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With the official legalization of cannabis in New Mexico, fears of drugged drivers taking to the streets became palpable overnight. But questions remain about how impaired cannabis users actually are and what role marijuana tolerance should play in the conversation. State law enforcement officials seem to be keeping a cool head and appear to be approaching the problem with their eyes firmly on the science.

The state Courts, Corrections and Justice Committee heard from Department of Public Safety (DPS) officials at a recent hearing concerning the safety of New Mexico’s roads following the legalization of adult-use cannabis.

Acting Deputy Secretary of DPS Troy Weisler told lawmakers that cannabis DWI could be on the rise and that the department was concerned. The problem is that experts have yet to agree on standards for judging impairment levels in cannabis users, and lab tests for THC levels aren’t as useful as they seem. The department is reportedly looking to hire consultants to help create those standards.

In the meantime researchers are already trying to answer some basic questions about cannabis tolerance.

Tolerating Pot

Assistant Professor of Economics at UNM Sarah Stith says she and her colleagues are in the beginning stages of examining the links between level of cannabis use and level of effect. She says it’s too early to determine exact correlations, but we do know one thing. “There definitely is tolerance,” she says.

The researchers will be using the ReLeaf app to examine data from cannabis users, including dosage per session, level of experience and preferred levels of THC. “There’s a central story here,” says Stith. “It seems that THC makes everything more intense, so at baseline, you’re going to get more symptom relief with higher THC, but you’re also looking at more negative side effects. What seems to be happening with tolerance—although it’s certainly not peer reviewed or published yet—is that it lessens both of those effects.”

For cannabis users with a higher tolerance for the drug, all of its effects—positive or negative—are dulled. This might explain the noted phenomenon of climbing THC levels found in the cannabis being sold legally in the U.S. As frequent users begin to notice a lessening of the effects that they are pursuing, they begin looking for higher and higher concentrations of THC, pushing the industry to produce more potent products. If this consistent rise in potency is correlated to rising tolerance among users, then the perceived dangers surrounding the issue are likely worse than the reality.

What Causes Tolerance?

The lack of available data on cannabis tolerance means that scientists don’t actually know what causes it to build in some individuals. Increased tolerance to drugs has been observed in humans interacting with a number of compounds, including alcohol and opioids. In most cases, when a person uses a drug often enough, the metabolism learns to process the drug more efficiently. This leads to a lessening of effects and the need to use higher doses.

While the mechanism behind cannabis tolerance is unclear, it doesn’t seem like a long shot to assume that it’s similar to other forms of drug tolerance and is linked to frequency of use. If that’s the case, questions will still remain about whether tolerance is also affected by the pace at which higher levels of THC are introduced. The tolerance of other cannabinoids will have to be examined as well.

Once we have a better grasp on tolerance, better treatment plans will become available to medical cannabis patients. But it will also have the added benefit of helping to identify marijuana-impaired drivers.

Tolerance and Driving

Currently, the most commonly used method for detecting cannabis impairment is a drug test. The only problem is that THC can stay detectable in an individual’s system for months, depending on their level of use. Relying on traditional testing methods, officers involved in a suspected impairment case might identify a driver as impaired, even if they haven’t consumed pot in months.

And as we’ve seen, a user’s cannabis tolerance will play a huge role in how they respond to the drug when it comes to experiencing both negative and positive effects. Presumably, this will apply to levels of intoxication as well. “The experienced user at a high THC level or even after having just consumed is going to look very different, from an impairment perspective, than someone who is a novice user,” says Stith.

The answer appears to be the employment of comprehensive field sobriety examinations and educating officers about the signs of cannabis impairment rather than cannabis consumption. “Keeping people safe on the streets and making sure that there are no impaired drivers is priority,” says Stith. “The conclusion I’ve come to from what I know about the research is that the best option is field sobriety tests—a functional test rather than some sort of lab value.”

NM State Police Ahead of the Game

Last month the state gave DPS $750,000 to fund the Drug Recognition Expert certification program as state police revise their marijuana policies to reflect legalization laws. State police are reportedly being instructed that the smell of cannabis should no longer be considered a sign of illegal activity. To determine if a driver is impaired, officers are being trained to watch for erratic driving, the presence of bloodshot eyes and impaired speech as well as the smell of cannabis smoke. Drivers suspected of being impaired by drugs or alcohol are to be subjected to a field sobriety test.

State police Capt. Micah Doering signaled to reporters that law enforcement is aware of the tolerance conundrum and acting accordingly. “There are those who use regularly and are far less impaired, or not impaired, with cannabis in their system,” he said. “We need to treat people individually based on their personal impairment level, not just a number.”

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