Placebo

A placebo is a procedure that contains no active substance but resembles the active intervention (such as a pill, capsule, or injection). In randomized controlled trials, it is used as a comparator to separate the effect of a drug or supplement from the effects of expectation, natural recovery, and measurement (RCT).

How large is the placebo effect?

The Cochrane review “Placebo interventions for all clinical conditions” compiled randomized trials comparing a placebo with a group given no treatment at all: 234 trials, 60 clinical conditions; outcome data were available in 202 trials and only 16 (8%) were considered at low risk of bias. The authors reported that they found no important clinical effects of placebo interventions overall. However, in certain settings, placebo could influence patient-reported outcomes, particularly pain and nausea; it was difficult to separate the patient-reported placebo effect from biased reporting. The effect on pain varied from insignificant to clinically important, even in trials at low risk of bias. ^[1]

Why does it matter?

Improvement seen in a study without a control group cannot be attributed to the intervention, because the cause of the improvement cannot be distinguished. On this site, “case series without a control group” therefore receive at most grade D (Levels of evidence).

For the difference between study types: How is a study read?.