Why Breathalyzers for Cannabis Won’t Work

byMichael Warford5 minutes

The argument against cannabis prohibition is often compared to the case against alcohol prohibition. That argument goes something like this: While people may object to alcohol or cannabis for health or moral reasons, ultimately adults should be able to decide for themselves whether they can drink alcohol or consume marijuana.

Based on that logic, banning cannabis consumption makes about as much sense as banning alcohol.

But while comparing alcohol to cannabis is helpful in highlighting the problems with prohibition, it can get in the way of how we think about cannabis in other areas. Case in point: breathalyzers and drugged driving.

Most experts agree that laws need to be put in place targeting drivers who are impaired by marijuana, just as we punish drivers who drink and drive. The way that governments—both in the U.S. and Canada—have approached drugged driving is by treating it similarly to how we currently treat drunk driving. This approach sounds logical, but it’s actually getting in the way of creating smart drugged driving laws.

What Is Drugged Driving Anyway?

First, let’s take a look at what the federal government is proposing for its marijuana DUI laws. Like most U.S. states that have legalized marijuana, Canada has drafted legislation to make 5 nanograms (ng) of tetrahydrocannabinol (THC) per milliliter (mL) of blood the threshold at which a driver can be found guilty of impaired driving. Driving with 2.5 ng/mL along with more than 50 milligrams of alcohol per 100 mL of blood will also be considered impaired driving.


In other words, the government is proposing treating marijuana DUI laws the same way it treats alcohol DUI laws. If you drive above a certain chemical threshold (0.08 blood-alcohol concentration or 5 ng/mL for THC), then you’ll be charged with impaired driving.

On the surface, this approach sounds like it makes sense. But dig a little deeper, and you quickly realize that such thresholds aren’t so straightforward.

THC & Alcohol: A False Comparison

The problem is that THC doesn’t behave in the body the same way alcohol does. When somebody registers a blood-alcohol concentration level of 0.10, for example, then that person is almost certainly too impaired to drive safely.

However, the same can’t be said of a person who’s driving with more than 5 ng of THC in their blood. Why? Because THC’s effect on the body depends on a number of variables, including:

  • Weight
  • Body fat percentage
  • Gender
  • The state of a person’s endocannabinoid system (ECS)
  • Cannabis tolerance levels

Somebody who only consumes marijuana infrequently, for example, may be extremely impaired at just 1 ng/mL. More frequent users could have THC levels five times higher and show no signs of impairment.


Another factor is that THC is absorbed by body fat and so it stays in the body for much longer than alcohol does. For infrequent consumers, THC can turn up in their blood for about 24 hours after they last took marijuana.

Meanwhile frequent consumers can test positive for THC up to three months after last taking cannabis. So, it’s possible to break the government’s proposed marijuana DUI limits without even being impaired.

Even worse, the government has proposed making driving with a THC concentration between 2 and 5 ng/mL punishable by a summary offence. A summary offence is technically not considered impaired driving, but it could land you a fine of up to $1,000 and make it hard to travel to the U.S.


Why Cannabis Breathalyzers Won’t Work

These problems with measuring impairment on the basis of a driver’s THC levels are the main reason why so-called breathalyzers for cannabis won’t work. Since a number of states began legalizing marijuana, tech companies have been barreling ahead to create a breathalyzer-type device that will help police determine if a driver is too impaired by marijuana to drive. These companies have spent millions of dollars developing prototypes, with some claiming to have solved the problem.

All of the breathalyzers in development are trying to answer one question: What’s a person’s level of THC? And that question isn’t always the same as asking: Is this person too impaired to drive?

Even if a breathalyzer is developed that can accurately measure a person’s THC levels, using THC levels alone isn’t a guarantee that the person being tested is impaired—or has even consumed cannabis within the past day, week or month.

So, What’s the Solution to Test for Drugged Driving?

Right now, the best way to determine if a person is too high to drive is by training police to better recognize the signs of impairment. To be clear, drug recognition training is riddled with its own problems, including implicit bias, and it has been blamed for many false arrests.

Even while Canada is spending $81 million on training police to become drug recognition experts, in the U.S. the American Civil Liberties Union (ACLU) of Georgia is launching a lawsuit against the use of drug recognition experts. According to the ACLU, the judgment calls of drug recognition experts are about as good as “flipping a coin.”

Anti-bias training could go some way towards addressing problems with drug recognition tests, but such tests will always be prone to an uncomfortably large margin of human error.


So, the best thing to do is to wait for more research. Marijuana contains more than 500 chemical compounds, very few of which have been extensively studied. Better understanding of how those compounds interact with the body will go a long way towards eventually developing better tools for measuring impairment.

While the research process will be long, it ultimately offers better hope of creating clear and sensible cannabis laws than the current focus on arbitrary THC thresholds does.

Photo credit: Easton Oliver