Dose response relationship between blood concentrations of THC and crash risk: meta-regression of culpability studies

Recent meta-analyses on the risk associated with the detection of tetrahydrocannabinol (THC) in blood and crash risk indicate a ~10-80% increase in crash culpability for detection of THC. However, the meta-analyses did not analyse a dose-response relationship between blood concentrations of THC and risk necessary to better understand the crash risk associated with THC and to inform public risk tolerances surrounding THC and driving. Here, we leverage these recent meta-analyses to understand whether there is a blood concentration crash-culpability risk relationship. We show that crash culpability risk increases with increasing THC concentration, with an inflection around 2 ng/ml where risk begins to increase. There is a doubling of risk around 4-5 ng/ml, and a potential tripling of risk around 10 ng/ml. Conversely, blood THC concentrations below ~1-2 ng/ml indicate practically no risk (<20% increase). More studies are needed to better define risk for the lower and higher bands of blood THC concentrations. Consideration of blood concentration risk relationships are necessary for public discussions surrounding THC and crash risk.