Cannabis and driving

After many senseless deaths and serious injuries as a result of alcohol impaired driving, Canada has implemented strict laws prohibiting driving under the influence of even a relatively small amount of alcohol.

In BC, it is well publicized that a blood alcohol content of even 0.05 could result in the loss of your license as well as some pretty stiff fines. Hardly anyone protests these laws because they are in place to protect the public from a real danger. Since the penalties have been implemented, alcohol related crashes have dropped significantly.

Although not as common, drug impaired driving has also gained attention. In many places, drugged driving laws are being implemented. Although some of these enforce a zero tolerance policy, others are setting legal limits as in the case of alcohol.

Marijuana is by far the most widely used illicit drug around the world. It is also associated with vehicle crashes. According to US data, cannabis users have a 10-fold increase in car crash injury after adjusting for blood alcohol concentration. However, until recently, it was unknown how long cannabis remains in the blood stream or causes impairment, making it difficult to set any kind of reasonable limit in driving laws.

A recently published study examined 30 male chronic daily cannabis smokers over a month of supervised abstinence to determine how long the active chemical from the drug remained in their blood (THC).

Of the 30 participants, 27 were positive when the study began. The chemical decreased gradually over the course of the study - 95 percent and then 85 percent were still positive on days eight and 22 respectively. After 33 days, one person still had detectable THC in his blood.

We know that acutely intoxicated cannabis smokers exhibit significant impairment in cognitive, perceptual and psychomotor tasks including the areas of attention, complex decision-making and reaction time. Some studies have also shown cognitive impairment that can last anywhere from seven to 28 days after use of the drug.

This study is the first to actually quantify the persistence of the chemical in the bloodstream and its findings are consistent with studies on the potential for lasting impairment. It was a relatively small study of chronic users and all participants were male, so more research will likely need to be done to see how the results compare in different populations.

Determining an acceptable blood limit for THC and driving is a more complex task than with alcohol. However, given the strong association between cannabis use and vehicle accidents, it is important to get a clear picture of the course and persistence of impairment so the laws can be set accordingly.


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