Relating blood concentrations of tetrahydrocannabinol and metabolites to pharmacologic effects and time of marijuana usage.

Pharmacokinetic and pharmacodynamic analyses of marijuana data have provided new insights into the relationship of blood concentrations of tetrahydrocannabinol (THC) and metabolites to drug-induced effects. THC is rapidly absorbed and distributed to tissues; initial changes in blood concentrations are out of phase (hysteresis) with physiological and behavioral changes. Once blood/tissue equilibrium is established, a direct correlation of THC blood concentration and effect is observed. Various pharmacodynamic models provide concentration estimates in the range of 7-29 ng/ml for amount of THC in blood necessary for production of 50% of maximal subjective high effect. Also, models have been proposed for predicting the time of marijuana exposure from plasma concentrations of THC and THC-carboxy acid metabolite (THCCOOH). These models were based on data from a controlled clinical study of marijuana smoking. Such models allow prediction of the elapsed time since marijuana use based on analysis for cannabinoids from a single plasma sample and provide accompanying 95% confidence intervals around the prediction. These models may be beneficial to forensic scientists in their interpretation of cannabinoid blood data associated with accidents, criminal investigations, and traffic violations.