Divided attention performance of cannabis users and non-users following cannabis and alcohol.

The effects of delta9-tetrahydrocannabinol (delta9-THC) and alcohol, and their combination, on divided attention performance were compared for cannabis users and non-users of both sexes. Performance by all subjects was significantly impaired following 2.6 and 5.2 mg delta9-THC but not at blood alcohol concentrations of 48 and 96 mg/100 ml. The combined effect of the 2 drugs depended upon prior experience with cannabis. A synergistic action occurred in non-users while an antagonistic effect occurred in the group of users. Differences in the alcohol effects between users and non-users provided evidence of cross-tolerance between cannabis and alcohol.