Marijuana is well on its way to joining America’s best-selling drug, alcohol. Booze has been legal for adults in most parts of the country since 1933; pot is now legal for recreational or medicinal use in 20 states.
Both products make people feel good, both can be abused, both can make it dangerous to drive. But there’s no doubt in 50 states about when someone’s legally drunk. When they have a blood alcohol concentration of .08, they’re considered too impaired to drive.
There’s no such bright line for marijuana impairment.
Zero tolerance for pot has been the norm for decades for workplace drug testing, and, in most states, for policing drugged driving.
But with millions of Americans now legally able to use pot for either medical purposes or outright, there’s growing demand to know how much is too much to safely drive or perform on the job.
Scientists generally admit they don’t know the answer, in part because studies have been limited, but also because marijuana and the ways people use it have changed faster than the pace of research.
Pot today is many times stronger than it was in the 60s, the active ingredient (delta-9-tetrahydrocannabinol) present at two to four times greater levels, sometimes more. And people use the drug differently, not just smoking it or baking it into brownies, but through an array of foods, drinks and concentrates. Medical users generally tolerate a much higher amount of THC than occasional tokers, but metabolism varies as do levels of body fat – the tissue where breakdown products from pot linger longest.
Marijuana users struggle to do multiple tasks at once and to maintain concentration, says Marilyn Huestis, a senior investigator and toxicologist at the National Institute on Drug Abuse. “You have divided attention when doing the complex tasks required to drive,’’ Huestis said.
In both Colorado and Washington, where recreational use and possession of pot have been legal since late 2012, DUID arrests have increased.
Denver attorney Sean McAllister says he’s seeing more cases of drugged driving than at any time in his 15 years of practice. “It seems like I get a call (about a DUID) every day, when a few years ago it was a few each month.”
The Colorado State Patrol only started tracking marijuana use among drugged driving citations in January. That month, about half of the 61 drugged driving arrests involved people who had used pot by itself or with other drugs; in February, the percentage had jumped to 74.
At the Arapahoe House treatment centers in Colorado, the share of patients held on DUI charges who list marijuana as their drug of choice is up, according to director Art Schut. From late August to late January, 10 percent fit that category compared to 8.2 percent during the same period the previous year. “This is the main impact we’ve seen since legalization,” said Schut.
In Washington, the State Patrol recorded 1,362 drugged driving arrests in which drivers tested positive for marijuana last year, 25 percent greater than in 2012.
Washington and Colorado now use a threshold of 5 nanograms of THC per liter of blood, while almost half the states count any amount of THC as evidence of impaired driving.
“A level that might be accurate for a newbie user is probably meaningless for a chronic user, and this is particularly unfair for medical marijuana users,’’ said McAllister, who was a leader in the 2012 campaign for legalization in Colorado.
Levels of active THC in the bloodstream tend to drop in a few hours after someone smokes pot, but peak at around 4 hours when people eat marijuana. Low levels may persist in the blood for up to 24 hours, but the scales may be different for people who use pot every day.
Huestis says “we know that people are impaired at lower levels than 5, but the balancing act is finding a number that can reliably separate impaired from non-impaired, which is almost impossible to do.” She’s been working with tests for several years trying to establish dose levels relevant to driving performance and has helped to develop a saliva/breath test that could be used to identify recent pot use. But development of precision tests still has a way to go.
Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws, said the focus of drug testing has always been on finding evidence someone’s used the substance “rather than determining whether someone is under the influence.”
With culture and laws changing, “there clearly is a need for new approaches to determine impairment,’’ he added.
NORML has a “strong position” that responsible marijuana consumers should not consume and drive, Armentano said. The organization recently helped a software developer design a phone app that lets people do a self-check of their mental and physical skills related to driving and store the information to later test themselves for possible impairment after using drugs.
“We stepped in to support this because it didn’t seem anyone else was trying to address this problem,’’ he said.