A new study published this month adds further evidence that levels of THC detected in the blood or breath of cannabis users is not a reliable indicator of impairment. Researchers also found that levels of THC in blood and breath did not provide reliable evidence of how recently a test subject had used cannabis.
In their introduction to the study, the researchers noted that “finding an objective measure of recent cannabis use that correlates with impairment has proven to be an elusive goal.” Some states have enacted laws that set per se legal limits on the amount of THC a driver may have in their blood, similar to the 0.08% blood alcohol concentration limit in effect nationwide.
Critics of per se limits on THC concentrations in blood or breath have argued that such limits have little bearing on the level of impairment or intoxication, which can vary widely from person to person despite similar levels of THC concentration.
“These findings provide further evidence that single measurements of specific delta-9-THC blood concentrations do not correlate with impairment, and that the use of per se legal limits for delta-9-THC is not scientifically justifiable at the present time,” wrote the authors of the study published by the journal Scientific Reports.
To conduct the study, the researchers recruited a group of test subjects, most of whom were daily cannabis users. The scientists then determined the THC levels in their blood and breath prior to and after inhaling cannabis.
Before inhaling cannabis, most subjects had residual THC levels of 5ng/ml or higher, which exceeds the per se legal limit in several states. The authors noted that THC at such levels was detected despite “the absence of any impairment.” After the test subjects inhaled the cannabis, the researchers noted an inverse relationship between THC blood levels and impairment of performance.
“Our findings are consistent with others who have shown that delta-9-THC can be detected in breath up to several days since last use,” they wrote. “Because the leading technologies for breath-based testing for recent cannabis use rely solely on the detection of delta-9-THC, this could potentially result in false positive test outcomes due to the presence of delta-9-THC in breath outside of the impairment window.”
New Study Backed by Previous Research
The results are consistent with the findings of a study published late last year in the journal Neuroscience & Biobehavioral Review. In that study, researchers affiliated with the University of Sydney analyzed all the available studies on driving performance and THC concentrations in blood and saliva.
“Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users while no significant relationship was detected in regular cannabis users,” wrote lead author Dr. Danielle McCartney of the Lambert Initiative for Cannabinoid Therapeutics. “This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment.”
To conduct the study, the researchers reviewed data from 28 publications that studied the consumption of inhaled or ingested cannabis. They then analyzed the association between THC concentration and driving performance, using measures of driving-related skills such as reaction time and divided attention.
The researchers documented “weak” associations between THC levels and impairment among infrequent cannabis users. But they observed no significant association between blood or saliva THC levels and impairment among regular pot users, defined as those who used cannabis weekly or more often.
“Of course, this does not suggest there is no relationship between THC intoxication and driving impairment,” McCartney said. “It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”
The authors noted that the findings in the study call into question the validity of widespread random mobile testing for THC in saliva in Australia and the reliance on THC levels by law enforcement in the United States.
“Our results indicate that unimpaired individuals could mistakenly be identified as cannabis-intoxicated when THC limits are imposed by the law,” said McCartney. “Likewise, drivers who are impaired immediately following cannabis use may not register as such.”
Professor Iain McGregor, the academic director of the Lambert Initiative, a long-term research program studying the medical potential of cannabis, said that “THC concentrations in the body clearly have a very complex relationship with intoxication. The strong and direct relationship between blood-alcohol concentrations and impaired driving encourages people to think that such relationships apply to all drugs, but this is certainly not the case with cannabis.”
“A cannabis-inexperienced person can ingest a large oral dose of THC and be completely unfit to drive yet register extremely low blood and oral fluid THC concentrations,” McGregor added. “On the other hand, an experienced cannabis user might smoke a joint, show very high THC concentrations, but show little if any impairment. We clearly need more reliable ways of identifying cannabis-impairment on the roads and the workplace.”
In April 2022, New York Gov. Kathy Hochul and a coterie of other state lawmakers and public-safety officials launched a firm yet nebulous public-safety campaign warning people that they shouldn’t be driving high. The initiative pulled off the neat trick of informing citizens that certain behavior is prohibited, without telling citizens exactly what that behavior is.
Called “Cannabis Conversations,” the campaign will be emphasized in an upcoming series of billboards, commercial sports and other public service announcements (PSAs) to complement similar warnings against drunk driving. But what, exactly, is “driving high?” Unlike drunk driving, that’s not something Hochul—nor anyone else in states where cannabis is legal—has been able to satisfactorily define.
