The Online Encyclopedia and Dictionary






Placebo effect

The placebo effect (also known as non-specific effects) is the phenomenon that a patient's symptoms can be alleviated by an otherwise ineffective treatment, apparently because 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.

The opposite effect, in which a patient who disbelieves in a treatment receives less benefit, is known as the nocebo effect.


Placebo-controlled studies

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

It is still controversial if a placebo effect exists. A part of the controversy may be due to the fact that patients who have been given a drug (or a placebo for that matter) will often 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. Hence, there is uncertainty over the real size of the placebo effect. An often quoted number is that about one third of patients improve on a placebo, but a recent study has called that number into question, 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.

It has been argued that the placebo effect may be simply an experimental artefact that arises because the patients in any study are selected to show a particular characteristic (symptom of illness). Since the sample is thus initially skewed away from the mean, it is likely that during the course of the study there will be a natural tendency for the sample to regress towards the mean, i.e. show an improvement of health condition.

A recent study also questioned the placebo effect (A. Hrˇbjartsson & P. C. G÷tzsche, 2001). The authors found 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. Most studies however only use a placebo group as control. The authors concluded that the placebo effect is overrated, and that studies in the future should have a no-treatment group when possible, to make sure that effects would not be attributed to a placebo effect while they are totally natural effects that would have occurred anyway. In a follow-up study (A. Hrˇbjartsson & P. C. G÷tzsche, 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".

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.



  • The Placebo Effect: An Interdisciplinary Exploration. 1997. Edited by Anne Harrington. Cambridge: Harvard University Press. ISBN 067466984-X
  • "The Placebo Prescription" by Margaret Talbot in The New York Times, January 9, 2000

Does the placebo effect exist

Placebo and pain

External links

Last updated: 06-02-2005 13:03:17
Last updated: 08-17-2005 14:06:58