How Much Does Marijuana Impair Driving?

F. Perry Wilson, MD, MSCE


December 01, 2020

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This transcript has been edited for clarity.

Welcome to Impact Factor, your weekly dose of commentary on a new medical study. I'm Dr F. Perry Wilson from the Yale School of Medicine.

This week, we consider a question that has plagued humankind for ages: Why am I spending my time running clinical trials of digital health technologies when I could be smoking people up in Denmark and setting them on the highway to see how straight they drive?

I have been beaten to the punch, unfortunately, by this study, which is really an elegant approach to quantifying how much driving is impaired by marijuana.

The thing that makes marijuana tricky to study — aside from the fact that the Controlled Substances Act still ridiculously lists it as a Schedule I agent — is that it [doesn't contain] just one thing.

There are two major psychoactive chemicals in cannabis: tetrahydrocannabinol (THC) and cannabidiol (CBD). You'll know CBD from that store that opened up on your drive to work, with a clever name like "The High Life" or "Planet of the Vapes."

To test how these chemicals affect driving, researchers from the Netherlands conducted a within-participant crossover trial. What that means is that each individual (26 total were enrolled) experienced all four of the following experimental scenarios, administered via vape pen:

  • Placebo cannabis

  • 13.75 mg THC

  • 13.75 mg CBD

  • A 13.75-mg mixture of 50% THC and 50% CBD

That last one is to examine some claims that the effects of CBD may somehow decrease the effects of THC.

Now, to test how well people drive under the influence, you could put them into some kind of cool video game simulation. But not these researchers, no; they conducted a real-world driving test: 100 km on the roads around Maastricht. There was a driving instructor in the car with a separate set of controls if things got dicey.

The primary outcome? Standard deviation of lateral position.

Verster JC, Roth T. Int J Gen Med. 2011;4:359-71. doi: 10.2147/IJGM.S19639. Epub 2011 May 17.

Basically, a measure of how much you swerve while driving. A camera mounted on top of the car measured how far from the lane line the individual was, four times every second. Most people stay about 50 cm away from the lane line, but that varies a bit depending on how swervy you are. At a blood alcohol content of 0.05%, just under the federal limit, swerviness increases by about 2.4 cm. Remember that now.

Okay, on to the results. When individuals vaped the THC, they were more swervy than when they vaped placebo. CBD didn't have much of an effect, and the mix of THC and CBD looked pretty much like THC. After the first 100 minutes of driving, the scores all equaled out.

Arkell TR, et al. JAMA. 2020;324:2177-2186. doi:10.1001/jama.2020.21218

By and large, people knew they were impaired; scores on a self-assessment of driving ability tended to parallel the swerviness results.

Arkell TR, et al. JAMA. 2020;324:2177-2186. doi:10.1001/jama.2020.21218

There's one thing we can conclude from this study — and a bunch of things we definitely can't.

The thing we can conclude is that THC leads to levels of driving impairment that are similar to what we see with alcohol intoxication.

But we can't conclude that the impairment goes away after 100 minutes or that CBD won't have similar effects, because the doses used in this study (13.75 mg) are pretty small — which, of course, they would have to be because these people were driving on real roads.

A typical marijuana cigarette — what the kids call a "joint" — contains about 10 times the dose of THC given in this study. I can buy gummy bears on Amazon with twice the CBD dose this study provided.

Which is all to say that we are a long way from saying that CBD is safe, or that the negative effects of THC wear off quickly.

To be fair, how swervy you drive is not the only measure of dangerous driving. People under the influence of alcohol may make other bad choices while driving — speeding, dangerous lane changes, etc. — that people under the influence of THC may not. In fact, speed and variance of speed were no different in the groups in this trial.

Nevertheless, for now, DUI is still DUI, regardless of which influence you are under. If you live in one of the several states that have now legalized recreational marijuana, and if you partake, stay home with some snacks and a rug that really ties the room together.

F. Perry Wilson, MD, MSCE, is an associate professor of medicine and director of Yale's Clinical and Translational Research Accelerator. His science communication work can be found in the Huffington Post, on NPR, and here on Medscape. He tweets @fperrywilson and hosts a repository of his communication work at

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