Marijuana’s Not-So Scientific Legal Limit

States with legalized marijuana are struggling to figure out how much marijuana is too much for drivers.

While Massachusetts has legalized medical marijuana, and is on the cusp of complete legalization next year, the state’s legislature and law enforcement officials are grappling with this problem as well. For alcohol, states have settled on a legal blood alcohol limit ranging from .05 to .08 – roughly the difference between two and three drinks for a 140-lb. person.

But there is no clear range for marijuana – there hasn’t been enough research on the drug’s effects to settle on a number, its concentrations vary widely, and everyone responds differently to THC, marijuana’s psychoactive component.

“There have been people who I have seen with 20 or 21 nanograms of THC where you can’t see any impairment, and people with two nanograms who clearly should not be driving,” said Sergeant Don Decker, the Massachusetts State Coordinator for Drug Recognition Experts, a group of law enforcement officers trained to recognize drug impaired driving and evaluate the driver.

Despite these challenges, and a lack of knowledge, marijuana driving laws—so called “drugged driving laws”—have followed the drug’s legalization in Colorado, Washington, Alaska, Oregon and Washington, D.C.

“In many ways, unfortunately, policy has outpaced science and laws are passed before we really know the impact of certain things,” said Dr. Staci Gruber, the Director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital. “We’re trying a little bit to play catch up at this point and I think it’s important to do because [marijuana]’s not going anywhere.”

Marijuana affects each person differently depending on frequency of use, the strain of marijuana and delivery method. Users slowly build up a tolerance to the drug. An experienced user could be fine with a THC concentration in their blood that would impair a non-frequent user.

Genetics also play a role in how two different people are affected by the same level of THC.

And this is a problem for medical marijuana users. Those legal users can’t always get the same product because of stringent laws about growing and distributing marijuana. The strain and potency can vary each time they go to purchase their supply. Gruber said, the state is “still in its infancy with regard to medical marijuana.” She compares this to buying a bottle of Advil – each bottle is going to contain pills of comparable dose. But for medical marijuana, “it’s going to be different wherever you get it, perhaps even by batch.”

Dr. Herbert Hill, a chemistry professor at Washington State University, is developing an in-field breathalyzer for law enforcement to test THC levels of suspected drugged drivers; but he said it is “not ready for the field yet, and still has at least a year to go before the police will be able to test it.”

Law enforcement officers are also using a behavioral test to evaluate drivers, and states like Massachusetts and Colorado are increasing the number of officers who are qualified to assess marijuana impairment in the field. In programs like Decker’s the Drug Recognition Experts (DRE), officers get weeks of specialized training to be able to determine if a subject is under the influence of drugs and what kind of drugs by observing and testing the subject in the field. The DRE test is 12 steps. The officers first administer a breath alcohol test to see if the impaired driver is drunk. If the blood alcohol level doesn’t suggest drunk driving, then the DRE will perform the 12 step test. This includes an in-depth interview, eye examination, the One Leg Stand test, the Finger-to-Nose test, examination of the subject for injection sites, their muscle tone and multiple pulse rates taken throughout the process. Currently there at 108 such experts in MA, and program has just graduated about 30 moreDecker is actively working to increase this number to prepare for the possibility that Massachusetts voters will legalize recreational marijuana next fall.

Bill O’Leary, a Highway Safety Specialist at the National Highway Traffic Safety Administration, said that the problem any state faces when legalizing marijuana can be compared to seatbelt laws. Once marijuana becomes legal, people who said no to it before, just so they didn’t break the, law will now smoke it. Then they will potentially get behind the wheel of a car not understanding when the drug will kick in as well as recurring users.

“I think people will begin to think more clearly about what we mean by ‘impairment,’” said Gruber. “Is it just [this limit] and if you have that, do we just yank you out of your car? I think much more will depend on sobriety testing.” And she said that Massachusetts will turn to states like Colorado and Washington to see what lessons they have learned, “before making any grand decisions.”

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