By Shefali Luthra, Kaiser Health News, California Healthline

It used to be the stuff of stoner comedies and “Just Say No” campaigns. Today, marijuana is becoming mainstream as voters across the country approve ballot questions for legalization or medical use.

In response, state governments are testing ways to ensure that the integration of this once-illicit substance into everyday life doesn’t create new public health risks. These efforts are sparking a difficult question: At what point is someone too high to get behind the wheel?

The answer is complicated. Brain scientists and pharmacologists don’t know how to measure if and to what extent marijuana causes impairment.

The reason: Existing blood and urine tests can detect marijuana use, but, because traces of the drug stay in the human body for a long time, those tests can’t specify whether the use occurred earlier that day or that month. They also don’t indicate the level at which a driver would be considered “under the influence.”

“It’s a really hard problem,” said Keith Humphreys, a psychiatry professor and drug policy expert at Stanford University in California, the first state to legalize medical marijuana and where recreational pot use among adults became legal in 2016. “We don’t really have good evidence — even if we know someone has been using — [to gauge] what their level of impairment is.”

Marijuana is now legal for recreational use in 10 states and the District of Columbia — including Michigan, where a ballot initiative passed in November took effect Dec. 6. In New York, the governor said Dec. 17 that legalization would be a top priority for 2019. And nearly three dozen states have cleared the use of medical cannabis.

For alcohol, there is a clear, national standard. If your blood alcohol content (BAC) is 0.08 percent or higher, you’re considered cognitively impaired at a level that is unsafe to drive. Extensive research supports this determination, and the clarity makes enforcement of drunken driving laws easier.

Setting a marijuana-related impairment level is a much murkier proposition. But states that have legalized pot have to figure it out, experts said.

“You can’t legalize a substance and not have a coherent policy for controlling driving under the influence of that substance,” said Steven Davenport, an assistant policy researcher at the nonprofit Rand Corp., who specializes in marijuana research.

Marijuana, after all, weakens a driver’s ability to maintain focus, and it slows reflexes. But regulators are “playing catch-up,” suggested Thomas Marcotte, a psychiatry professor at the University of California-San Diego and one of a number of academics around the country who is researching driving while high.

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