Relationship Between Plasma Delta-9-tetrahydrocannabinol Concentration and Pharmacologic Effects in Man

The relationship between each of two pharmacologic effects (tachycardia and psychological "high") of delta-9-tetrahydrocannabinol (THC) and plasma THC concentration was investigated in three male and three female experienced marihuana smokers. Each subject smoked 1% THC cigarette on two occasions separated by 2 h. Heart rate and subjective psychological self-rating were determined frequently throughout the 4 h study period. Data were analyzed by calculating the area under the parameter versus time curves, constructing hysteresis plots, and calculating the decay rate constants from pharmacologic effect versus time plots. In both males and females, dose inhaled and psychological response were apparently equivalent for the first and second cigarettes. While all subjects exhibited marked tachycardia in response to the first cigarette, heart rate in both male and female subjects was not increased as markedly during the second cigarette. Interestingly, female subjects had less tachycardiac response than males during the second cigarette. Hysteresis plots revealed that both heart rate and subjective psychological effect were elicited in an effect compartment which was "deep" relative to the reference plasma compartment. The time courses of tachycardiac and psychological responses lagged behind the plasma THC concentration-time profile. Zero-order decay rate constants for subjective psychological rating did not change substantially from the first to second cigarette. This study suggest that plasma THC concentration is a poor predictor of simultaneously occurring physiological and psychological pharmacologic effects.