Drug tolerance and personality: Some implications for Eysenck's theory

Two previous studies, one of nitrous oxide and the other of amylobarbitone sodium, had shown that tolerance of these drugs, assessed by the ‘sedation threshold’ procedure, was influenced by an interaction between introversion-extraversion and neuroticism: tolerance was negatively correlated with E in High N subjects and positively with E in low N subjects. In a study described in the first part of this paper the same result was found for thiopentone among a group of 50 surgical patients, tested as part of their routine pre-operative procedure. The remainder of the paper is devoted to a more detailed analysis of this interactive effect, undertaken by combining the personality and sedation threshold scores from three studies. In this larger sample the previously observed results were broadly maintained, in that neurotic extraverts and normal introverts proved to have very low drug tolerance; the highest tolerance was found, however, in introverts with moderate, rather than very high, neuroticism. Comparison with similar data for a large group of neurotic patients revealed a somewhat different picture: the sedation thresholds of patients were sometimes predictable from their personality scores and sometimes from their diagnoses, the two not always coinciding. The results as a whole are discussed with reference to the possible neurophysiological mechanisms mediating depressant drug action and personality differences. Specifically it is suggested that some parts of the observed variation in drug tolerance might be explained by combining certain features of the modifications to Eysenck's theory of extraversion and neuroticism proposed, respectively, by Gray and by Claridge. However, it is considered that not all of the variation can be explained in this way and that other personality characteristics, probably those associated with ‘psychoticism’, are likely to prove important determinants of drug tolerance differences.