Does Cannabis impair driving like alcohol does?

Cannabis produces little or no car-handling impairment in driving studies. This is consistently less than moderate doses of alcohol and many legal medications. However, potential risks could be amplified by mixing cannabis and alcohol together. Surveys have shown that when THC is detected in the blood of fatally injured drivers, alcohol is often detected too.

Ethanol, in the form of beverage alcohol, is extremely water-soluble and easily diluted in aqueous solutions such as cocktails. Once imbibed, alcohol distributes to all physiological compartments quickly and evenly in predictable ways, since the human body is mostly composed of water. Biological samples from blood or breath (which contains high amounts of water) reflect the amount of alcohol imbibed and the amount present in the brain, which in turn reflects current levels of intoxication and impairment. 

Rather than being hydrophilic (affinity for water) like alcohol, THC is extremely lipophilic (affinity for lipids or fats). It distributes quickly into organs with higher blood supplies including the brain, heart and liver, moving later into body areas with less perfusion. Due to its fat solubility, it leaches into and persists in body regions with high fat content, including the brain and adipose (fat) tissues.

With chronic use, significant accumulation in these tissues can occur with gradual release, even if cannabis is not used for a period of time. This release and redistribution can lead to its subsequent metabolism and detection in bio-samples including urine days to weeks following last cannabis use. 

It is never a good idea to get behind the wheel, even if affected by cannabis alone. For some people, cannabis may play a role in their bad driving. However, the overall rate of accidents does not appear to be significantly affected by use, and people who frequently use cannabis appear to develop a tolerance to its impairing effects.

Harm prevention measures that exist for alcohol consumption, safe use guidelines, formal intoxication levels, designated driver education and information about how alcohol impacts physical and mental functioning are not widely available for cannabis as a result of its criminalisation.

With medical cannabis more readily available, the Lambert Initiative for Cannabinoid Therapeutics‘ research is contributing to the ongoing policy debate regarding issues such as safety, impairment and detection when driving across Australia.

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