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Malcolm S. Dec. 22, 2009

Marijuana is by far the most commonly used illicit substance. The recent decriminalization of marijuana possession and the constitutional amendment to Colorado’s constitution to authorize medical marijuana use have invariably caused an increase in consumption of the drug. While the use of marijuana has become increasingly common and socially accepted by many, it is still illegal as it is with all intoxicants to operate a motor vehicle when under its influence.

Given the increased use of marijuana, it is becoming more common for the police to charge individuals with driving under the influence of marijuana. Unlike in driving under the influence of alcohol cases, specific standards or quantification of levels of intoxication have not been formulated in the law for driving under the influence of marijuana. As a consequence, the prosecution of such cases is much more difficult than with alcohol impairment. In the DUI — alcohol case, nearly all toxicologists agree that at a .08 Blood Alcohol Concentration one’s ability to safely operate a motor vehicle is impaired. In the DUI — marijuana case, there is no such consensus among toxicologists as to what level of THC concentration in one’s blood constitutes impairment such that one is unsafe to operate a motor vehicle.

In Colorado, many law enforcement agencies attempt to prove marijuana-impaired driving by the use of urine tests. As most of us know, marijuana may be detected in urine up to about a month after last use for a chronic user. Even those that use only rarely may still have THC metabolites in their urine or blood days following their last use. More significantly, the presence of THC metabolites in one’s urine or blood in no way proves impairment. At most, such evidence establishes that the individual consumed marijuana at some point in the past.

Given the long shelf life of THC in urine and because the impairment effects of marijuana typically subside three to four hours after last use, the continued use of urine to “prove” impairment is just simply absurd. If you are charged with Driving Under the Influence of Cannabis or Marijuana based on urine or blood test results, it is critically important that such results be independently examined by qualified forensic toxicologists to ensure accurate testing and interpretation.