Stacey Sigmon of the University of Vermont College of Medicine and co-author Roland Griffiths of The Johns Hopkins University School of Medicine used a choice procedure to identify participants as caffeine "choosers" and "non-choosers" for the study.
Choosers were those who chose caffeine over placebo seven out of 10 choices per session and non-choosers chose placebo over caffeine in the majority of choice sessions. There were no significant differences regarding pre-study caffeine intake or other characteristics between the two groups.
During the second phase of the study, all participants received various doses of d-amphetamine and rated how much they liked or disliked each dose.
The study, published in the journal Drug and Alcohol Dependence, found caffeine choosers reported significantly more positive subjective effects and fewer negative/unpleasant effects of d-amphetamine compared to non-choosers, particularly at the highest doses.
Caffeine non-choosers reported fewer positive effects and more unpleasant effects of d-amphetamine compared to choosers.
"People differ dramatically in how they respond to drugs," Sigmon said in a statement. "For example, a single dose of a drug can produce completely opposite effects in two people, with one absolutely loving and the other hating the drug's effects."