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What is the placebo effect?

The most important point to remember about the placebo effect is that it is simply the name given to a collection of observed events, the causes of which are unknown. Indeed it is not even known how many causes are involved. As such the placebo effect is not a scientifically defined process, and the terms 'placebo' and 'placebo effect' can often be used in ways which cause confusion. One common and serious example of this confusion is the fact that the term 'placebo effect' can be used to mean two different things.


Actual changes

It has been observed that real changes in health can occur in people who believe that they are having a treatment, when in fact the procedure or medication is considered to be inactive. Because this was first observed in patients who felt that they had benefited, the response was called ‘the placebo effect’ (from the Latin ‘I shall please’). Similarly, symptoms of damage can occur when a person believes that they are being harmed, and this is sometimes called ‘the nocebo effect’. In both these cases, if the person is subsequently made aware of the fact that they have received an inactive treatment, the changes can revert to the former state.


The important point about this effect is that real changes take place.


Imagined changes

The term is also often used to refer to a similar but significantly different effect, where a person feels better after an ‘inactive’ treatment, but presents no observable change.



The confusion of these two different types of reaction not only shows a lack of scientific clarity about the placebo effect, but is often deliberately used to mislead people by

  • Implying that the placebo effect is illusory;

  • Equating the placebo effect with other emotional responses;

  • Ignoring the need to explain how it leads to highly specific real changes in health.

The last of these is of particular importance, since the placebo effect is often used as a 'scientific' explanation of the success of complementary and alternative medicine.


In fact scientists are unable to offer an explanation of the actual mechanism by which a person’s expectations of the results of the treatment can produce an enormously wide range of specific changes. Furthermore they are unable to distinguish between the placebo effect and the effect of an active substance other than by knowing which treatment was given in each case. In other words this 'scientific' explanation is itself scientifically inexplicable.


Its use in trials

The existence of the placebo effect has meant that trials of medical treatments have to take into account benefits not attributable to the intervention being tested. As a result the randomised controlled trial (RCT) has been developed, where the effects of a treatment are compared with those of an ‘inactive’ version in order to establish whether the treatment produces a benefit over and above any placebo effect. The problem is that because the mechanism of action of the placebo effect is unknown, there can be no certainty about whether all the relevant factors have been taken into account.


When it comes to testing homeopathic treatment, there are further complications. Homeopathy is practised according to 'laws' which govern the relationship of treatment to the patient's illness, including what can actually be treated at a particular time. Thus:

  • If the treatment is not appropriately individualised in some cases, then in those cases a placebo would be being tested instead of a treatment.

  • If the treatment was trying to treat conditions in the wrong order in some cases, then in those cases a placebo would be being tested instead of a treatment.

In other words, unless the trial takes these ‘laws’ fully into account, the treatment being tested may actually be a mixture of ‘active’ and ‘inactive’ (placebo) treatments, reducing the possibility of a difference between the results from the test group and the placebo group.


Finally, as an explanation of the successes of homeopathic treatment, the placebo effect can have no scientific credibility since it is itself inexplicable. Indeed homeopaths are able to distinguish between the effects of successful treatment, those of the placebo effect, and those of imagined benefit, and homeopathy is far better able to explain these results than the placebo effect is able to explain homeopathy.


Related pages:

Why it works

Law of cure



Orthodox medicine

What is effectiveness?

What are efficacy and effectiveness?

What is evidence?

What are randomised controlled trials (RCTs)?

What factors affect RCTs of homeopathy?


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