Cannabis on the Road: The Future of Determining Cannabis Impairment


One of the issues plaguing cannabis legalization has always been how to identify a user who is currently under the influence of marijuana. There is currently no way to prove that an individual, let’s say a vehicle driver, has used the product recently enough to still be under its influence.

Inventing a Marijuana Breathalyzer

There is little surprise that a rush is underway to develop a “marijuana breathalyzer” that can quickly and accurately identify recent use. And while it may sound like such a device is not at all in the interest of medical marijuana users, the truth is a properly calibrated marijuana breathalyzer would recognize whether the user is still under the influence of cannabis when tested. Such accuracy could protect cannabis users by showing definitively they were no longer impaired by the substance when pulled over or when an accident occurred. In other words, a breathalyzer device could mean the difference between an accident charge, and a more serious DWI.

UC Berkley on the Case

The forerunner in the breathalyzer game seems to be a California-based company who, with the help of UC Berkley scientists, has created a device that detects “impairment from recent THC consumption.”1 A test of the device did accurately represent recently the the subjects had consumed cannabis.

Using the breath may be preferable to blood tests because the presence of marijuana remains on the breath for only a few hours – approximately equal to the length of time the user is affected by the substance – whereas traces may remain in the blood for as long as a few months. Unless blood tests (or urine tests) can show, based THC amounts, how recently the subject consumed marijuana, they are essentially useless proof of impairment.

Can a Breath Sample Accurately Show How High You Are?

The other side of the breath/blood debate also has a good point, however. While a breathalyzer test may be able to show the amount of cannabis in the system, that statistic does not relate to the effect of the cannabis on the brain. As we know, cannabis affects each user differently, and the specific strain, quality of the strain, and ingestion method all play as much of a role, if not more, than time in determining just how “impaired” a person is.

driving under influence cannabis seeds for sale
How long should you wait before driving after taking cannabis? The jury’s still out.

Another argument against testing the breath cites Henry’s Law, the 19th century formulation that explains how alcohol breathalyzers work. Basically, when a volatile substance (aka alcohol) mixes with a liquid (that’s the blood), the amount of the substance can be measured in the vapor that floats above the liquid – the breath.2 (If you find this angle of cannabis investigation interesting, check out the source of this historical info – it’s long, but enlightening!)

But cannabis isn’t a volatile substance, so it interacts with the blood differently than does alcohol. This isn’t to say a marijuana breathalyzer couldn’t be useful, but the same logic cannot be applied to both devices.

Like cannabis, alcohol affects individuals differently, and despite that, a minimum blood alcohol level has been officially paired with driving impairment (0.08 percent). So the question, then, is how do scientists determine a reliable THC level that denotes impairment? This rate must be accurate enough to apply to a wide sampling of people, regardless of weight, age, gender, or tolerance. AAA has recommended that two tests be administered to drivers – one, like the breathalyzer in production, that establishes recent use, and a second that determines “behavioral and physiological evidence” of intoxication.3

Is It Safer To Drive High Than Drunk?

Always in the background is the question of whether statistics comparing accident rates by impairment should be considered in this argument. A 2011 study published in the journal Oxford Academic cited evidence that on a scale of 1 to 10, with 1 being the crash risk of a sober driver, driving under the influence of marijuana was ranked at 2; driving under the influence of alcohol was ranked 7, and driving while texting: 23.4

A Marijuana Driving Test Is Still a Ways Away

Before any device can be trusted to accurately and fairly determine a driver’s level of cannabis impairment, scientists need to learn a lot more about THC and how it interacts with the body on a chemical level. Chemical engineers at the National Institute of Standards and Technology (NIST) are on the trail. The team is exploring the plant from a variety of less-researched angles, with the goal of designing materials to improve the marijuana breathalyzer technology. 5

Until then, it seems best to assume that if you’ve ingested marijuana in the last couple of hours, you shouldn’t be behind the wheel of a car. Even if you’re certain you are ok to drive, if you’re pulled over and a test shows cannabis in your system, you could be facing serious legal penalty.

At Growers Choice, we always recommend using our cannabis seeds with care. Any medication or drug that can cause changes in your brain should be treated with respect and always used in moderation. If you have any questions about our products or how to ensure you’re medicating safely, please don’t hesitate to contact us.

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The Cannabist
Oxford Academic
Forensic Magazine

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