Mescaline, LSD-25, psilocybin, and a combination of these three drugs were each administered in moderate doses at 7-day intervals to 24 volunteer subjects according to a latin square design. Primary suggestibility, that type closely related to hypnosis, was measured both before and 2 hrs following each administration of drugs, using the 17-item Stanford Suggestibility Scale. A Trance Indicator Score was also obtained, representing both behavioral observations and subjective reports concerning the degree of trance-like or hypnotic-like experiences which may have accompanied each suggestibility testing session.
Major findings were that mescaline, LSD-25, and the combination of drugs each produced an average enhancement in primary suggestibility closely comparable to that produced by an induction of hypnosis, the latter being tested for each subject after the completion of the drug series. Psilocybin did not enhance primary suggestibility. Associated trance phenomena were also substantially increased during all of the drug trials, but significantly less so for psilocybin. Significant correlations were obtained between suggestibility and Trance Indicator Scores following each drug and after induction of hypnosis, although psilocybin produced substantially less correlation.
Although these results confirm speculations previously made in the literature that LSD-25 may increase suggestibility, it must be emphasized that they cannot be generalized beyond the specific type of suggestibility measured. Second, the differences between psilocybin and the other drugs on primary suggestibility restrict generalizations regarding the action of otherwise similar psychotomimetic drugs.