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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

  1. A study was designed to examine the effects of marijuana and alcohol when taken alone and in combination on human skills performance and mood.
  2. 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).
  3. Twenty subjects were used for each dosage group, the experiment employing 320 subjects in all. Each subject attended the laboratory on one occasion only.
  4. 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.
  5. The performance battery included tests of human skills related to those considered necessary to drive a motor vehicle with safety.
  6. 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.
  7. 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
  8. 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
  9. Both drugs produced significant dose-dependent effects on the performance measures, on the intoxication rating scales and on some of the mood measures.
  10. 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.
  11. By far the major effects on these tests were those produced by alcohol.
  12. 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.
  13. 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.
  14. 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.
  15. 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.
  16. 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.
  17. Evidence is presented which suggests that marijuana produces periodic attentional lapses.
  18. The results strongly suggest that the performance deficits and mood changes produced by alcohol are of a greater magnitude than those produced by marijuana.
  19. Recommendations for directions of further research are made.


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