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Last updated: Thu, Aug 1, 2024
A placebo is a fake treatment such as a sugar pill. Placebos are often used in clinical trials, such as in the trial of a new drug. The active drug is given to some of the subjects and the sugar pill is given to the others. The effectiveness of the new drug is the difference between the first and second groups of patients.
In other trials there are three groups of subjects. One of the groups receives no treatment at all, while the second and third groups receive the sugar pill (placebo treatment) and the real pill. In such trials, the placebo group often does better than the no-treatment group. This phenomenon sheds some additional light on the role of the mind in pain.
The reported size of the placebo effect is highly variable. A 2004 study reviewed 114 reports of clinical studies in which there was a no-treatment group. (The no-treatment group is needed to estimate the effect of the placebo treatment.) The review found little evidence for a strong placebo effect.1 On the other hand, the placebo effect tends to be four to five times as large in studies that are designed specifically to investigate the placebo effect.2 In one study, 35% of subjects reported a marked reduction in pain after being given a placebo. This is remarkable since morphine relieves severe pain in only about 70% of patients. Another study showed that a standard morphine dose was effective for only 54% of people who didn't respond to a placebo, but for 95% of people who did.3
Studies like those mentioned above tend necessarily to rely on the self-report of patients in estimating the effectiveness of treatments, whether they are "real" or placebo treatments. These self-reports are subject to a number of types of uncertainty, which in the context of a study are generally assumed to work unconsciously: 1) the subject's pain "anchor" point, the level against which he or she makes comparisons, may drift; 2) the subject may overweigh or underweigh moments of high pain or of low pain; 3) the subject may believe that there is a "correct" level of pain and tend to bias toward that level; 4) the subject may feel a need to be consistent with his/her own prior reports; 5) the subject may bias his/her report toward what he/she would like to happen.4 These uncertainties are present whenever pain levels are assessed. Perhaps placebo effects are simply the result of these uncertainties in measurement?
Re-enter the high-tech sleuthing devices of the modern neuroscientist, the PET, MEG, and fMRI studies. A large number of such studies show that three things occur within the central nervous system when the subject reports placebo-caused reduction in pain. First, the areas of the brain that make up the "pain matrix" show reduced activation. Second, brain circuits that modulate (in this case, reduce) pain show increased activation. Third, the endogenous opioid-based pain modulating system is engaged. This third finding is further confirmed by studies in which naloxone, which defeats the action of endogenous opioids, is injected into the subject. With naloxone present, the placebo effect is much reduced or eliminated. All of this strongly suggests that the subject doesn't merely believe that he or she is experiencing less pain, but in fact he or she is experiencing less pain.
Researchers have come to believe that the placebo effect is the combined result of expectation, environmental cues, and social cues. The expectation of less pain may be conscious or subconscious. Experimenters have found that subjects can be conditioned to experience less pain. For example, experimenters may apply an inert (placebo) cream to the subject's skin and surreptitiously turn down the intensity of the shock or the heat that the subject will feel. The subject thus learns to associate the cream with lower pain. If the subject is subsequently tested with cream, the subject will report less pain for an equal level of shock or heat. This effect will persist longer if the subject has been conditioned more thoroughly (more trials for a greater length of time).5
The term “nocebo” is sometimes used to mean a negative placebo. A nocebo generates an expectation of increased pain. In one study, experimenters were able, using only verbal instructions, to cause nitrous oxide (“laughing gas”) to increase rather than decrease dental pain. Nocebo treatments are said to have more powerful and longer-lasting effects than placebo treatments.6
It isn't clear just what conditions are necessary and sufficient to create a significant placebo effect. Researchers have looked for personality traits that may be common among those who respond strongly to placebo treatments. They have not found consistent results. Recent researchers have used the names "suggestibility," "optimism," "expectation," "behavioral activation," and "desire for relief" to label the supposed trait that makes an individual susceptible to/capable of placebo relief. One study evaluated a group of women for their placebo responses to three different kinds of pain, and found that the placebo response differed for different kinds of pain. A more recent study found that the response of subjects to a placebo pill was different depending on the brand name that was used.7 Those who respond strongly to placebo in one context and with one type of pain don't necessarily respond strongly in another.
Research into placebo effects will continue. One of its motivations is the hope that it may be used to improve the effectiveness of clinical treatments. Although, as with most pain research, most placebo research deals with brief stimulus and brief effects, and so may not be directly applicable to chronic pain states, the hope is that it will be. The very real phenomenon of placebo pain relief lends some support to some of the cognitive-behavioral ideas that are discussed in The CBT Model of Pain and Pain Behavior. These same phenomenon may also be the small kernel of truth behind the largely-discredited ideas that back up the concept of psychogenic pain.
If a patient does not consent to therapy [acupuncture] with positive engagement, the physician should not proceed as the therapy will not succeed.