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The Interaction Between Alcohol and Marijuana
A Dose Dependent Study of the Effects on Human Moods and Performance Skills
By Gregory B. Chesher, Helen Dauncey, John Crawford, Kim Horn
Psychopharmacology Research Unit
Department of Pharmacology
University of Sydney, NSW.
for the Federal Office of Road Safety (Australia)
Executive Summary
- A study was designed to examine the effects of marijuana and alcohol when taken alone
and in combination on human skills performance and mood.
- Four dosage conditions were employed for each drug (placebo and three active doses). All
possible combinations of these dosage conditions were tested (ie 16 dosage groups).
- Twenty subjects were used for each dosage group, the experiment employing 320 subjects
in all. Each subject attended the laboratory on one occasion only.
- Data collected were for psychomotor performance using a battery of computer-presented
tests, mood effects, subjective assessments of the nature and degree of intoxication, and
the subjective assessment of the effects of the drugs on driving skills and willingness to
drive a motor vehicle.
- The performance battery included tests of human skills related to those considered
necessary to drive a motor vehicle with safety.
- The population sample were recruited by advertisements on two Sydney "Rock
music" FM radio stations. All volunteers were non-naive as regards marijuana use and
were indeed heavy to very heavy users of this drug. The extent of alcohol use by the
volunteers was considered to be within the normal range of use of this drug within the
community.
- The attitudes expressed concerning the dangers associated with the use of the two drugs
indicated that the population sample was heavily biased against alcohol and in favor of
marijuana
- The subjective assessment of the doses of each drug employed indicated that they were
comparable. The subjects assessed the degree of intoxication by marijuana as being of a
similar intensity as that produced by alcohol. The doses selected therefore appear to be
relevant to those used within the social experience of the volunteer population
- Both drugs produced significant dose-dependent effects on the performance measures, on
the intoxication rating scales and on some of the mood measures.
- However, there were both quantitative and qualitative differences between these effects,
both on the performance measures and on the subjective mood effects of the two drugs.
- By far the major effects on these tests were those produced by alcohol.
- The effect on skills performance of alcohol and marijuana when taken in combination was
essentially one of addition. Marijuana tended to increase the intensity of the performance
impairment produced by alcohol. However there was evidence to suggest that the lowest dose
of marijuana produced a degree of antagonism of the effects of alcohol.
- Marijuana had no effect on the absorption or metabolism of alcohol. The blood alcohol
concentration was not affected by any of the doses of marijuana used.
- The results of this study indicate clearly that alcohol and marijuana are distinctly
different drugs. The effects produced on the performance measures were qualitatively and
quantitatively different. In addition, the differences in the nature of the drug-induced
subjective intoxication and the self-reported changes in mood effects such as anxiety and
alertness, strongly suggested different drug actions.
- The ability to discriminate and assess the degree of intoxication with alcohol was not
affected by marijuana. However, the ability to assess the intoxication due to marijuana
was greatly affected by alcohol. The subjective intoxication produced by marijuana appears
to be of a more subtle nature than that produced by alcohol.
- Evidence is presented which suggests that under the influence of alcohol, subjects
engage in a "speed-accuracy trade-off". They are prepared to make a hasty
response to a question rather than to spend more time to ensure a correct answer. This
effect could be related to a risk-taking behaviour. The results with marijuana on the
other hand suggested a slower and more careful approach to the problem, though as with
alcohol, an increased error rate in responses was recorded.
- Evidence is presented which suggests that marijuana produces periodic attentional
lapses.
- The results strongly suggest that the performance deficits and mood changes produced by
alcohol are of a greater magnitude than those produced by marijuana.
- Recommendations for directions of further research are made.
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