under the influence

THC “breathalyzers” are getting better, but will they do more harm than good?

The highs and lows of THC “breathalyzers.”

Originally Published: 
marijuana leaf
Seth McConnell/The Denver Post via Getty Images

It’s illegal to drive under the influence of marijuana, but that influence is hard to prove.

If you are and you’re pulled over and asked to pass a breathalyzer test, nothing will be detected.

But in the near future, an officer could try another test: This time for THC, the psychoactive cannabinoid being metabolized in your body at that moment.

Researchers are racing to create the first mainstream THC “breathalyzer” and there’s a number of devices in various stages of development. Most recently, scientists reported the creation of a new rapid THC detector called EPOCH. They described its sensitivity and accuracy in the study published this October in the journal Science Translational Medicine.

The application of this device, however, is debated. While advocates argue it can make roads safer for everyone, critics note that cannabis can be detected in a person’s body after the high is gone. This could lead to unwarranted citations and arrests.

A woman smokes marijuana before the annual Hemp Parade in Berlin.

Sean Gallup/Getty Images

For a white person, this offense may just be an inconvenience or a slap on the wrist. But for Black people, who are already four times more likely to be arrested for possession despite similar rates of use to their white counterparts, such interactions could be much more serious and even potentially deadly.

The creators behind this new THC detector argue it has uses far beyond detecting who is driving stoned (though the problems associated with roadside testing could follow the device to these other applications).

“We envision that our device will be beneficial for on-site testing of THC use,” Hojeong Yu, a post-doctoral fellow at Harvard Medical School, and Hakho Lee, associate professor of radiology at Harvard Medical School, tell Inverse by email.

“This is not only for roadside testing but more so for testing at workplaces and public areas.”

As of this summer, weed is legal for medical and recreational use in 18 states (and D.C.).

Volodymyr Bondarenko / EyeEm/EyeEm/Getty Images

While cannabis legalization is rapidly spreading across the U.S., not all states have legalized or even decriminalized the drug. Such a rapid test may enable law enforcement to potentially persecute cannabis users even when they’re not directly in possession of the drug or even necessarily detrimentally impaired.

Paul Armentano, deputy director of nonprofit advocacy group NORML, tells Inverse technology like this could be misleading.

“Such tools may possess value in identifying those who have had relatively recent exposure to cannabis,” Armentano says. “But I’ve cautioned that such detection technology is unable to determine whether someone is under the influence from cannabis and, therefore, a positive test result should not be perceived as prima facie evidence of impairment.”

How does it work? — Tests to measure THC levels already exist but they’re often slow and inaccurate. For example, the kind of drug test you might be required to take for a new job today typically works by analyzing the content of a urine sample at a lab.

“[These are] generally performed in specialized laboratories and could take days to process,” Yu, Lee, and colleagues write in the study. “Test results could also be ambiguous in determining the last time of cannabis consumption [because] residual THC and its metabolites are often present in bodies weeks after cannabis intake, and it is challenging to separate acute recent from chronic use.”

This new test, called EPOCH (Express Probe for On-site Cannabis Inhalation) instead works by collecting and concentrating your saliva to evaluate it for current levels of THC. It evaluates whether or not THC levels are above one nanogram of THC per milliliter of saliva within a twelve-hour consumption window.

The EPOCH system has 3 modules for the on-site THC test, including a sample processing kit, a sensor cartridge, and a detection cradle to detect optical signals.

H. Yu, et al., Science Translational Medicine (2021)

To test their device, the researchers recruited 43 cannabis users (who self-administered either via smoking or consuming gummies for the trials) and 43 control participants. During different consumption intervals, the users spit into EPOCH’s collection tool and waited while it concentrated their saliva for evaluation.

Throughout their trials, the researchers report that their device was able to not only accurately measure the participants’ THC levels but did so without any false positives. However, Lee and Yu do acknowledge that participant size in these trials should be expanded in the future to determine how accurate those results are.

Yu and Lee are confident that these tests will play a crucial role in regulation in the future, even as cannabis use becomes increasingly legalized.

“Repealing prohibition did not make the need for alcohol tests go away,” Yu and Lee say. “The same can be applied to the increased access to cannabis. Luckily, we have a quick test for alcohol, but no such test exists for cannabis.”

What’s next — This tech is not yet ready to hit the streets — or the office lounge or playground — in the near future. In addition to expanding their sample size, Yu and Lee say that there also needs to be more work done to establish the “baseline difference between frequent and occasional marijuana users.”

In the meantime, however, Armentano suggests behavioral tests might benefit society more than THC tests, at least when it comes to identifying who is driving impaired.

“NORML has long advocated moving away from drug detection technology — which only has the ability to determine whether one has had past exposure to a controlled substance,” Armentano says. “[NORML] instead suggested the more widespread use of performance-based technologies, like DRUID.”

Driving under the influence of marijuana can be deadly but a path for policing this behavior is murky.

Carlos Osorio/Toronto Star/Getty Images

DRUID, which stands for “driving under the influence of drugs,” is an app that works similarly to the kind of roadside balance tests that are used to evaluate drunk drivers.

By reacting to different game-like stimuli from DRUID, the app determines if a user has impaired response time, coordination, or balance — signs of impairment that could be deadly when driving or operating heavy machinery.

Unlike drug detection technology, Armentano says apps like DRUID are better at determining true impairment and not just whether or not someone has smoked recently.

Ultimately, both efforts — THC detection and behavioral tests — are designed to mitigate risks. But the exact parameters of these risks are also difficult to identify: While studies suggest cannabis use may lead to unsafe driving, how this actually translates to the number of actual accidents has been difficult to determine (a 2017 study, for its part, did not find fatal collisions have risen in states where recreational marijuana use has been made legal).

It may be a chicken-and-egg situation, depending on who you’re talking to. The CDC, for example, states in a 2017 fact sheet that “it is unclear whether marijuana use actually increases the risk of car crashes” possibly because “an accurate roadside test for drug levels in the body doesn’t exist.”

This article was originally published on

Related Tags