With some states in the USA and regions in other countries legalizing marijuana for medical use, recreational use, or both the ability to determine blood THC levels to prevent impaired driving has become an important priority. Impaired driving after the use of any substance, even marijuana, puts the safety of the general public and that of everyone on the road at risk. 
After using marijuana the blood concentration of THC and other metabolites will disappear quickly but it takes longer for the impairment caused by the drug use to disappear. This means that determining impairment by using blood concentration samples of THC are not always reliable in order to determine whether impaired driving occurred. A partnership between researchers at the University of Iowa and the National Institute on Drug Abuse’s Intramural Research Program allowed THC blood concentration testing on adult participants before they drove, while they drove, and after they finished driving.
The participants in the study on impaired driving and THC blood levels after marijuana use did not drive on the public roads, the participants used the National Advanced Driving Simulator instead. The goal was to determine and evaluate how THC blood concentration level changes affect the toxicology result interpretations. There is a wide degree of variability in how THC is metabolized as well. Frequent marijuana users may metabolize the drug differently than users who only indulge occasionally. The current methods used for testing do not reflect the actual impairment that the individual may have, and this could be a safety risk. Newer and more effective ways of determining impaired driving after marijuana use must be developed to identify this issue and protect the public.
