The Placebo Effect
What is the placebo effect?
The placebo effect describes changes associated with receiving an intervention that cannot be attributed to its specific active treatment component alone.
Placebo-controlled research
In controlled research, a placebo condition gives investigators something to compare against. Without it, an improvement observed during a study could reflect the intervention — or the passage of time, the act of being observed, the expectations of participants, or simple statistical variation.
Blinding — concealing which condition a participant has been assigned to — is a common method for reducing the influence of expectation on both participants and investigators.
A response with many contributors
What is observed in a placebo arm is rarely a single mechanism. It typically reflects a combination of influences, some psychological, some methodological, some purely statistical.
Mechanisms remain an active area of scientific inquiry. No single explanation currently accounts for the range of findings reported across the literature.
Total observed response
A conceptual illustration — not a quantitative formula. What a study measures can include influences from several sources at once.
- 01
Specific intervention effect
- 02
Natural history
- 03
Regression toward the mean
- 04
Expectation & context
- 05
Measurement effects
- 06
Other variables
Conceptual illustration. Relative contributions vary by study design, population and outcome measure.
What researchers have examined
- Expectation
- What a person anticipates an intervention will do.
- Conditioning
- Learned associations formed by prior treatment experience.
- Treatment ritual
- The form, frequency and setting of administration.
- Patient–provider interaction
- Communication, attention and the clinical relationship.
- Branding and presentation
- How an intervention looks and is described.
- Open-label placebo
- Administration without deception.
- Nocebo response
- Unfavourable outcomes associated with expectation.
Open-label placebo
Placebo without deception.
Unlike a conventional blinded placebo condition, participants receiving an open-label placebo are explicitly informed that the intervention is inert.
Researchers are investigating whether, when and why responses may still occur under these conditions. Evidence is evolving, and reported findings differ between subjective outcomes — such as self-reported symptom scales — and objective measures.
Read the research, not the marketing.
Independent sources. Citation does not imply endorsement of Efficacy Labs by these authors, journals or institutions, and none of this research was conducted using Efficacy Labs products.
National Center for Complementary and Integrative Health (NIH)
Placebo Effects: What the Science Says
A public-health overview of how placebo responses are studied and what researchers currently understand about them.
Ongoing
(opens in a new tab)Scientific Reports / PubMed
Open-label placebo treatment: a systematic review and meta-analysis
A pooled analysis of trials in which participants were told they were receiving a placebo. Reported effects vary by population, outcome and methodology.
2021
(opens in a new tab)PLoS ONE / PubMed
Placebos without deception: a randomized controlled trial in IBS
An early open-label placebo trial that prompted much of the subsequent research interest in non-deceptive placebo administration.
2010
(opens in a new tab)The New England Journal of Medicine
Placebo and nocebo effects
A review article describing mechanisms researchers have proposed for placebo and nocebo responses, and the limits of current evidence.
2020
(opens in a new tab)Beth Israel Deaconess Medical Center / Harvard Medical School
Program in Placebo Studies (PiPS)
An academic research program dedicated to the study of placebo effects and the patient–clinician relationship.
Ongoing
(opens in a new tab)National Library of Medicine
PubMed: open-label placebo literature
The primary index for peer-reviewed literature on placebo and open-label placebo research.
Ongoing
(opens in a new tab)
Common questions
- What is the placebo effect?
- The placebo effect describes changes associated with receiving an intervention that cannot be attributed to its specific active treatment component alone. It is studied as a phenomenon in its own right, not simply as noise in a trial.
- Why do studies use placebos?
- A placebo condition helps investigators distinguish effects attributable to an intervention from effects associated with expectation, context, natural variation and other influences.
- What is an open-label placebo?
- A placebo administered without deception: participants are explicitly told the intervention is inert. Researchers have reported placebo-associated effects in some open-label studies, while findings vary by population, outcome and methodology.
- What is the nocebo effect?
- The nocebo effect refers to unfavourable outcomes or side effects reported after an inert intervention, often studied in relation to negative expectation and information framing.