Nevertheless, Hochul is the latest public official to highlight a curious situation that’s proven one of the more complex and nagging problems to arise during the marijuana legalization era.
Burden of Proof, Body of Doubt
In one sense, driving while high is not unlike pornography: You know it when you see it—if “you” are a law-enforcement official who’s a drug-recognition expert, determining whether to write a ticket or make an arrest for a misdemeanor offense.
Under New York state law, drunk driving and driving high are outlawed under the same criminal statute. But unlike the first wave of states to legalize cannabis, there’s no strict “legal limit” for cannabis impairment in New York. This is because other states such as Colorado have ditched limits like the initial standard of five nanograms of cannabis metabolite per milliliter of blood—because, unlike alcohol, cannabis metabolites are detectable in the human body long after the effects have worn off. For that reason, New York state law has no “per se” standard for impairment.
So, while this standard may be workable while out on the road, where a law enforcement officer can use various metrics, i.e., erratic driving, to make a stop and other metrics to determine impairment—slurred speech, red eyes, the scent of cannabis—it’s unclear what will happen in court, where defense attorneys were winning too many “stoned driving” cases.
In an e-mail, Jason Gough, a spokesman for New York’s governor, reiterated what Hochul and other officials have said: It’s illegal to drive under the influence of cannabis, to consume cannabis while driving or to have your friends burn a blunt in the back when you’re driving them around.
“We’re undertaking a public education campaign to help make sure New Yorkers know that if they drive high or impaired, they could be charged or hurt others,” said Gough, who added that the state would devote “cannabis revenue funds” towards the police: to train more drug-recognition experts to suss out the above, and to develop “emerging tools” such as cannabis breathalyzers “that could be used to accurately detect whether a driver is impaired by cannabis.”
OK, but what’s impairment? Gough referred Cannabis Now back to his original statement—which acknowledged, indirectly at least, that there’s no cut-and-dry standard, and it will be up to individual law-enforcement officers to decide. But will their word be enough to stand up in court? And what can a responsible, safety-minded citizen do?
What is Driving While High?
For one, people should be honest with themselves. If you feel too stoned to drive—if you feel impaired—you probably are. But what if you’ve had your wake-and-bake, and followed that up with coffee and a relaxing morning—and you feel fine?
According to recent research, cannabis users can expect their driving abilities to return to normal about three-and-a-half hours after getting stoned—or, in a laboratory setting, using cannabis to achieve the satisfactory effect. There’s a brief period where users feel a false sense of security, at about the ninety-minute mark, and then abilities return at around the three-hour mark before baseline returns at hour four.
That doesn’t do much good for someone who microdoses—that is, never used cannabis “to satisfaction” like the lab-test subjects. Nor does it give you a clear and satisfactory answer to the initial problem.
Legal experts say that the determining factor may be a driver’s ability prior to the stop. That is, if they were driving like a high person, and then the drug-recognition expert determines they looked like a high person, then a judge and/or a jury may be more likely to decide that yes, they were, in fact, driving high.
“I think drug-recognition experts, in addition to other things, such as glassy or red eyes and slurred speech, are going to have to say, ‘We saw them swerving, or making illegal lane changes,’” said David C. Holland, a New York City-based criminal defense attorney and executive director of Empire State NORML.
That may change, of course, if the driver was involved in an accident. In that case, tacking on an impaired driving charge may become axiomatic—or at least an easier sell in court. Which highlights a convenient truth: If you don’t want to get busted for driving high, don’t do it. In the meantime, getting busted for driving high while you’re not, will remain a very unsatisfying and quite real possibility.
We’ve all familiar with checkpoints and breathalyzers, even if we never had to deal with one ourselves. And we all know that drinking with a blood alcohol content over .08 is illegal, and leads to a DUI. But what about driving stoned? While driving under the influence can relate to many things, new laws to introduce blood THC content limits are putting a new spin on the standard practice of getting behind the wheel high.
