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A placebo (/pləˈsiːboʊ/ plə-SEE-boh; Latinplacēbō, "I shall please" from placeō, "I please") is a substance or treatment with no active therapeutic effect. A placebo may be given to a person in order to deceive the recipient into thinking that it is an active treatment. In drug testing and medical research, a placebo can be made to resemble an active medication or therapy so that it functions as a control; this is to prevent the recipient(s) and/or others from knowing (with their consent) whether a treatment is active or inactive, as expectations about efficacy can influence results. This psychological phenomenon, in which the recipient perceives an improvement in condition due to personal expectations, rather than the treatment itself, is known as the placebo effect or placebo response. Research about the effect is ongoing.
Placebos are an important methodological tool in medical research. Common placebos include inert tablets (like sugar pills), vehicle infusions, sham surgery, and other procedures based on false information. There is also some evidence that patients who know they are receiving a placebo still report subjective improvement in their condition if they are told that the placebo can make them feel better. It has further been observed that use of therapies about which patients are unaware may generally be less effective than using ones that patients are informed about, regardless of whether or not a placebo is involved.
Placebo effects are the subject of scientific research aiming to understand underlying neurobiological mechanisms of action in pain relief, immunosuppression, Parkinson's disease and depression. Brain imaging techniques done by Emeran Mayer, Johanna Jarco and Matt Lieberman showed that placebo can have real, measurable effects on physiological changes in the brain. Placebos can produce some objective physiological changes, such as changes in heart rate, blood pressure, and chemical activity in the brain, in cases involving pain, depression, anxiety, fatigue, and some symptoms of Parkinson’s. In other cases, like asthma, the effect is purely subjective, when the patient reports improvement despite no objective change in the underlying condition.
The placebo effect is a pervasive phenomenon; in fact, it is part of the response to any active medical intervention. The placebo effect points to the importance of perception and the brain's role in physical health. The use of placebos as treatment in clinical medicine (as opposed to laboratory research) is ethically problematic as it introduces deception and dishonesty into the doctor–patient relationship. The United Kingdom Parliamentary Committee on Science and Technology has stated that: "...prescribing placebos... usually relies on some degree of patient deception" and "prescribing pure placebos is bad medicine. Their effect is unreliable and unpredictable and cannot form the sole basis of any treatment on the NHS."
In 1955, Henry K. Beecher proposed that placebos could have clinically important effects. This view was notably challenged when, in 2001, a systematic review of clinical trials concluded that there was no evidence of clinically important effects, except perhaps in the treatment of pain and continuous subjective outcomes. The article received a flurry of criticism, but the authors later published a Cochrane review with similar conclusions (updated as of 2010). Most studies have attributed the difference from baseline until the end of the trial to a placebo effect, but the reviewers examined studies which had both placebo and untreated groups in order to distinguish the placebo effect from the natural progression of the disease.
The placebo effect can be produced by inert tablets, such as a sugar pills
In a 1983 article, Clement J. McDonald and others defined a placebo as "a substance or procedure... that is objectively without specific activity for the condition being treated". Under this definition, a wide variety of things can be placebos and exhibit a placebo effect. However, the placebo effect may be a component of legitimate pharmacological therapies: Pain-killing and anxiety-reducing drugs that are infused secretly without an individual's knowledge are less effective than when a patient knows they are receiving them. Likewise, the effects of stimulation from implanted electrodes in the brains of those with advanced Parkinson's disease are greater when they are aware they are receiving this stimulation. Sometimes administering or prescribing a placebo merges into fake medicine. Common placebos include pills ("sugar pills") or saline injections. Fake surgeries have also seen some use. An example is the Finnish Meniscal Legion Study Group’s trial published in The New England Journal of Medicine, which found a sham meniscal surgery to be equally effective to the actual procedure. While examples of placebo treatments can be found, defining the placebo concept remains elusive.
The placebo effect has sometimes been defined as a physiological effect caused by the placebo, but Moerman and Jonas have pointed out that this seems illogical, as a placebo is an inert substance that does not directly cause anything. Instead they introduced the term "meaning response" for the meaning that the brain associates with the placebo, which causes a physiological placebo effect. They propose that the placebo, which may be unethical, could be avoided entirely if doctors comfort and encourage their patients' health. Ernst and Resch also attempted to distinguish between the "true" and "perceived" placebo effect, as they argued that some of the effects attributed to the placebo effect could be due to other factors.
Research suggests that for psychological reasons, some placebos are more effective than others. Large pills seem to work better than small pills, colored pills work better than white pills, an injection is more powerful than a pill, and surgery gives a stronger placebo effect than injections do.
Research has also shown when it comes to specific psychological disorders, such as mild or moderate depression, placebos have the same effects compared to antidepressants.
The placebo effect has been controversial throughout history. Notable medical organizations have endorsed it, but in 1903 Richard Cabot concluded that it should be avoided because it is deceptive. Newman points out the "placebo paradox" – it may be unethical to use a placebo, but also unethical "not to use something that heals". He suggests to solve this dilemma by appropriating the meaning response in medicine, that is make use of the placebo effect, as long as the "one administering... is honest, open, and believes in its potential healing power".
