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Research on THC blood levels sheds light on difficulties of testing for impaired driving

Science Spotlight

January 28, 2016

Driving under the influence of alcohol in a tunnel©Shutterstock/Ralf Kleemann

Driving under the influence of drugs is a major public safety issue, and marijuana is the illicit drug most commonly found in the blood of drivers. There is an ongoing challenge to develop valid tests of driver intoxication with appropriate legal limits for blood concentration of THC, a primary ingredient in marijuana, or its metabolites. Due to the chemical profile of THC, its concentration in the blood diminishes rapidly after smoking, but impairment does not diminish as rapidly, rendering THC concentration in blood samples collected after driving an imperfect means for the person’s level of impairment while driving.

Researchers at the National Institute on Drug Abuse’s (NIDA) Intramural Research Program and the University of Iowa tested the blood THC concentrations of adults before, during, and after driving in the National Advanced Driving Simulator, to evaluate how changes in THC concentration may affect the interpretation of toxicology results.  Results showed that even though blood THC concentrations while driving were at levels that affected driving ability, they had decreased to under commonly used impairment testing thresholds after a few hours. Consumption of alcohol, along with marijuana, did not significantly affect the blood results for marijuana.

The findings show that THC concentrations in blood collected after driving may be much lower than concentrations closer to the time of the actual marijuana use, and thus, may not reflect the resulting impairment. In addition, the wide variability in how THC is metabolized by frequent users versus infrequent users makes accurate testing challenging. Current testing methods do not necessarily accurately reflect impairment.  Development of improved methods for assessing cannabis intoxication in real time will be important for giving law enforcement officers the tools they need to address drugged driving, and will enable legislators to develop more accurate impairment guidelines.

For a copy of the abstract, “Effect of Blood Collection Time on Measured Δ9-Tetrahydrocannabinol Concentrations: Implications for Driving Interpretation and Drug Policy” published in Clinical Chemistry, and co-authored by Drs. Marilyn Huestis and David Gorelick from the National Institute on Drug Abuse, go to http://www.clinchem.org/content/62/2/367.abstract.

To learn more about marijuana and drugged driving, go to: https://www.drugabuse.gov/publications/research-reports/marijuana/letter-director

For more information, contact the NIDA press office at media@nida.nih.gov or 301-443-6245.

NIDA Press Office

About the National Institute on Drug Abuse (NIDA): The National Institute on Drug Abuse (NIDA) is a component of the National Institutes of Health, U.S. Department of Health and Human Services. NIDA supports most of the world’s research on the health aspects of drug use and addiction. The Institute carries out a large variety of programs to inform policy, improve practice, and advance addiction science. Fact sheets on the health effects of drugs and information on NIDA research and other activities can be found at www.drugabuse.gov, which is now compatible with your smartphone, iPad or tablet. To order publications in English or Spanish, call NIDA’s DrugPubs research dissemination center at 1-877-NIDA-NIH or 240-645-0228 (TDD) or email requests to drugpubs@nida.nih.gov. Online ordering is available at drugpubs.drugabuse.gov. NIDA’s media guide can be found at www.drugabuse.gov/publications/media-guide/dear-journalist, and its easy-to-read website can be found at www.easyread.drugabuse.gov. You can follow NIDA on Twitter and Facebook.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.

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    NIDA (4622). Research on THC blood levels sheds light on difficulties of testing for impaired driving. Retrieved , from https://www.drugabuse.gov/news-events/news-releases/2016/01/research-thc-blood-levels-sheds-light-difficulties-testing-impaired-driving

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