Interactions of cannabis with other drugs in man.

Only THC, of all cannabinoids, has a significant pharmacodynamic interaction with ethanol. Effects in man are additive as expected. THC, but not CBD, showed a similar interaction with barbiturates. Interactions with stimulants were weakly additive, but the former drugs do not reverse impairments from THC. Interactions between cannabinoids are controversial. Some evidence consistently suggests that CBD may block actions of THC, while other evidence could not show a clinically significant interaction. CBD did not alter the kinetics of THC, but it decreased metabolism of hexobarbital. Preliminary studies of interactions between THC and drugs affecting activity of neurotransmitters have not provided good tests of the mechanism of action of the drug, showing, at best, subtle effects of questionable clinical significance. The variable responses of subjects to THC is neither explained by differences in metabolism of drugs nor by differences in the setting in which the drug is taken.