Comparison of no medication, placebo, and hyoscyamine for reducing pain during a barium enema

Placebo interventions for all clinical conditions

BACKGROUND

Placebo interventions are often claimed to substantially improve patient-reported and observer-reported outcomes in many clinical conditions, but most reports on effects of placebos are based on studies that have not randomised patients to placebo or no treatment. Two previous versions of this review from 2001 and 2004 found that placebo interventions in general did not have clinically important effects, but that there were possible beneficial effects on patient-reported outcomes, especially pain. Since then several relevant trials have been published.

OBJECTIVES

Our primary aims were to assess the effect of placebo interventions in general across all clinical conditions, and to investigate the effects of placebo interventions on specific clinical conditions. Our secondary aims were to assess whether the effect of placebo treatments differed for patient-reported and observer-reported outcomes, and to explore other reasons for variations in effect.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library Issue 4, 2007), MEDLINE (1966 to March 2008), EMBASE (1980 to March 2008), PsycINFO (1887 to March 2008) and Biological Abstracts (1986 to March 2008). We contacted experts on placebo research, and read references in the included trials.

SELECTION CRITERIA

We included randomised placebo trials with a no-treatment control group investigating any health problem.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trial quality and extracted data. We contacted study authors for additional information. Trials with binary data were summarised using relative risk (a value of less than 1 indicates a beneficial effect of placebo), and trials with continuous outcomes were summarised using standardised mean difference (a negative value indicates a beneficial effect of placebo).

MAIN RESULTS

Outcome data were available in 202 out of 234 included trials, investigating 60 clinical conditions. We regarded the risk of bias as low in only 16 trials (8%), five of which had binary outcomes.In 44 studies with binary outcomes (6041 patients), there was moderate heterogeneity (P < 0.001; I(2) 45%) but no clear difference in effects between small and large trials (symmetrical funnel plot). The overall pooled effect of placebo was a relative risk of 0.93 (95% confidence interval (CI) 0.88 to 0.99). The pooled relative risk for patient-reported outcomes was 0.93 (95% CI 0.86 to 1.00) and for observer-reported outcomes 0.93 (95% CI 0.85 to 1.02). We found no statistically significant effect of placebo interventions in four clinical conditions that had been investigated in three trials or more: pain, nausea, smoking, and depression, but confidence intervals were wide. The effect on pain varied considerably, even among trials with low risk of bias.In 158 trials with continuous outcomes (10,525 patients), there was moderate heterogeneity (P < 0.001; I(2) 42%), and considerable variation in effects between small and large trials (asymmetrical funnel plot). It is therefore a questionable procedure to pool all the trials, and we did so mainly as a basis for exploring causes for heterogeneity. We found an overall effect of placebo treatments, standardised mean difference (SMD) -0.23 (95% CI -0.28 to -0.17). The SMD for patient-reported outcomes was -0.26 (95% CI -0.32 to -0.19), and for observer-reported outcomes, SMD -0.13 (95% CI -0.24 to -0.02). We found an effect on pain, SMD -0.28 (95% CI -0.36 to -0.19)); nausea, SMD -0.25 (-0.46 to -0.04)), asthma (-0.35 (-0.70 to -0.01)), and phobia (SMD -0.63 (95% CI -1.17 to -0.08)). The effect on pain was very variable, also among trials with low risk of bias. Four similarly-designed acupuncture trials conducted by an overlapping group of authors reported large effects (SMD -0.68 (-0.85 to -0.50)) whereas three other pain trials reported low or no effect (SMD -0.13 (-0.28 to 0.03)). The pooled effect on nausea was small, but consistent. The effects on phobia and asthma were very uncertain due to high risk of bias. There was no statistically significant effect of placebo interventions in the seven other clinical conditions investigated in three trials or more: smoking, dementia, depression, obesity, hypertension, insomnia and anxiety, but confidence intervals were wide.Meta-regression analyses showed that larger effects of placebo interventions were associated with physical placebo interventions (e.g. sham acupuncture), patient-involved outcomes (patient-reported outcomes and observer-reported outcomes involving patient cooperation), small trials, and trials with the explicit purpose of studying placebo. Larger effects of placebo were also found in trials that did not inform patients about the possible placebo intervention.

