Why Devising a ‘Driving While Stoned’ Limit Proves Difficult for Law Enforcement, Legislators | Marijuana

Why Devising a ‘Driving While Stoned’ Limit Proves Difficult for Law Enforcement, Legislators


Law enforcement procedures surrounding driving under the influence of alcohol is relatively cut and dried.

If the blood alcohol content level of 0.08 is reached, you are considered legally drunk. According to the U.S. Department of Transportation’s National Highway Traffic Safety Administration (NHTSA), typical effects of a 0.08 BAC include poor muscle coordination and impaired judgement.

But things aren’t quite as simple when it comes to marijuana.

“Technically, is it possible to measure THC in breath? Yes,” said Barry Sample, a board-certified toxicologist, senior director of science and technology for New Jersey-based Quest Diagnostics and manager of the lab’s employer drug testing division. “There’s been some early work on that. The problem is [there is]no consensus level even in blood as to what constitutes impairment.”

According to a July 2017 NHTSA “Marijuana-Impaired Driving” report to Congress, “While ethyl alcohol is readily soluble in water, and hence blood, THC is fat-soluble. This means that once ingested, THC is stored in fatty tissues in the body and can be released back into the blood sometimes long after ingestion.”

The report went on to state that some studies have detected THC in the blood at 30 days past ingestion. And while THC can be detected in the blood a month after ingestion, the acute psychoactive effects of marijuana last for only hours, not the days or weeks it can show up in blood.

“It is certainly more challenging,” Sample said of the contrast between alcohol and marijuana testing.

For workplace drug testing, Sample said three main specimen types are used: urine, which is the most common, oral fluids, and hair. He said there is usually a one- to three-day detection window in urine drug testing for marijuana – however, depending on usage frequency by the individual, metabolism and other factors, that window could run up to a month. In other words, a casual user could show up negative in a few days, but a heavy user could test positive 30 days later.

Sample said using oral fluids, or saliva, for workplace drug testing for marijuana, the second most common test, has a detection window of about 24 to 36 hours for typical patients.

The third test used is for hair. Whereas urine and saliva tests determine more recent use, the hair test can detect patterns of repetitive use, according to Sample. It has a 90-day detection window.

In the case of urine and hair, laboratories test for metabolic breakdown products created by the body, or THC metabolites, Sample said. Oral fluid are tested for THC directly.

He said that while frequency of use, a person’s size, metabolism and other factors can affect test results, how marijuana is used also has an effect.

When it is smoked, 10 percent of marijuana is converted to active metabolite. But in edible form, 50 percent is converted to active metabolite. This means when metabolites are tested in the urine or hair, an edible product would result in more metabolites. Variables like these are why there is a lack of consensus on testing parameters to determine what constitutes intoxication. Recreational-use statutes designate levels for THC limits in the blood, but these vary by state, Sample said.

“Another reason why testing for marijuana impairment is not like testing for alcohol impairment,” Sample said.  

Despite the problems inherent in defining marijuana intoxication, some companies are coming up with marijuana breathalyzer-type products, including the Cannabix Marijuana Breathalyzer for law enforcement and the workplace, and the Hound marijuana and alcohol breathalyzer.

About Author

Jessica Peralta is a multimedia journalist with experience covering a wide range of topics, including courts and crime, police and fire, health, education and community news for outlets like the Los Angeles Times and Orange County Register.


  1. Cannabis is not alcohol so none of the regimes currently used are effective. THC is not ethanol and the body reacts to it very differently. The per se limits envisioned by the legislators in their ignorance will never work. It is a mater of personal tolerance. That varies with every individual. It has to be judged at the scene.

  2. If a person drinks heavily for years they build a tolerance and might not be impaired at .08. But that limit stays the same. Set a limit for marijuana and stick with it. If you smoke too much pot and are constantly over that limit then maybe you have a problem. I agree with drug tests while at work or driving… but I do also believe that the urine drug test should be eliminated, someone could be around marijuana smoke over time and get enough in there system to fail a urine test without even being impaired.

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