Detection time of regular THC use in urine shorter than often assumed

According to a review in the current issue of the journal Drug Court Review "it is uncommon for occasional marijuana smokers to test positive for cannabinoids in urine for longer than seven days using standard cutoff concentrations. Following smoking cessation, chronic smokers would not be expected to remain positive for longer than 21 days, even when using the 20 ng/mL cannabinoid cutoff." By using a cut-off of 50 ng/ml in drug screening assays the detection window would typically be not longer than ten days for regular users and between 3-4 days for occasional users.

The author, Dr. Paul Cary of the University of Missouri, noted that it is usually assumed by scientists, the legal system and users of cannabis that the use of cannabis is detectable in the urine by drug screenings 30 days or longer after last consumption. However, he points out that many studies that found a long detection time had major methodical weaknesses. The most serious of these limiting factors would be "the inability to assure marijuana abstinence of the subjects during the studies."

Despite these limitations of the available studies his analysis revealed that very long cannabinoid detection times (30 days or more) are rare. The average detection window for the THC metabolite THC-COOH in urine of regular cannabis users at a cut-off concentration of 20 ng/ml in the studies reviewed by Dr. Cary was 14 days. In many of the studies "only one single subject was the source of the maximum cannabinoid detection time." He concluded that "these rare occurrences have had a disproportional influence" on the perception on the length cannabis use can be detected in urine after last consumption.