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Marijuana DUI Testing Equipment


Cannabis is the most commonly used drug worldwide, with 5% of 15 to 64 year-olds (about a quarter of a billion people) admitting to consuming cannabis at least once in 2010.[1] By 2012, some studies estimate 36% of American high school students have smoked cannabis at least once.[2] As reported by the National Highway Traffic Safety Administration and a 2007 U.S. Roadside Survey, Delta-9-Tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, was found in 8.6% of nighttime drivers’ blood and/or oral fluid, and in 15% of all fatal car accidents.[3] As of 2018, cannabis is legal in the State of California, and these numbers have only increased.

Despite the increasingly legal use of cannabis in many states, including California, law enforcement still does not have the equivalent of a reliable alcohol breathalyzer or blood test — a chemically based way of estimating what the drug is doing in the brain – for Cannabis. Though a blood test exists that can detect some of marijuana’s components, there is no widely accepted, standardized amount in the breath or blood that gives police, courts or anyone else a good sense of who is impaired. Currently, your impairment from Cannabis can only be subjectively assessed based on blood tests, field sobriety testing, pupil size, speech patterns, and other general behavior.         

Assuming scientist can invent a reliable chemical test for cannabis impairment, there remains the secondary problem of defining what, exactly, the “legal level of impairment” would actually be. Coming up with a standardized blood or breath test for cannabis proposes a tricky chemistry problem because of the properties of the main psychoactive chemical in cannabis: delta-9-tetrahydrocannabinol, or “THC”.


In states like Colorado, there is a THC blood test that law enforcement can use to show “presumed” impairment. According to Colorado law if a person has more than 5 nanogramsof delta-9-THC per milliliter of blood, a court or jury can infer that they are impaired, (this is called a “permissible inference”)[4]. According to Tara Lovestead, a chemical engineer at the National Institute of Standards and Technology in Boulder, Colorado “We just don’t know whether or not that means they’re still intoxicated, or impaired or not,” she said. “There’s no quantitative measure that could stand up in a court of law.”

Ethanol, the chemical in alcoholic drinks which causes impairment when driving is small and dissolves in water. Because humans are mostly water, it gets distributed expeditiously throughout the body and therefore, also expeditiously leaves the body. Normal ethanol from alcoholic drinks is usually digested and expelled from the human body within a matter of hours. THC, the main chemical in cannabis discussed above, is different – it dissolves in fat. This means the length of time it lingers in the body can differ from person to person even more than alcohol — influenced by things like gender, amount of body fat, frequency of use, and the method (smoked or ingested) and type of cannabis product consumed (flower, hash, concentrate, tincture, edible, etc.).

In one study, researchers had thirty frequent marijuana users stay at a research facility for a month without any access to drugs of any sort and repeatedly tested their blood for evidence of cannabis. “It shocked everyone, including ourselves, that we could measure, in some of these individuals, THC in the blood for 30 days,” says Marilyn Huestis, a toxicologist with the University of Maryland School of Medicine who recently retired from leading a lab at the National Institute on Drug Abuse. The participants’ bodies had built up stores of THC that were continuing to slowly leech out, even though they had abstained from using marijuana for a full month. In some of those patients who regularly smoked large amounts of pot, researchers could measure blood THC above the 5-nanogram level for several days after they had stopped smoking. [5]

On the other hand, a different study was conducted on people who had never smoked cannabis. They were given a medicinal grade cannabis joint to smoke, and their blood was repeatedly tested thereafter. Interestingly, these patients who had smoked a joint right directly in front of scientists showed no evidence of cannabis in their blood.[6]

So, in addition to being invasive and cumbersome, the blood test can be misleading and a poor indicator of whatever is happening in the brain. [7]


Due to the problems discussed above regarding blood test, several law enforcement agencies have commissioned private laboratories to create an accurate breath test for cannabis consumption. Further, a number of companies, like Cannabix Technologies, Drägerwerk AG, and Hound Labs, are in the process of developing their own breath detection devices. Even though breath provides an incomplete and indirect picture of what’s happening in the brain, it’s the measure law enforcement turns to as a benchmark based on their historical use of alcohol breath testing devices.

THC appears in blood plasma differently than it appears in breath samples. As discussed above, THC appears in the blood plasma immediately after the first puff and peaks before the last puff of a cannabis cigarette. Concentrations in the blood slowly increase after the first hour after smoking, and plateau after two to four hours. This is not the same for breath testing.

In breath testing, biomarkers from smoked cannabis degrade quickly and disappear rapidly after smoking cannabis is completed. Further, scientist do not even know which biomarkers they are even looking for, the exact half life of said biomarkers, how to identify or single out those biomarkers, and most importantly, whether the cannabis was directly consumed, or if it was “second hand smoke”.


You may receive a DUI for driving under the influence of cannabis, however in most instances it will be extremely difficult for the District Attorney to actually prove this at trial if you conduct yourself reasonably. If you are polite, well spoken, don’t smell like cannabis, don’t have any burned joints in the vehicle, and follow the officer’s directions, they will be left will little evidence to say you are “high”. Keep in mind you also have the right to refuse field sobriety tests such as the walk and turn. It is a good idea to in fact refuse these tests, as more often than not, no matter what you do, the officer will find a way to hold it against you.

On the other hand, if your eyes are red, your pupils are large, your vehicle smells of burned cannabis, there is half a doobie in the center console, you are off balance, you can’t count to 30, and you say “dude” more than a hundred times during your traffic stop, chances are this will provide the officer evidentiary ammunition to claim you are “high” and therefore “impaired”, culminating in your arrest for a DUI.

[1] United Nations Office on Drugs and Crime

[2] “National Results on Drug Use” September 2013 by Johnston LD et al. in Monitoring the Future.

[3] National Highway Traffic Safety Administration Office of Behavioral Safety Research

[4] Colorado Revised Statue 42-4-1301

[5] “Impact of Prolonged Cannabinoid Excretion in Chronic Daily Cannabis Smokers’ Blood on Per Se Drugged Driving Laws” February 2013 by Robert S. Goodwin, et al. in the Journal of Clinical Chemistry

[6] “Phase I and II Cannabinoid Disposition in Blood and Plasma of Occasional and Frequent Smokers Following Controlled Smoked Cannabis” February 2014 by Nathalie A. Desrosiers et al. in the Journal of Clinical Chemistry

[7] “Scientists Still Seek A Reliable DUI Test For Marijuana” July 30, 2017 by Rae Ellen Bichell at NPR