Placebo effects and the common cold: a randomized controlled trial

Abstract

Purpose: We wanted to determine whether the severity and duration of illness caused by the common cold are influenced by randomized assignment to open-label pills, compared with conventional double-blind allocation to active and placebo pills, compared with no pills at all.

Methods: We undertook a randomized controlled trial among a population with new-onset common cold. Study participants were allocated to 4 parallel groups: (1) those receiving no pills, (2) those blinded to placebo, (3) those blinded to echinacea, and (4) those given open-label echinacea. Primary outcomes were illness duration and area-under-the-curve global severity. Secondary outcomes included neutrophil count and interleukin 8 levels from nasal wash at intake and 2 days later.

Results: Of 719 randomized study participants, 2 were lost and 4 exited early. Participants were 64% female, 88% white, and aged 12 to 80 years. Mean illness duration for each group was 7.03 days for those in the no-pill group, 6.87 days for those blinded to placebo, 6.34 days for those blinded to echinacea, and 6.76 days for those in the open-label echinacea group. Mean global severity scores for the 4 groups were no pills, 286; blinded to placebo, 264; blinded to echinacea, 236; and open-label echinacea, 258. Between-group differences were not statistically significant. Comparing the no-pill with blinded to placebo groups, differences (95% confidence interval [CI]) were -0.16 days (95% CI, -0.90 to 0.58 days) for illness duration and -22 severity points (95% CI, -70 to 26 points) for global severity. Comparing the group blinded to echinacea with the open-label echinacea group, differences were 0.42 days (95% CI, -0.28 to 1.12 days) and 22 severity points (95% CI, -19 to 63 points). Median change in interleukin 8 concentration and neutrophil cell count, respectively by group, were 30 pg/mL and 1 cell for the no-pill group, 39 pg/mL and 1 cell for the group binded to placebo, 58 pg/mL and 2 cells for the group blinded to echinacea, and 70 pg/mL and 1 cell for the group with open-label echinacea, also not statistically significant. Among the 120 participants who at intake rated echinacea's effectiveness as greater than 50 on a 100-point scale for which 100 is extremely effective, illness duration was 2.58 days shorter (95% CI, -4.47 to -0.68 days) in those blinded to placebo rather than no pill, and mean global severity score was 26% lower but not significantly different (-97.0, 95% CI, -249.8 to 55.8 points). In this subgroup, neither duration nor severity differed significantly between the group blinded to echinacea and the open-label echinacea group.

Conclusions: Participants randomized to the no-pill group tended to have longer and more severe illnesses than those who received pills. For the subgroup who believed in echinacea and received pills, illnesses were substantively shorter and less severe, regardless of whether the pills contained echinacea. These findings support the general idea that beliefs and feelings about treatments may be important and perhaps should be taken into consideration when making medical decisions.

Figure 1.

Participant flowchart: entry, randomization, and follow-up of participants

Figure 2.

Standardized effect size of pill groups compared with no-pill group. AUC = area-under-the-curve global severity; IL-8 = interleukin 8; WURSS = Wisconsin Upper Respiratory Symptom Survey. Note: Central black boxes represent Cohen’s d standardized effect size; Error bars are 95% confidence intervals. These results come from a mixed general linear model adjusting for potential confounders: duration before entry, cold severity at entry, age, sex, ethnicity, education, income, smoking status, physical and mental health at entry, and assignment to the 3 clinician-related groups. Raw unadjusted results were similar.