Can a Breathalyzer Detect Weed?

Sophia Delphi May 12, 2022 - 8 min read
Fact Checked
Image of a Breathalyzer That Can Detect Weed

It doesn’t matter how well a drinker can “hold their alcohol” if the cops pull them over. Once the breathalyzer comes out, the game is up.

But smokers who can “hold their weed” have had a free pass for years if they’re stopped for suspicion of DUI. (And yes, in most states driving under the influence of marijuana carries the same penalties as drunk driving.)

Breathalyzers are designed to measure the percentage of alcohol that’s in a driver’s breath when they exhale; the machines then use that information to estimate how much alcohol is in the driver’s blood.

Psychoactive THC, of course, is a completely different substance. So unless the driver has also been drinking while toking up, a breathalyzer reading will be useless for law enforcement officers who suspect someone has been “driving while high.”

Driving when stoned can be dangerous, though, and police will make an arrest if they can prove that someone is under the influence. Right now, they have to use other methods to check for marijuana use – but it looks like that will change in the very near future.

Soon, a breathalyzer may be able to detect weed.

What Happens During A DUI Stop

When police think someone may be driving under the influence and pull them over, they first look for obvious signs of intoxication, drinking, or drug use. Needless to say, the smell of alcohol or weed, an open beer can, or a weed pipe on the passenger’s seat isn’t helpful to the driver. Neither are bloodshot eyes, slurred speech, or unnatural behavior.

The cops will probably ask the driver to step out of the car, ask questions about recent alcohol or drug use, and administer standard field sobriety tests like unusual eye movements, walking a straight line, or standing on one leg. Unless they’re convinced the driver is sober, that’s when the breathalyzer comes out. (If you’re ever in this situation, remember that you have the right to refuse testing – but you’ll be arrested and could face serious penalties including loss of your license.)

What happens if they suspect the person is driving while high? That depends on the state.

In California, for example, Highway Patrol officers do a saliva test by swabbing the driver’s cheek; they claim the test is more than 90% accurate for weed use within the past few hours. However, it doesn’t measure how much THC is in the driver’s system – so the swab test still doesn’t provide conclusive evidence.

In other states, a “drug recognition specialist” may be called to the scene to do further physical and cognitive testing. More often, the suspect will simply be taken into custody under suspicion of DUI, and a blood or urine test will be performed at the police station. Of course, drug charges are also a possibility in non-legal states.

Here’s the problem for police, though. Urine tests will show whether their suspect has used weed in the last few days or weeks, and blood tests will show if they’ve used it in the last few days. But those tests won’t show when the driver smoked up, and whether they were impaired while driving.

That’s understandably increased law enforcement’s desire for a breathalyzer that can detect cannabis intoxication in real-time. And a few of those machines are currently being developed.

Cannabis Breathalyzers Are On The Horizon

Several different models are in the works.

Hound Labs

This company has worked with scientists at the University of California, San Francisco to develop a machine that is supposed to hit the market in 2022.

The manufacturer doesn’t go into detail on exactly how their breathalyzer works, saying only that it uses “ultra-sensitive technology [that is] 1 billion times more sensitive than an alcohol breathalyzer”, and can detect weed use within the past 2-3 hours.

The Hound Labs machine collects two samples, one for immediate THC detection and another for later lab analysis if desired. The company says that THC content is measured at levels less than three-trillionth of a gram, an impressive achievement.

University Of Pittsburgh

Another “promising” breathalyzer that can detect weed is the brainchild of researchers at the University of Pittsburgh. (We’ve put promising in quotation marks because it’s certainly not promising for smokers who get behind the wheel.)

The scientists have released more details than Hound Labs on how their machine works, and it’s fascinating. In fact, building a machine like this one wouldn’t even have been possible a few years ago, because it uses technology that’s only recently been developed: carbon nanotubes and advanced machine learning.

The tubes are so small that they’re only 1/100,000th the size of a human hair. THC in a driver’s breath binds to the carbon, electrical signals change the makeup of the THC molecules so they can be detected, and information is then sent to sensors that have learned to detect the presence and level of THC.

Researchers say this process is even more sensitive than the gas chromatography/mass spectrometry technique that laboratories use to test for weed, and the presence of other substances like alcohol doesn’t interfere with the breathalyzer’s ability to detect THC.

There’s no word on when the Pittsburgh machine may be available to law enforcement agencies (or employers). However, even when police have the ability to use a breathalyzer to detect weed, there’s still a big problem. 

When Is A Weed Smoker Impaired?

For many years, the standard for proving drunk driving was a blood alcohol content of 0.10% or higher. In recent years, it’s been lowered to 0.08%. When someone blows more than that number on a breathalyzer, case closed.

But what level of THC would show that someone was driving while high? There’s no legal answer to that question.

In fact, current approaches to weed testing – blood or urine THC content – usually just show if someone has used cannabis over a period of days or weeks. More sophisticated tests can determine how much THC is in a person’s system, but can’t determine when it “got there.”

For that reason, there are no laws that define “cannabis intoxication.” And even if a breathalyzer proves that a driver tested at four nanograms/nanoliter of THC, that doesn’t mean anything in legal terms.

Breathalyzers are only the first step in allowing cops to arrest people for driving under the influence of weed. States or the federal government will have to pass laws that specify a driver’s maximum allowable THC content, before a weed breathalyzer’s results will be legally incriminating.

So you have a little more time to try to talk your way out of a DUI charge if you’re driving while blazed. Take our advice, though: don’t do it. You could still be arrested based on other evidence – and you could cause serious damage, injury, or death. The risk simply isn’t worth it.

Can A Breathalyzer Detect Weed? FAQ

Q: How much would you have to smoke to be considered “impaired?”
A: There’s no real answer to that question. One person could blow a 0.08 and be legally drunk after two beers, while it might take another person five or six beers to hit that level. In the same way, everyone’s weed tolerance is different. The issue isn’t how much you’ve smoked, it’s whether your physical or mental abilities are impaired to a level where you’d be a danger to yourself or others on the road.

Q: A “drug recognition specialist” sounds like bull. What does that even mean?
A: Most large police departments now have them; they’ve undergone training designed by the International Association of Chiefs of Police and the National Highway Transportation Safety Administration. The specialists (also called “drug recognition experts”) look for obvious signs of impairment like eye movements, dilated pupils and tremors, and administer more specialized field sobriety tests. Believe it or not, the good ones are also able to tell if drivers are still sober after recent weed use, and whether weed odor in a car is old and no longer relevant.

Q: Does this mean you’ll have to take two different breathalyzers when you’re stopped?
A: Probably not. At least one of the machines in development also tests for alcohol at the same time.