We screened 287 volunteers and found 214 eligible. Ineligibility was related to medical history (17), blood test results (12), daily caffeine intake (11), heavy smoking (6), and education level/VAS test (27). 34 of the 214 eligible volunteers did not show up for the study, thus 180 were equally randomized to caffeine or placebo cross-over arms. We excluded from analysis participants who later withdrew from the study (3 randomized to placebo, 2 to caffeine) or did not adequately abstain from caffeine (baseline caffeine levels in the study periods differed by ≥1 μg/ml (2 randomized to placebo and 5 to caffeine). A flow chart is presented in Figure . Study coordinators guessed that 52%, 51%, 41%, and 44% of participants who received, caffeine described as caffeine, caffeine described as placebo, placebo described as placebo, and placebo described as caffeine, respectively, received caffeine; indicating the success of blinding. Baseline characteristics of study groups are shown in Table .
Flow diagram of study procedures. *Excluded because of failure to abstain from caffeine as reflected on baseline level.
Baseline characteristics of study participants.
Placebo effect on subjective endpoints
The estimated drug, placebo, and total effects are presented in Figure ( &) and Table . There was significant placebo effect on energy level, and placebo effect and placebo+interaction effect on sleepiness level. The placebo effect on energy and sleepiness levels and placebo+interaction effect on sleepiness level were still significant using Wilcoxon Signed Ranks test without adjustment for baseline (P = 0.03, P = 0.01, and P = 0.03, respectively), indicating robustness of the results. There was significant correlation between placebo or placebo+interaction effects on energy and sleepiness (placebo effect: r = -0.62, p < 0.001; rho = -0.64, p < 0.001 and placebo+interaction effect: r = -0.62, p < 0.001; rho = -0.58, p < 0.001). There was no significant placebo or placebo+interaction effect on nausea.
Figure 3 Self-reported energy and sleepiness levels on continuous and binary scales over four hours after intervention. A and B: Time course of estimated unadjusted total effect (closed diamond), drug effect (closed squares), placebo effect (closed triangles), (more ...)
Drug, placebo, placebo+interaction, and total effects on systolic blood pressure, energy, sleepiness, and nausea levels.
Figure ( &) depicts unadjusted mean (SE) VAS scores of energy and sleepiness determined over 4 hours after intervention. Participants also provided binary answers about feeling not-energetic, sleepy, or nauseated, at zero time and at 0.5, 1, 1.5, 2, 2.5, 3, and 4 hours after intervention (Figure ( &)). The binary data were consistent with the continuous data. Using percentage of time symptoms were reported over 4 hours as a summary measure, there was significant placebo effect on feeling not-energetic and sleepy (Table ) that was still significant using Wilcoxon Signed Ranks test (1-tailed P = 0.006 and P = 0.04, respectively). The placebo+interaction effect was not significant.
Drug, placebo, placebo+interaction, and total effects on self-classification as not-energetic, sleepy, and nauseated.
The placebo and drug effects were comparable in size on the three subjective endpoints (Tables &). Head to head comparison of the ANCOVA-adjusted means of the placebo and drug effects, using t test did not show a significant difference (p = 0.66 to 0.75).
Interaction between drug and placebo effects
As shown in Table , the placebo effect was larger than the placebo+interaction effect on energy and sleepiness levels, and the combination of drug and placebo effects was larger than the total effect. Further, while the placebo effect was significant on binary endpoints, the placebo+interaction effect was not (Table ). We also estimated the drug+interaction effect by comparing receiving caffeine described as caffeine to receiving placebo described as caffeine, using ANCOVA. The drug+interaction effect was not significant on energy level 11.3 [CI, -4.3 to 27.0] mm*hr, p = 0.15) and of borderline significance on sleepiness level (geometric mean ratio 0.56 [CI, 0.32 to 1.00), p = 0.05). The data combined strongly suggest the presence of a negative interaction between caffeine drug and placebo effects.
Drug and placebo effects followed similar time course with a peak at 2 to 2.5 hours, whereas, the interaction effect on sleepiness level appeared to increase over the study period (Figure ( &)).
Placebo effect on objective outcomes
We examined whether caffeine plasma level depends upon whether participants knew they were getting caffeine. We conducted a 14 hour bioavailability study on 22 participants nested in the 83 who received caffeine described as caffeine or as placebo in a balanced randomized cross-over design, under controlled food and fluid intake. Mean plasma caffeine levels before and after natural logarithmic transformation are shown in Figure ( &). They were lower in the terminal part of the curve after receiving caffeine described as placebo compared to receiving caffeine described as caffeine. Caffeine pharmacokinetics is shown in Table . Using ANOVA (model included group, subjects nested in groups, period, and intervention), there was no significant difference in maximum measured plasma level or its time. However, caffeine AUC was significantly lower and caffeine terminal half-life was significantly shorter after receiving caffeine described as placebo. These differences continued to be significant using ANOVA after logarithmic transformation of data or Wilcoxon Signed Ranks test (Table ). There was no significant period (p = 0.45 to 0.80) or group effect (p = 0.47 to 0.79). Furthermore, there was a significant difference in mean 3-hour caffeine level (Table ) in the entire group of 83 participants who received caffeine when caffeine was described as caffeine compared to when it was described as placebo (0.3 [CI, 0.04 to 0.57] μg/ml, p = 0.02).
Figure 4 Placebo effect on caffeine plasma levels and systolic blood pressure. A & B: Mean plasma caffeine levels over 14 hours after administration of caffeine to 22 participants in a balanced cross-over design before (A) and after natural logarithmic (more ...)
Pharmacokinetics of caffeine when described as caffeine or placebo.
We did not find a significant placebo effect or placebo+interaction effect on systolic blood pressure despite the presence of a clear drug effect (Table and Figure ).