What is the placebo effect, and how strong is it?

The placebo effect is when a person's health appears to improve after a dummy treatment, such as a sugar pill, a saline injection or a fake procedure, because they expect it to help. A large Cochrane review found no important clinical effects in general, though placebos can modestly influence patient-reported outcomes such as pain and nausea. Popular figures for how often placebos work are higher than that review suggests. [1] [2]

A placebo is an inactive substance or action that resembles a real treatment. The word is Latin for 'I will please'. The effect is attributed to the person's belief and expectation rather than to anything in the placebo itself. [betterhealth.vic.gov.au](https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/placebo-effect) describes it this way, and [michiganmedicine.org](https://www.michiganmedicine.org/health-lab/studies-and-real-life-placebos-have-powerful-healing-effect-body-and-mind) says researchers link it to positive expectations. [1] [3]

The NIH explains that expecting relief can switch on the body's natural mechanisms, since the brain makes substances that lessen pain, stress and anxiety. According to [newsinhealth.nih.gov](https://newsinhealth.nih.gov/2023/01/powerful-placebo), this can help with pain, fatigue, depression, anxiety or nausea, but it cannot shrink tumors, lower cholesterol or clear infections. [4]

In clinical trials, one group gets the treatment being tested and another gets a placebo, often without participants knowing which. Both groups frequently improve, so a treatment counts as effective only if it does significantly better than the placebo. [health.clevelandclinic.org](https://health.clevelandclinic.org/placebo-effect) outlines this logic. [5]

Estimates differ by source. The Victorian government's health site says around one third of people taking placebos for complaints such as pain, headache and seasickness experience relief. An older textbook account on [ncbi.nlm.nih.gov](https://www.ncbi.nlm.nih.gov/books/NBK11141/) cites a study in which 75 percent of patients with postoperative wound pain reported satisfactory relief after a saline injection. Such figures count everyone who improved, which can include natural recovery, not only the placebo's own effect. [1] [6]

The 2022 Cochrane review is more cautious because it compared placebo with no treatment in 202 trials covering 60 conditions. It found placebos produced no major health benefits in general. For continuous outcomes the overall effect was small (standardised mean difference -0.23), with a slightly larger effect on patient-reported outcomes than observer-reported ones. Pain showed an effect of -0.28, and nausea a small but consistent one. For binary outcomes, the pooled effect was a relative risk of 0.93, and effects on pain, nausea, smoking and depression were not statistically significant, though confidence intervals were wide. [2]

The reviewers stress that results varied. The effect on pain ranged from negligible to clinically important even in well-run trials, and only 16 trials (8 percent) had low risk of bias. They also say it is hard to separate a true patient-reported effect from biased reporting. Larger effects were linked to physical placebos such as sham acupuncture, small trials, and trials that did not tell patients a placebo might be used. [2]