The Interaction Between Alcohol and Marijuana
A Dose Dependent Study of the Effects on Human Moods and Performance Skills
Gregory B. Chesher
Psychopharmacology Research Unit
Department of Pharmacology
University of Sydney, NSW.
for the Federal Office of Road Safety (Australia)
- A study was designed to examine the effects of marijuana and alcohol
when taken alone and in combination on human skills performance and
- Four dosage conditions were employed for each drug (placebo and three
active doses). All possible combinations of these dosage conditions were
tested (i.e. 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
- 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
- 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
- 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
- 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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