Science Acknowledges Marijuana Impairment is Extremely Complex


I’ve frequently argued that there’s no scientific correlation between blood THC concentration and a person’s ability to drive without being impaired. Often, opponents will respond with a bevy of medical articles suggesting the complete opposite, while supporters will do their own research and respond in kind with contradictory literature. 

It’s really a question of who is right. Who is believable. Why are studies contradictory? Since well before marijuana was widely legalized in the United States and certainly before Canada announced its intentions to legalize it, scientists the world over have been busy at work. They were trying to determine, what, if any, correlation having marijuana in the system had to traffic incidents, regardless of whether these incidents caused injury or death. 


So I was pleasantly surprised when I saw a meta-analysis completed by researchers in Romania published on Feb. 12, 2018. These researchers really took it to the next level by first identifying 1,878 documents across the world to find the best among them for analysis. 

They sought out studies that in their view was high quality, removing irrelevant items such as case-reports, articles that were too old, studies that didn’t include enough data, and so on. 

Eventually, these researchers identified 24 articles for their analysis, among them five research studies made in Canada. The rest were from the United States, New Zealand, Brazil, Norway, Australia, the Netherlands, France, Spain, Thailand and Switzerland. So, truly, a sample of the world’s latest research on the topic of marijuana’s impact on traffic safety. 

Many of these studies, as expected, conflicted with one another and had different research methodology. But still, some of the things they found weren’t entirely surprising. 

​For example, it’s simply difficult to have a good measure of how and if cannabis impairs. A few studies acknowledged this directly. One study suggested that THC in blood decreases quickly after use, but can still be clinically detected for a much longer period, as I have often suggested. Additionally, the same amount of cannabis can affect one person differently from another. One person may completely be impaired while another displays a normal driving ability.

Some studies had problems such as relying on drivers who self-reported, or admitted to consuming cannabis, which infers that the sample is not really that representative of the general population. Other groups didn’t taken into account the fact that men and youth use cannabis more frequently. Still others didn’t account for the effect time of day had on traffic incidents – Friday nights, as one example, would be when traffic incidents occur more frequently and would affect the accuracy of incident results. 

Where studies used geography as a measure, that introduced another weakness. Simply speaking, traffic incidents occur more in urban areas compared to rural areas, and it’s very difficult to account for every factor to come up with a truly accurate result. 

Some studies even sought to eliminate all the variables, or confounders, by adjusting results. When they did that, they found that cannabis had much less of an impact on driving than the results would have previously suggested. In one case, once these “confounders” were accounted for, the odds ratio of marijuana use and traffic incidents decreased to 0.8 from 11.4.

That’s a huge difference caused by things like time of day, age and sex of drivers, geography, and all the other factors that can possibly affect how likely someone is to get into a traffic collision.

The studies simply are not thorough enough. Not factual enough. There are too many variables that none of the studies completely account for to provide a definitive conclusion. At least according to these researchers from Romania, if we want to solve this question, what we really need is a study with such a massive scale that it’s corroborated with measurable data about how much cannabis was used, or perhaps a clinical assessment of every driver, prior to establishing whether someone is fit to drive.

Marijuana impairment is a complex topic. We know at some point, marijuana may impair the ability to drive. But the law should be based on science. And when the science itself is so contradictory and so unsure of itself, we cannot have definitive laws that pretend there is some baseline where, when reached, would mean a driver is too impaired to operate a vehicle. 

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