Researchers behind the study suggest that the finding can provide real help in the public safety need for a drugged driving policy that reduces the number of drugged driving accidents on the road.
Cannabis is second only to alcohol for causing impaired driving and motor vehicle accidents. In 2009, 12.8 percent 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 study, 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 micro g/L (0.3-6.3). THC decreased gradually with only 1 of 11 participants negative at 26 days; 2 of 5 remained THC-positive (0.3 micro g/L) for 30 days.
These results showed, for the first time, that cannabis could be detected in blood of chronic daily cannabis smokers for a month after last intake.
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 author on the paper.
"These data add critical information to the debate about the toxicity of chronic daily cannabis smoking," Dr. Huestis added.
The research appeared online in Clinical Chemistry, the journal of AACC.