DWI Marijuana: When Every Day is 420

With the legalization of marijuana and reduced penalties for its use, it logically follows that there will be an increase in DWI marijuana cases. The number of drivers with marijuana in their systems increased from 8.6 percent in 2007 to 12.6 percent in 2014. Although there is a plethora of research on DWI alcohol, the opposite is true for marijuana. Additionally, field sobriety and blood alcohol concentration (BAC) tests do not translate to effective indicators for marijuana impairment.
Use of marijuana can: (1) impair a person’s problem solving ability (2) can reduce a person’s ability to focus on several things at once, (3) can inhibit the part of the brain which initiates and coordinates movement (4) affects concentration and (5) reduces peripheral vision. But studies show that drivers who have ingested marijuana tend to be aware that they are impaired and try to compensate by driving slowly and avoiding risky actions.
DWI marijuana and alcohol drivers have different deficits and drive differently. Alcohol impaired drivers tend to drive faster than normal and to overestimate their skills. The opposite is true for marijuana impaired drivers who could pass simple tests of memory, addition and subtraction, whereas alcohol impaired drivers were much more likely to fail.
Field sobriety tests such as walking heel to toe (nine steps one way, turning on one foot and walking back), standing and balancing on one leg and the nystagmus test (tracking a pen with your eyes) will snag 88% of drivers under the influence of alcohol but will only catch 30% of drivers who use marijuana regularly.Additionally, accurate measurements of impairment are illusory. With alcohol, blood tests will measure the amount of alcohol in the blood at the time the blood is drawn. Breath tests measure the alcohol which has passed into the membranes of the lung’s air sacs and then into the air. The concentration of the alcohol in the air exhaled is related to the concentration of the alcohol in the blood and can be detected by the breath alcohol testing device. The alcohol concentration in the breath is related to that in the blood by the ratio of 2,100:1.
However, when marijuana is ingested, the active ingredient, THC, leaves the blood stream quickly and is absorbed by the brain and fatty tissues. So by the time a person is actually impaired and the THC levels are the highest (10 to 30 minutes after ingestion); the THC has left the blood. Blood tests will show recent use but not necessarily impairment. Urine tests are not an accurate indicator of impairment since the THC stored in fat cells slowly releases metabolite days and weeks after use.
To compound this conundrum, scientists do not agree on how much THC is necessary to cause impairment. Several states have set a numeric limit for the amount of THC in the blood from 1 nanogram per milliliter to 5 nanograms per milliliter. Scientists argue that the 5ng/ml is too high but zero tolerance is erroneous since the existence of THC metabolite is not proof of impairment. Some states have one standard for THC in the blood and another for THC metabolite.
Since nobody seems to agree about anything and until a new breathalyzer is created which can detect recent marijuana use, you may want to consider pushing the case to trial.



1. The National Highway Traffic Safety Administration studies 2013 and 2014.
2. Psychopharmacology, “A placebo-controlled study to assess Standardized Field Sobriety Tests performance during alcohol and cannabis intoxication in heavy cannabis users and accuracy of point of collection testing devices for detecting THC in oral fluid,” Bosker et. al, 2012.
3. “Driving While High”, E. Gray, Time, 10/13/14, p.56, Dr. Marilyn Huestis, National Institute on Drug Abuse, Psychopharmacology, Drummer et al. 2004; Grotenhermen et al. 2007.
4. “Stoned driving on the rise, but is it as risky as drinking and driving?” CBS News, 2/11/15.
5. “Driving under the Influence, of Marijuana, Koerth-Baker, NY Times, 2/17/14.
6. Freudenrich, How Breathalyzers Work,
7. “Driving While High”, E. Gray, Time, 10/13/14, p.54, 56.
8. Id at p. 56.