alcohol on driving ability watch my YouTube video at https://youtu.be/cRShEenjW6M

References

Brubacher, J.R., Chan, H., Martz, W., Schreiber, W., Asbridge, M., Eppler, J., Lund, A., Macdonald, S., Drummer, O., Purssell, R., Andolfatto, G., Mann, R., and Brant, R., "Prevalence of Alcohol and Drug Use in Injured British Columbia Drivers."

Wigmore on Alcohol

"Wigmore on Alcohol, Courtroom Alcohol Toxicology for the Medicolegal Professional", Irwin Law Publishing, 559 pp, 2011, available at www.wigmoreonalcohol.com





Wigmore on Cannabis

My newest book, Wigmore on Cannabis The Forensic Toxicology of Marijuana for Lawyers and Other Medicolegal Professionals is also available on my website.