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How much does weed affect your driving? Why is it on the news so much? This is Healthcare triage News.

For those of you who want to read more, here's the report I reference: http://www.nhtsa.gov/staticfiles/nti/pdf/812117-Drug_and_Alcohol_Crash_Risk.pdf

John Green -- Executive Producer
Stan Muller -- Director, Producer
Aaron Carroll -- Writer
Mark Olsen -- Graphics

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How much does weed affect your driving? Why is it on the news so much? This is Healthcare Triage news.

[intro music]

We've done our own video on marijuana, and one of the things I stressed was that compared to alcohol, the stuff is hard to classify as dangerous.

A new study from the National Highway Traffic Safety Administration drives that point home. Hard. They conducted a study where they collected data from drivers both in and not in car crashes over 20 months in Virginia. They matched them on a lot of factors. Then they looked to see how things differed between those who got in crashes and those who didn't.

Lots of accidents happen. People often test positive for various substances when they do. For instance, about 7.6% of drivers in crashes tested positive for marijuana. But so did 6.1% of people who didn't.

The key analysis looked at how different drugs changed the odds of you being in a crash. And the real analysis had to be adjusted. Why? Cuz there are other factors that are known to impact the odds of you being in a crash.

Male drivers are more likely to be in a crash. So are young drivers. They're also more likely to smoke pot. See, you gotta account for that. And when you do, marijuana was not associated with an increased risk of having an accident.

The adjusted odds ratio was 1.05, but it wasn't statistically significant. In fact, none of the drugs tested reached statistical significance. But if you want to look at a chart, here's how they stack up.

Remember, none of these reach statistical significance. But from the point estimate, you may see stories talking about how antidepressants or stimulants are protective, but sedatives and narcotics and marijuana are not, so they're bad. Bad's relative. And we haven't added in alcohol yet.

What if we do the same analysis, but just adjust for alcohol? Cuz people who drive under the influence of certain drugs are also more likely to drive under the influence of alcohol at the same time. We need to tease that out. And then things sorta look different.

There's still no statistical difference. But as you can see here, even the point estimate for marijuana is now zero. No effect of marijuana independent of alcohol. But still, we're ignoring the elephant in the room: alcohol itself. How does it compare?

By the way, that's all people with a breathalyzer result of 0.05 or above, which many places don't even describe as drunk driving. That's usually, what, 0.08? 0.05 is just impaired in most places. And the effect of alcohol is so huge on this chart, you really can't even see the other drugs anymore.

And the more alcohol you have in you, the more you're impaired. At a breathalyzer of 0.05, your odds are twice that of sober drivers of getting in a crash. At 0.10, your odds are about five and a half that of sober drivers. At 0.15, it's twelve times that of sober drivers. And once your blood alcohol hits 0.2 or more, your odds of having an accident are more than twenty-three times higher.

That's 2,300% on the chart I just showed you. Compared to negligible for most other drugs. But we have to acknowledge that our tests for pot are nowhere near as sophisticated as that for alcohol. We could be measuring use that's not acute, maybe in the past. We don't know. Lots of THC positive people might not have been impaired. Better tests are needed.

And please hear me here: I'm not advocating that you use pot and drive. Far, far from it. Do not drive while impaired. Do not do it! That's not the take-home message from this video. Be safe.

But this study showed, regardless of all the news stories you hear about the dangers of pot in driving, after adjusting for demographic factors and alcohol use, the impact of most drug use--including pot--is very tiny with respect to accidents. The effect of alcohol, though? Massive. Don't drink and drive.

[outro music]