Is driving high as dangerous as driving drunk?

Q: I know all about the dangers of driving while drunk. But is driving while high on marijuana as dangerous?

A: Any form of driving while intoxicated is obviously a bad idea, but smoking a joint or taking an edible before getting behind the wheel can pose distinctive risks. That, experts say, is because of the particular ways that marijuana affects the brain and the fact that there’s no standard dose for a federally criminalized drug.

With alcohol, there are universally accepted quantities for what constitutes a single drink: 12 ounces of regular beer, 5 ounces of wine and 1.5 ounces of distilled spirits. There’s no agreed-upon equivalent for cannabis, said Marlene Lira, a senior research manager at the Clinical Addiction Research and Education Unit at Boston Medical Center. This makes it tricky for consumers to measure how much they’ve actually consumed.

During the past decade, car accidents involving cannabis have been rising, and recreational use of the drug continues to climb. A recent analysis of U.S. public safety data showed that from 2000 to 2018, the percentage of motor vehicle fatalities involving cannabis more than doubled from 9% to about 22%. By contrast, the percentage of fatalities involving alcohol stayed roughly the same.

While there’s a dearth of data directly comparing the hazards of driving while high versus driving while drunk, the research that does exist suggests that marijuana may be less likely than alcohol to lead to deadly car crashes. In one 2017 examination of more than 4,000 drivers from a police database in France, researchers found that drivers under the influence of alcohol were roughly 17.8 times more likely to be responsible for fatal car crashes than drivers who were sober, while drivers under the influence of marijuana were 1.65 times more likely to cause deadly accidents.

Studies like these often rely on drivers who were drug tested after car crashes. But because cannabis can hang around in fatty tissues for up to 28 days, a positive drug test does not necessarily mean that a driver was high at the moment they were driving.

It’s also tricky for marijuana users to predict how exactly they’ll be impaired and for how long. If you smoke a joint or take a bong hit, you’ll feel high within minutes and then return to a base level after three or four hours, Dr. Godfrey Pearlson, a neuroscience professor at Yale University who has researched marijuana and car safety, said. If you opt for a pot brownie or a gummy, it takes longer for the high to kick in, since the marijuana in edibles has to be absorbed into your gut and be metabolized through your liver.

“If someone takes an edible, the classic scenario is they say, ‘Hey, I’m not feeling this,’ and then 40 minutes goes by, and they get in a car and start driving,” said Dr. Collin Reiff, a psychiatrist at NYU Langone Health.

THC, the intoxicating substance in cannabis, impairs a number of behaviors related to driving, including the ability to weave between lanes. It also delays response times in general, Lira said.

The psychoactive effects of cannabis can also make driving dangerous. The paranoia that some people experience while high could induce a panic attack while on the road, Reiff said. Weed also can create an altered sense of time, the sludgy sense that everything around you is moving slower than it actually is. “There’s that joke: Why did you get pulled over?” he said. “If you’re speeding, it’s because of alcohol. If you’re going too slow, it’s because of cannabis.”

This article originally appeared in The New York Times.