This paper presents some results of an Australian study, to determine the effects of combinations of alcohol and marijuana on driving-related psychomotor skills in a group of regular alcohol and marijuana users. The objective was to define the nature and extent of the interaction between these two drugs over a range of doses. A 4 x 4 randomised, placebo-controlled, single-blind on-crossover design was used. Four doses were used for each drug: (1) 0, 2.5, 5 & 10mg of THC; (2) 0, 0.25, 0.5 & 0.75g/kg of alcohol; all possible combinations of these doses were tested. There were 20 volunteer subjects for each dosage combination, and each subject was tested only once. In seven of the combinations, the subjects received not more than one drug in combination with a placebo, and the data from them provided the dose responses for the drugs in isolation. The other nine cells provided data about the interaction between the drugs. For each subject, performance in psychomotor tests was measured on four occasions, and a question was asked about how safely he felt he could drive a car. None of the three specific interaction models tested was statistically significant, but all three showed substantial trends in the same direction, strongly suggesting de-intensification or antagonism between the effects of the drugs. Other similar studies are discussed.
Abstract