A new study shows that cannabis can be detected in the blood of daily smokers for a month after last intake. The scientific data, published in the journal Clinical Chemistry, might assist efforts to adopt public policies aimed at curbing drugged driving.

Clinical Chemistry is the journal of the American Association for Clinical Chemistry (AACC).

Cannabis is second only to alcohol as a cause of impaired driving and motor vehicle accidents. In 2009, 12.8% of young adults reported driving under the influence of illicit drugs and in the 2007 National Roadside Survey, more drivers tested positive for drugs than for alcohol. These cannabis smokers had a 10-fold increase in car crash injury compared with infrequent or nonusers after adjustment for blood alcohol concentration.

In this research paper, 30 male chronic daily cannabis smokers resided on a secure research unit for up to 33 days, with daily blood collection. Twenty-seven of 30 participants were THC-positive on admission, with a median (range) concentration of 1.4 mcg/L (0.3–6.3). THC decreased gradually with only one of 11 participants negative at 26 days; two of five remained THC-positive (0.3 mcg/L) for 30 days.

These results, AACC said, demonstrate for the first time that cannabinoids can be detected in the blood of chronic daily cannabis smokers during a month of sustained abstinence. This is consistent with the time course of persisting neurocognitive impairment reported in recent studies and suggests that establishment of 'per se' THC legislation might achieve a reduction in motor vehicle injuries and deaths.

This same type of 'per se' alcohol legislation improved prosecution of drunk drivers and dramatically reduced alcohol-related deaths.

"These data have never been obtained previously due to the cost and difficulty of studying chronic daily cannabis smoking over an extended period," said Dr. Marylin Huestis of the National Institutes of Health and an author of the paper. "These data add critical information to the debate about the toxicity of chronic daily cannabis smoking."