Fig. 1

Placebo pill ingestion diminishes back pain intensity while trial participation non-specifically decreases qualitative pain outcomes. a Experimental design and time line: CBP patients entered a six-visit (V1-6) 8-week randomized controlled trial, including baseline (BL), treatment (T1, T2), and washout (W1, W2) periods. Participants entering and completing the study are indicated. b Example time series of smartphone app pain ratings in 2 patients (values in parentheses are numbers of ratings). c The patients receiving placebo treatment (PTx) showed lower pain levels during the last week of each treatment period, compared to the patients in the no treatment arm (NoTx). d Within-subject permutation tests between pain ratings entered during BL and T1 or T2 identified responders in PTx and NoTx groups. The group-averaged %change in phone app ratings of back pain intensity (2 bins/week) is displayed in d. e The magnitude of response represents the stronger response between the two treatment periods and is displayed in original score (left) and after conversion in %analgesia (right). f Placebo analgesia was present the first day after placebo pill ingestion. g, h Pain intensity decreased by about 20–30% for numerical rating scale (NRS) and memory during both treatment periods. i Qualitative outcome measures (McGill pain questionnaire, MPQ-sensory) decreased in time similarly in all groups, including the NoTx and PTxNonR. j Principal component analyses clustered pain outcomes into 2 factors, segregating intensity and quality. Group by time by Pain (intensity vs quality) interaction indicated that only the pain intensity was diminished by placebo pill ingestion. In c–i number of subjects are in parentheses, ^&p < 0.12; *p < 0.05; **p < 0.01; ***p < 0.001, n.s. not significant. Error bars indicate SEM. For each figure, a description of the analyses and p-values are reported in Supplementary Table 7