Bet you didn’t think you’d have to worry about having your blood THC content checked? New laws for this make the need to be careful in states where such laws exist. Luckily, you can always enjoy your favorite cannabis products at home. Plus, with the new cannabinoid market, who said it has to be THC? With additions like delta-8 THC, HHC, and THCV, there are tons of ways of experiencing the cannabis plant. Check out our deals for all compounds, and remember to get high responsibly. Remember to subscribe to The THC Weekly Newsletter for deals on legal cannabis products, as well as all the latest news and industry stories. Also save big on Delta 8, Delta 9 THC, Delta-10 THC, THCO, THCV, THCP & HHC products by checking out our “Best-of” lists!
What is a blood alcohol content limit?
This is a good question, and before getting into blood THC content, it’s best to start here. Blood THC content sounds oddly familiar, yet is new to most. It sounds oddly familiar because of the term ‘blood alcohol content’ which is well known, especially for the driving community. It’s the line between legal driving, and going before a judge. The breathalyzer test given by cops measures the amount of alcohol in the air being breathed out by a suspected drunk driver. Ever get in close to a drunk guy and smell the heavy scent of alcohol coming out of them? Well, this is what a breathalyzer measures.
When a person registers at .08% or above, its considered that they passed the legal limit if 21 or above. For those below 21, the limit is significantly lower, at 0-.02% depending on location. While .08 is a federal mandate, individual states are allowed to institute more restrictive laws. What does the .08% measurement actually mean?
It’s measured in grams per 100ml of blood. Meaning eight grams of alcohol per 10 deciliters of blood = .08%. Because this cut-off line exists, it allows law enforcement the presumption of guilt when a person registers this amount, regardless of actual driving ability. So if a person who is driving okay is stopped at a checkpoint, and does not pass a test, their actual ability to drive will not help them. This isn’t meant to be an argument against measures to stop drunk driving, its just to explain that the limit represents a law that is independent of actual behavior.
What is a blood THC content limit?
While we’re all familiar with the idea of drunk driving, and why it’s illegal (see below for statistics), the reality is that all states have something along the line of a ‘drugged driving’ law as well. Alcohol isn’t the only substance that can cause impairment behind the wheel, many drugs can. So drugged driving laws exist to cover the idea of being impaired on any substance. The difference between a vague ‘drugged driving’ law, and a law with specifics like a .08% limit, is that there isn’t a legal line that differentiates legal from non-legal amounts.
With illicit drugs, its obviously illegal to be caught possessing and using them, however, they can also incur a driving violation if it’s decided that the person was impaired while driving on them. Did you get the language? If it’s ‘decided’ by law enforcement. As in, in order to make a charge like that stick, impairment must be proven somehow. Given how life tends to work, this can create precarious situations as cops have the ability to get a person arrested based on their opinion of the situation, without something to point at to show an actual problem. Obviously in cases where there is an actual problem – a person very much compromised based on whatever drug, this is okay. However, particularly for legal compounds, or situations where cops might feel the need to go beyond their jurisdiction, this can create dicey situations.
And this is where testing for blood THC content comes in. Perhaps its been hard for law enforcement to make such charges stick against potheads, well with the institution of legal limits, the same presumption of guilt is allowed for cannabis, regardless of driving ability. I want to take a second here to state that I’m a cannabis user, and have been hanging out and driving in cars with other users for half my life. I’d never get in a car knowingly with someone who’s been drinking, but I don’t bat an eye at driving with a stoner.
So basically, a blood THC content is the measure of THC in the blood. Unlike a breathalyzer test, this can’t be picked up by someone breathing out, and requires a blood test. In the US, this is generally measured in nanograms per milliliter of blood, but only done on a state level as cannabis is federally illegal. Different legalized states have set their own limits, and some go by the idea of impairment only, without a limit to test for.
Which states test for blood THC content?
Right now there are 18 states that have legalized recreational cannabis, which means these states can institute limits if they choose to separate legal use with driving, from illegal use with driving. Of the 18 states that are legal, the following have laws for blood THC content limits.
Washington uses a maximum THC level for driving, and set the amount at five nanograms per milliliter or higher of THC in the blood stream. This same level is used by Colorado and Montana. Nevada is the one state going further than this right now. In this state, it’s considered under the influence if a driver’s blood test shows two nanograms of THC and five nanograms of metabolite when driving.
Vermont has not opened its market yet, but holds that any amount of illicit substance found in the system constitutes the ability for arrest when driving. It was not confirmed how this zero-tolerance policy effects a non-illicit substance like cannabis. Massachusetts also has a zero-tolerance policy, making any amount of cannabis found in the system of a driver enough for arrest.
