If you’ve spent time with marijuana—any time at all, really—you know that the high can be rather unpredictable. It depends on the strain, its level of THC and hundreds of other compounds, and the interaction between all these elements. Oh, and how much you ate that day. And how you took the cannabis. And the position of the North Star at the moment of ingestion.

OK, maybe not that last one. But as medical and recreational marijuana use spreads across the United States, how on Earth can law enforcement tell if someone they’ve pulled over is too high to be driving, given all these factors? Marijuana is such a confounding drug that scientists and law enforcement are struggling to create an objective standard for marijuana intoxication. (Also, I’ll say this early and only once: For the love of Pete, do not under any circumstances drive stoned.)

Sure, the cops can take you back to the station and draw a blood sample and determine exactly how much THC is in your system. “It's not a problem of accurately measuring it,” says Marilyn Huestis, coauthor of a new review paper in Trends in Molecular Medicine about cannabis intoxication. “We can accurately measure cannabinoids in blood and urine and sweat and oral fluid. It's interpretation that is the more difficult problem.”

You see, different people handle marijuana differently. It depends on your genetics, for one. And how often you consume cannabis, because if you take it enough, you can develop a tolerance to it. A dose of cannabis that may knock amateurs on their butts could have zero effect on seasoned users—patients who use marijuana consistently to treat pain, for instance.

The issue is that THC—what’s thought to be the primary psychoactive compound in marijuana—interacts with the human body in a fundamentally different way than alcohol. “Alcohol is a water-loving, hydrophilic compound,” says Huestis, who sits on the advisory board for Cannabix, a company developing a THC breathalyzer.1 “Whereas THC is a very fat-loving compound. It's a hydrophobic compound. It goes and stays in the tissues.” The molecule can linger for up to a month, while alcohol clears out right quick.

But while THC may hang around in tissues, it starts diminishing in the blood quickly—really quickly. “It's 74 percent in the first 30 minutes, and 90 percent by 1.4 hours,” says Huestis. “And the reason that's important is because in the US, the average time to get blood drawn [after arrest] is between 1.4 and 4 hours.” By the time you get to the station to get your blood taken, there may not be much THC left to find. (THC tends to linger longer in the brain because it’s fatty in there. That’s why the effects of marijuana can last longer than THC is detectable in breath or blood.)

So law enforcement can measure THC, sure enough, but not always immediately. And they’re fully aware that marijuana intoxication is an entirely different beast than drunk driving. “How a drug affects someone might depend on the person, how they used the drug, the type of drug (e.g., for cannabis, you can have varying levels of THC between different products), and how often they use the drug,” California Highway Patrol spokesperson Mike Martis writes in an email to WIRED.

Accordingly, in California, where recreational marijuana just became legal, the CHP relies on other observable measurements of intoxication. If an officer does field sobriety tests like the classic walk-and-turn maneuver, and suspects someone may be under the influence of drugs, they can request a specialist called a drug recognition evaluator. The DRE administers additional field sobriety tests—analyzing the suspect’s eyes and blood pressure to try to figure out what drug may be in play.