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The placebo effect is the phenomenon that a patient's symptoms can be alleviated by a sham 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 are
conducted.
If patients complaining of back pain are administered only a placebo, for example,
typically about a quarter of them will report a relief or diminution of the pain. Remarkably, not only do patients
report improvement, the improvements often are objectively measurable, and the same improvements typically are 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 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).
The size of the placebo effect is controversial. 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 oftentimes 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, the 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 (Beecher Hk, The Powerful Placebo. Journal of the American
Medical Association 1955;159:1602-6). 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 revert towards the
mean - i.e. show better than average levels of recovery. A recent study also questioned the placebo effect. 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.
The reverse effect, sometimes called the Nocebo
Effect, is the result of negative expectations by the patient. Some patients who believe that they should
be ill can exhibit genuine symptoms.
The Nocebo Effect may or may not be distinguished from older concepts such as sociogenic illness or
psychosomatic illness. In any case, distinguishing
between these effects can be a difficult problem for medical semiotics, or the
study of how to interpret the patient's description of their ailments.
The strictest definition would be the negative effect produced by an expectation that a medicine or treatment does not work in
general or for the patient in particular. Sociogenic illness and psychosomatic illness might therefore be broader terms where a
known or unknown cause with cultural or psychological factors is involved.
Example: I believe that cough medicine does not work. I suffer, therefore, from the Nocebo
effect--the medicine is less effective for me than for a patient who believes the medicine actually prevents coughs.
Neuroscience research on the placebo effect
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.
References
- Margaret Talbot: "The Placebo Prescription", The New York Times, January 9, 2000
- Gina Kolata: "Placebo Effect Is More Myth Than Science, Study Says", The New York Times, May 24, 2001
- Dana Desonie: "The Placebo Effect: Real or Imagined?", http://www.studyworksonline.com/cda/content/article/0,1034,NAV4-42_SAR1112,00.html
- "Placebo-Induced
Changes in fMRI in the Anticipation and Experience of Pain "
by T. D. Wager et al. in Science volume 303, pages
1162-1167, 20 February 2004.
- "Conscious
expectation and unconscious conditioning in analgesic, motor, and hormonal placebo/nocebo responses "
by F. Benedetti et al. in Journal of Neuroscience volume 23, pages 4315-4323, 15 May 2003.
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