THC concentrations drop rapidly during the time required to collect a blood specimen in the U.S., generally within two to four hours. Oral fluid (saliva) tests for THC can be performed roadside without this long wait. However, oral fluid THC showed a two to five fold greater variability than blood tests. This indicates that while oral fluid may be an effective screening tool for detecting recent marijuana use by a driver, it may not be a precise measure of the level of impairment.
In the study, more than 50 percent of participants controlled their marijuana inhalations (called titration) so they had consistent blood THC peak concentrations, regardless of the percentage of THC in the marijuana (2.9% vs. 6.7%). This shows that past driving studies based on cannabis dose rather than blood THC may have missed the importance of dose titration. In addition, it was found that low amounts of alcohol significantly increased peak THC concentrations.
To view the paper (published in Drug and Alcohol Dependence), go to: http://www.sciencedirect.com/science/article/pii/S0376871615003142. To learn more about drugged driving, go to www.drugabuse.gov/publications/drugfacts/drugged-driving.
NIDA. "Effects of marijuana – with and without alcohol – on driving performance." National Institute on Drug Abuse, 23 Jun. 2015, https://www.drugabuse.gov/news-events/news-releases/2015/06/effects-marijuana-without-alcohol-driving-performance.
NIDA. Effects of marijuana – with and without alcohol – on driving performance. National Institute on Drug Abuse website. https://www.drugabuse.gov/news-events/news-releases/2015/06/effects-marijuana-without-alcohol-driving-performance. June 23, 2015.