Doctors Struggling To Determine How Much Marijuana Impairs Driving Ability

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With marijuana being legalized in more and more states as a result of voters across the country, controlling the use and abuse of the substance has quickly become a priority for lawmakers in the U.S.

This includes, of course, determining how much is too much to be operating heavy machinery. As with alcohol, a legal limit should be slapped on marijuana usage to determine what the legal limit should be for people getting behind the wheel, according to a report by NBC News.

Legislators are concerned that the once-illegal substance could cause havoc and create new public health risks the country hasn’t dealt with before, and are now testing how they can integrate marijuana into society smoothly. Unfortunately, that process is not proving all that easy.

Neuroscientists and pharmacologists seem to be having a hard time pinning down exactly to what extent the drug causes impairment, or to find an appropriate way to measure it.

The reason for this is that while blood tests can show whether or not the drug exists in the body’s systems, marijuana traces stay in the body for approximately a month, and cannot show when it was ingested. 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. “We don’t really have good evidence — even if we know someone has been using — [to gauge] what their level of impairment is.”

California was the first state to legalize medical marijuana, as well as becoming the first state where recreational weed became legal for adults to use back in 2016. Since then, another 10 states have followed suit, including the District of Columbia. On top of that, nearly three dozen other states have already passed legislation that allows marijuana use for medical purposes.

But this has left lawmakers scrambling to find a way in which to measure when marijuana could be impairing a user who wants to drive. For alcohol, there are clear standards and tests, but weed is proving a far trickier substance.

“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.

The use of the drug is known to result in slower reflexes and lapsed concentration while under the influence, which makes it vitally important for researchers to set a clear standard as to its use.

So far, five states have set “per se” laws regarding the limit, which means that people are prohibited from driving if their blood tetrahydrocannabinol (THC) levels are above a certain predetermined level. Some have prohibited driving if any traces of THC are present in the blood.

Other states are simply relying on officers to be able to visibly see whether a driver is impaired because of drug use, but experts worry this is too subjective a method.

Even so, the tests are not proving to be highly accurate, and make it difficult for regular users when the drug stays in the system for weeks on end. Blood tests are also rather impractical, given that blood needs to be analyzed in a laboratory, and urine tests at road blocks are complicated.

In Canada, where recreational weed is also legal, law enforcement will test drivers with a saliva test called the Drager DrugTest 5000, which is also proving to be problematic.

There are some private companies who are attempting to develop a breathalyzer test, similar to the one used for alcohol at roadblocks, to test for the drug. However, they aren’t necessarily proving fruitful.

Jonathan Caulkins, a drug policy researcher at Carnegie Mellon University, said, “There are fundamental issues with the chemistry and pharmacokinetics. It’s really hard to have an objective, easy-to-administer roadside test.”