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What is the Placebo Effect?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on May 26, 2022.

A placebo (or dummy pill) is an inert (inactive) substance, typically a tablet, capsule or other dose form that does not contain an active drug ingredient. For example, placebo pills or liquids may contain starch, sugar, or saline. Physical placebos, or “sham” treatments have also been used, such as inactive acupuncture devices.

Placebos are often used in clinical trials as an inactive control so that researchers can better evaluate the true overall effect of the experimental drug treatment under study. In these clinical trials, one subset of patients would receive the placebo and one group would receive the experimental drug, but neither group is aware of which treatment they have received. In addition, researchers in the study would not know which patients have received active or placebo treatments.

These studies are called “double-blind” and “placebo-controlled” and are considered the gold standard for experimental drug research. However, unexpected high placebo rates in clinical trials can be detrimental, undermining the true effect of an active treatment.

Why are placebos used in drug studies?

Conducting a double-blind, placebo-controlled clinical trial helps to eliminate any bias that might occur due to knowledge of who receives which treatments. A patient or researcher would expect those who receive the active drug to have a better outcome than those who did not, and this knowledge can introduce bias into the study.

Typically, an experimental drug treatment needs to be statistically more effective than the placebo to be considered as a valid drug treatment. Including a placebo group in a study is also beneficial in evaluating treatment side effects.

However, many clinical trials, such as those in cancer research, do not include placebo groups because it would not be unethical to leave the patient’s cancer untreated. In these trials, the experimental drug may be compared to a treatment that is already FDA-approved instead of a placebo.

For example, in a study in The Lancet, researchers looked at 21 antidepressants used for the acute treatment of adults with major depressive disorder. The antidepressants were either studied against placebo or in head-to-head trials against each other.

  • Data were included from 522 trials with 116,477 participants in a systematic review and meta-analysis. The researchers found that all antidepressants were more effective than placebo in terms of effectiveness.
  • They also ranked different antidepressants based on effectiveness and safety. This data can inform clinicians who treat patients with antidepressants that these drugs are more useful than using a placebo. This study also helped to rank the active drugs based on effectiveness and safety.

What is a placebo effect?

Research has shown that a placebo treatment can have a positive therapeutic effect in a patient, even though the pill or treatment is not active. This is known as the “placebo effect” or “placebo response”.

Placebo effects have been reported to occur in 21% to 40% of patients depending upon the study type. Is there any proof that the placebo effect occurs?

  • In pain studies utilizing brain imaging, it has been shown that administration of a placebo to patients who believed they were receiving an analgesic (pain-relieving) medication led to activation of the endogenous opioid system in the brain.
  • Endogenous opioids, such as endorphins and enkephalins, are natural pain-relieving chemicals produced in the body. Analgesia (pain relief) due to the placebo effect is dependent upon the activation of theses endogenous opioids in the brain.
  • It has also been shown that the placebo response in patients with post-surgical pain could be blocked by the opiate antagonist naloxone, further lending support to the placebo effect. Naloxone is an opioid antagonist that would reverse the pain-relieving effects of opioids.
  • Dopamine, another central nervous system neurotransmitter, has also been shown to be activated in the brain after placebo administration to patients with Parkinson’s disease.

However, even when patients know they are receiving a placebo, there may be beneficial effects.

A study in Scientific Review found that an open-label placebo (OLP) may reduce fatigue in cancer survivors. They compared OLP to treatment as usual (TAU) for cancer survivors with fatigue in a 21-day randomized controlled trial.

The researchers found that participants in the placebo group reported a 29% improvement in fatigue severity and a 39% improvement in fatigue-disrupted quality of life compared to those randomized to TAU. Similar reductions in fatigue and boosts in quality of life were found when the TAU group was switched to the OLP for 21 days at the end of the study.

However, some studies have found differing results. A Cochrane Review of 202 trials comparing placebo treatment with no treatment reported that placebos produced no major health benefits, but did have a modest effect on patient-reported outcomes, such as in pain and nausea, although results were variable.

The authors explained that the observed variations in the placebo effect could be explained by trial design differences and how patients were informed of their treatments.

What is a nocebo effect?

A nocebo effect is the opposite of the placebo effect -- a negative psychological effect of a treatment with no therapeutic activity. This can occur when the placebo is administered and accompanied by the suggestion that the patients ailment will get worse. High nocebo effects can also interfere with interpretation of clinical trial results.

Negative effects of drugs may be due to psychological nocebo effects and not necessarily due to the drug itself.

It is ethical to use a placebo?

In clinical practice, physicians may prescribe placebo treatments with or without the patients knowledge that they are receiving an inactive therapy. The therapeutic use of placebo or sham treatments in medicine is very controversial.

Placebos have been used in treatment of sleep, anxiety, gastrointestinal disorders, chronic pain and other disorders.

Psychologically, the patient may be encouraged that they are receiving a treatment for their ailment that they believe will have beneficial effect, and in turn the placebo may actually provide some relief. However, the effect would not be due to a pharmacological action attributed to the chemical composition of the medicine.

In one survey, only 3% of U.S. physicians reported using actual sugar pills as placebos, but 41% said they had used over-the-counter painkillers and 38 percent said they had used vitamins as placebos for their patients. Sixty-eight percent of physicians described the placebo to their patients as a potentially beneficial medicine, and roughly two-thirds of the doctors felt the practice was ethical.

In another study, physicians used reduced doses of anti-inflammatory medications mixed in combination with a placebo to successfully treat psoriasis patients. Combining active drug with placebo may be effective in diseases that involve the mental state and immune system, including:

  • asthma
  • multiple sclerosis
  • chronic pain

Reducing doses of active drugs and combining with placebo treatment could also reduce side effects, addiction potential and cost.

Bottom Line

  • A placebo (or dummy pill) is an inactive substance, typically a tablet, capsule or other dose form that does not contain an active drug ingredient and has no pharmacologic action.
  • Research has shown that a placebo treatment can have a positive therapeutic effect in a patient, even though the pill or treatment is not active. This is known as the “placebo effect” or “placebo response”.
  • Studies differ on how effective the placebo effect can be as a therapeutic effect, but many studies show a positive effect. Placebos have been used in treatment of sleep, anxiety, gastrointestinal disorders, chronic pain and other disorders.

See also


  • Hoenemeyer TW, TKaptchuk TJ, Mehta TS. Open-Label Placebo Treatment for Cancer-Related Fatigue: A Randomized-Controlled Clinical Trial. Scientific Reports. Vol. 8, Article number: 2784 (2018). Accessed May 26, 2022 at
  • 21 Reviewed Antidepressants Top Placebo for Major Depression. February 22, 2018.
  • Enck P, Benedetti F, Schedlowski M. New insights into the placebo and nocebo responses. Neuron. 2008 Jul 31;59(2):195-206. doi: 10.1016/j.neuron.2008.06.030
  • Meissner K, Kohls N, Colloca L. Introduction to placebo effects in medicine: mechanisms and clinical implications. Philos Trans R Soc Lond B Biol Sci. 2011;366(1572):1783-1789. doi:10.1098/rstb.2010.0414
  • Hróbjartsson A, Gøtzsche PC. Placebo interventions for all clinical conditions. Cochrane Database of Systemic Review 2010, Issue 1. Art No: CD003974. DOI: 10.1002/14651858.CD003974.pub3. 
  • Tilburt JC, Emanuel EJ, Kaptchuk TJ, et al. Prescribing "placebo treatments": results of national survey of US internists and rheumatologists. BMJ. 2008 Oct 23;337:a1938. doi: 10.1136/bmj.a1938.

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