New Jersey is still working on its legislation, as legalization was in the last year, and the laws aren’t concrete yet. It has been argued repeatedly to keep out restrictions for maximum THC content, although whether this relates strictly to products, or to driving as well, is not clear.
New York, New Mexico, Virginia, and Connecticut are also newly legalized states where legislation has not been hammered out. We’ll have to wait and see what restrictions these states choose to institute when their laws are made clearer. States that go by judging impairment as the deciding factor of law enforcement, include: California, Oregon, Illinois, Michigan, Alaska, Maine, Arizona, and the states that still need to present legislation: New York, New Mexico, Virginia, Connecticut, and New Jersey.
Statistics for driving while under the influence of cannabis
Like I said before, I won’t get in a car with a drunk driver, but I don’t mind someone that just smoked up getting behind the wheel. And that’s because through decades of time, it has consistently been reinforced by life that drinking really can cause immense dangers, and cannabis, for the most part, really doesn’t. At least in my experience. Here are some basic stats to show the picture in the US.
According to the CDC, in terms of alcohol in the US, approximately every 50 minutes, a drunk-driving related death is occurring. 29 happen every day. In 2016, this equaled 10,497 deaths related to drivers impaired by alcohol, making up about 28% of all deaths on the road. 1,233 of the total deaths in 2016 were children, with 17% of those deaths related to alcohol impairment.
When it comes to accidents involving cannabis, there are virtually no actual statistics, and certainly none put out by government agencies. In a study from 2010 (before the cannabis market really exploded) called The Effect of Cannabis Compared with Alcohol on Driving, researchers concluded that “Epidemiological studies have been inconclusive regarding whether cannabis use causes an increased risk of accidents; in contrast, unanimity exists that alcohol use increases crash risk.” They went on to say
“The risk from driving under the influence of both alcohol and cannabis is greater than the risk of driving under the influence of either alone. Future research should focus on resolving contradictions posed by previous studies, and patients who smoke cannabis should be counseled to wait several hours before driving, and avoid combining the two drugs.” Of course, the problem with the last part, is that it then involves alcohol, which we already know causes many issues.
While the government provides no information, there are stories out about an increase in accidents related to cannabis use since legalizations. Perhaps this has more to do with testing for cannabis since legalizations occurred, since expecting that the actual smoking community changed so drastically by a legalization, is a little silly. Such headlines create the logical fallacy that the legalizations started the industry, and that this wouldn’t have been going on prior, which is incredibly mistaken. Logically, people were always smoking and driving, making these claims nonsensical, and sounding very much like fearmongering, especially when official numbers somehow can’t be provided.
In fact, in 2019, a USA Today article, citing research on crash fatalities in Colorado, Washington, and Oregon from 2019, found as little as a one per million people increase in fatalities after legalization. But then the numbers returned to normal the following year. Considering these aren’t even statistically significant results, and they didn’t hold, this backs up that there shouldn’t be any change due to legalizations, as smoking and driving habits would have been unlikely to be affected by legal measures.
Let’s be honest, cannabis and driving aren’t even remotely the issue that alcohol and driving is, and this shows up in all statistics. And so long as opiates are being doled out, and allergy medicines like Benadryl are available, making cannabis out to be the demon of driving, is ridiculous at best. Does this mean a person shouldn’t be careful when smoking and driving? Of course they should! But that’s always the case.
Alcohol will always be more dangerous, no matter how much information is distorted to try to sway opinions. My bet is these limits are being put in place to force fines, and that there isn’t any worry – and certainly not based on statistics – that would indicate this is really a problem worth responding to. Maybe further research will say otherwise. For now, best to be careful when driving with weed.
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Disclaimer: Hi, I’m a researcher and writer. I’m not a doctor, lawyer, or businessperson. All information in my articles is sourced and referenced, and all opinions stated are mine. I am not giving anyone advise, and though I am more than happy to discuss topics, should someone have a further question or concern, they should seek guidance from a relevant professional.
Bill C -46 It’s been two years since Bill C-46 became law, overhauling impaired driving rules nation-wide. Although Bill C-46 embodied a number of significant changes to the Criminal Code impaired driving offences, it did a couple of key things. First, it created specific offences for cannabis-impaired driving the first time. Second, it did away […]
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