Placebo effect

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The placebo effect (placebo, translating from Latin as "I shall please", is also known as non-specific effects and the subject-expectancy effect) is the phenomenon that a patient's symptoms can be alleviated by an otherwise ineffective treatment, since the individual expects or believes that it will work. Some people consider this to be a remarkable aspect of human physiology; others consider it to be an illusion arising from the way medical experiments were conducted.

In the opposite effect, a patient who disbelieves in a treatment may experience a worsening of symptoms. This nocebo effect (nocebo translates from Latin as "I shall harm") can be measured in the same way as the placebo effect, e.g., when members of a control group receiving an inert substance report a worsening of symptoms. The recipients of the inert substance may nullify the placebo effect intended by simply having a negative attitude towards the effectiveness of the substance prescribed, which often leads to a nocebo effect, which is not caused by the substance itself, but more the patient's mentality towards her or his ability to get well.


Placebo-controlled studies

It was reported that about a quarter of patients who were administered a placebo, e.g. against back pain, reported a relief or diminution of pain. Remarkably, not only did the patients report improvement, but the improvements themselves were often objectively measurable, and the same improvements were typically not observed in patients who did not receive the placebo.

Because of this effect, government regulatory agencies approve new drugs only after tests establish not only that patients respond to them, but also that their effect is greater than that of a placebo (by way of affecting more patients, by affecting responders more strongly or both). Such a test or clinical trial is called a placebo-controlled study. Because a doctor's belief in the value of a treatment can affect his or her behaviour, and thus what his or her patient believes, such trials are usually conducted in "double-blind" fashion: that is, not only are the patients made unaware when they are receiving a placebo, the doctors are made unaware too. Recently, it has even been shown that "mock" surgery can have similar effects, and so some surgical techniques must be studied with placebo controls (rarely double blind, for obvious reasons).

Does the placebo effect exist?

An alternate opinion attributes the false perception of a placebo effect to the fact that patients who have been given a placebo report improvement earlier and more eagerly in order to please and thank the care giver. These patients may even do this when there is no real physical improvement attained. One quoted figure is that about one third of patients improve on a placebo, but a recent study has called that number into question (A. Hróbjartsson & P. C. Götzsche, 2001), claiming that the effect is much smaller, if it exists at all. The 30 percent figure derives from a paper by Henry Beecher, published in 1955 (H. Beecher, 1955). Beecher was one of the leading advocates of the need to evaluate treatments by means of double-blind trials and this helps to explain why it has been so widely quoted.

The Hróbjartsson & Götzsche study demonstrated that in many studies where a control group was used that did not get any treatment at all, the effects in the no-treatment group were almost equal to the effects in the placebo group for studies with binary outcomes (e.g. well treated or poorly treated). The authors concluded that the placebo effect does not have "powerful clinical effects," and conceded that placebos have "possible small benefits in studies with continuous subjective outcomes and for the treatment of pain." Their study suggested that there was no use of placebos outside of clinical trials.

In a follow-up study (A. Hróbjartsson & P. C. Götzsche, 2004) the same authors were able to confirm their previous results and concluded: "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".

See also: Hawthorne Effect

Notable placebo effect absences

In psychological treatment, two disorders are known to have very low placebo effects: schizophrenia, and obsessive compulsive disorder.

Placebo and pain

Studies on humans are revealing the neural mechanisms of placebo effects. Some of the best research is in the area of pain research. People can be conditioned to expect analgesia in certain situations. When those conditions are provided to the patient, the brain responds by generating a pattern of neural activity that produces objectively quantifiable analgesia.

Evans (2004) argues that the placebo effect works through a suppression of the acute phase response, and as a result does not work in medical conditions that do not feature this. The acute phase response consists of inflammation and sickness behaviour:

  • Four classic signs of ‘inflammation’: tumor, rubor, calor and dolor – swelling, redness, heat and pain.
  • Sickness behaviour: lethargy, apathy, loss of appetite and increased sensitivity to pain.

Placebo and depression

A brain-imaging study (Leuchter, 2002) found that depressed patients who responded to the placebo effect showed changes in cerebral blood flow, which were different to the changes in brain function seen in patients who responded to anti-depressant medication. Other studies such as (Khan, 2000) have shown that up to 75% of the effectiveness of anti-depressant medication is due to the placebo-effect rather than the treatment itself.



  • The Placebo Effect: An Interdisciplinary Exploration. 1997. Edited by Anne Harrington. Cambridge: Harvard University Press. ISBN 067466984-X
  • Dylan Evans, The Placebo Effect: mind over matter in modern medicine 2004. HarperCollins (UK) / Oxford University Press (US)
  • "The Placebo Prescription" by Margaret Talbot in The New York Times, 9 January 2000

Does the placebo effect exist

Placebo and pain

Placebo and depression

External links

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