AUTHORS' CONCLUSIONS

We did not find that placebo interventions have important clinical effects in general. However, in certain settings placebo interventions can influence patient-reported outcomes, especially pain and nausea, though it is difficult to distinguish patient-reported effects of placebo from biased reporting. The effect on pain varied, even among trials with low risk of bias, from negligible to clinically important. Variations in the effect of placebo were partly explained by variations in how trials were conducted and how patients were informed.

Is the placebo powerless? Update of a systematic review with 52 new randomized trials comparing placebo with no treatment

BACKGROUND

It is widely believed that placebo interventions induce powerful effects. We could not confirm this in a systematic review of 114 randomized trials that compared placebo-treated with untreated patients.

AIM

To study whether a new sample of trials would reproduce our earlier findings, and to update the review.

METHODS

Systematic review of trials that were published since our last search (or not previously identified), and of all available trials.

RESULTS

Data was available in 42 out of 52 new trials (3212 patients). The results were similar to our previous findings. The updated review summarizes data from 156 trials (11 737 patients). We found no statistically significant pooled effect in 38 trials with binary outcomes, relative risk 0.95 (95% confidence interval 0.89-1.01). The effect on continuous outcomes decreased with increasing sample size, and there was considerable variation in effect also between large trials; the effect estimates should therefore be interpreted cautiously. If this bias is disregarded, the pooled standardized mean difference in 118 trials with continuous outcomes was -0.24 (-0.31 to -0.17). For trials with patient-reported outcomes the effect was -0.30 (-0.38 to -0.21), but only -0.10 (-0.20 to 0.01) for trials with observer-reported outcomes. Of 10 clinical conditions investigated in three trials or more, placebo had a statistically significant pooled effect only on pain or phobia on continuous scales.

CONCLUSION

We found no evidence of a generally large effect of placebo interventions. A possible small effect on patient-reported continuous outcomes, especially pain, could not be clearly distinguished from bias.

Placebo interventions for all clinical conditions

BACKGROUND

Placebo interventions are often claimed to improve patient-reported and observer-reported outcomes, but this belief is not based on evidence from randomised trials that compare placebo with no treatment.

OBJECTIVES

To assess the effect of placebo interventions.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 4, 2002), MEDLINE (1966 to 2002), EMBASE (1980 to 2002), Biological Abstracts (1986 to 2002), and PsycLIT (1887 to 2002). We contacted experts on placebo research, and read references in the included trials.

SELECTION CRITERIA

We included randomised placebo trials with a no-treatment control group investigating any health problem.

DATA COLLECTION AND ANALYSIS

Two reviewers independently assessed trial quality and extracted data. We contacted study authors for additional information.

MAIN RESULTS

Outcome data were available in 156 out of 182 included trials, investigating 46 clinical conditions. We found no statistically significant pooled effect of placebo in 38 studies with binary outcomes (4284 patients), relative risk 0.95 (95% confidence interval (CI) 0.89 to 1.01). The pooled relative risk for patient-reported outcomes was 0.95 (95% CI 0.88 to 1.03) and for observer-reported outcomes 0.91 (95% CI 0.81 to 1.03). There was heterogeneity (P=0.01) but the funnel plot was symmetrical. There was no statistically significant effect of placebo interventions in the four clinical conditions investigated in three trials or more: pain, nausea, smoking, and depression, but confidence intervals were wide. We found an overall effect of placebo treatments in 118 trials with continuous outcomes (7453 patients), standardised mean difference (SMD) -0.24 (95% CI -0.31 to -0.17). The SMD for patient-reported outcomes was -0.30 (95% CI -0.38 to -0.21), whereas no statistically significant effect was found for observer-reported outcomes, SMD -0.10 (95% CI -0.20 to -0.01). There was heterogeneity (P<0.001) and large variability in funnel plot results even for big trials. There was an apparent effect of placebo interventions on pain (SMD -0.25 (95% CI -0.35 to-0.16)), and phobia (SMD -0.63 (95% CI -1.17 to -0.08)); but also a substantial risk of bias. There was no statistically significant effect of placebo interventions in eight other clinical conditions investigated in three trials or more: nausea, smoking, depression, overweight, asthma, hypertension, insomnia and anxiety, but confidence intervals were wide.

REVIEWERS' CONCLUSIONS

There was no evidence that placebo interventions in general have clinically important effects. A possible small effect on continuous patient-reported outcomes, especially pain, could not be clearly distinguished from bias.