How the placebo effect works
Because the placebo response is simply the patient response that cannot be attributed to an investigational intervention, there are multiple possible components of a measured placebo effect. These components have varying relevance depending on study design and the types of observations. While there is some evidence that placebo interventions can alter levels of hormones,endocannabinoids or endogenous opioids, other prominent components include expectancy effects, regression to the mean, and flawed research methodologies.
What a placebo really is
The placebo effect is related to expectations
The placebo effect is related to the perceptions and expectations of the patient; if the substance is viewed as helpful, it can heal, but, if it is viewed as harmful, it can cause negative effects, which is known as the nocebo effect. In 1985, Irving Kirsch hypothesized that placebo effects are produced by the self-fulfilling effects of response expectancies, in which the belief that one will feel different leads a person to actually feel different. According to this theory, the belief that one has received an active treatment can produce the subjective changes thought to be produced by the real treatment. Placebos can act similarly through classical conditioning, wherein a placebo and an actual stimulus are used simultaneously until the placebo is associated with the effect from the actual stimulus. Both conditioning and expectations play a role in placebo effect, and make different kinds of contribution. Conditioning has a longer-lasting effect, and can affect earlier stages of information processing. Those that think that a treatment will work display a stronger placebo effect than those that do not, as evidenced by a study of acupuncture.
A placebo presented as a stimulant will have this effect on heart rhythm and blood pressure, but when administered as a depressant, the opposite effect. Perceived ergogenic aids can increase endurance, speed and weight-lifting ability, leading to the question of whether placebos should be allowed in sport competition. In addition, there are ethical and moral concerns related to their use and we must be cautious of policies and practices that exploit placebos to the detriment of athletes’ health and well-being.
Because placebos are dependent upon perception and expectation, various factors that change the perception can increase the magnitude of the placebo response. For example, studies have found that the color and size of the placebo pill makes a difference, with "hot-colored" pills working better as stimulants while "cool-colored" pills work better as depressants. Capsules rather than tablets seem to be more effective, and size can make a difference. One researcher has found that big pills increase the effect while another has argued that the effect is dependent upon cultural background.
Motivation may contribute to the placebo effect. The active goals of an individual changes their somatic experience by altering the detection and interpretation of expectation-congruent symptoms, and by changing the behavioral strategies a person pursues. Motivation may link to the meaning through which people experience illness and treatment. Such meaning is derived from the culture in which they live and which informs them about the nature of illness and how it responds to treatment. Research into the placebo treatment of gastric and duodenal ulcers shows that this varies widely with society. The placebo effect in treating gastric ulcers is low in Brazil, higher in northern Europe (Denmark, Netherlands), and extremely high in Germany. However, the placebo effect in treating hypertension is lower in Germany than elsewhere.
Though the placebo effect is typically associated with deception in order to invoke positive expectations, studies carried out by Harvard Medical School have suggested that placebos can work even without deception. In an attempt to implement placebos honestly, 80 patients suffering from IBS were divided into two groups, one of which receiving no treatment, while the other were provided with placebo pills. Though it was made clear the pills had no active ingredient, patients still reported adequate symptom relief. Another similar study, in which patients suffering from migraines were given pills clearly labeled placebo, but still reported improvements of their symptoms.
The higher brain works by regulating subcortical processes. High placebo responses link with enhanced dopamine and mu-opioid activity in the circuitry for reward responses and motivated behavior of the nucleus accumbens, and, on the converse, anti-analgesic nocebos responses were associated with deactivation in this part of the brain of dopamine and opioid release. (It has been known that placebo analgesia depends upon the release in the brain of endogenous opioids since 1978.) Such analgesic placebos activation changes processing lower down in the brain by enhancing the descending inhibition through the periaqueductal gray on spinal nociceptive reflexes, while the expectations of anti-analgesic nocebos acts in the opposite way to block this.
The brain is also involved in less-studied ways upon nonanalgesic placebo effects:
Depression: Placebos reducing depression affect many of the same areas that are activated by antidepressants with the addition of the prefrontal cortex
Caffeine: Placebo-caffeinated coffee causes an increase in bilateral dopamine release in the thalamus.
Glucose: The expectation of an intravenous injection of glucose increases the release of dopamine in the basal ganglia of men (but not women).
Methylphenidate: The expectation of intravenous injection of this drug in inexperienced drug users increased the release of dopamine in the ventral cingulate gyrus and nucleus accumbens, with this effect being largest in those with no prior experience of the drug.
Functional imaging upon placebo analgesia has been summarized as showing that the placebo response is "mediated by "top-down" processes dependent on frontal cortical areas that generate and maintain cognitive expectancies. Dopaminergic reward pathways may underlie these expectancies". "Diseases lacking major 'top-down' or cortically based regulation may be less prone to placebo-related improvement".
Brain and body
The brain has control over the body processes affected by placebos.
In conditioning, a neutral stimulus saccharin is paired in a drink with an agent that produces an unconditioned response. For example, that agent might be cyclophosphamide, which causes immunosuppression. After learning this pairing, the taste of saccharin by itself is able to cause immunosuppression, as a new conditioned response via neural top-down control. Such conditioning has been found to affect a diverse variety of not just basic physiological processes in the immune system but ones such as serum iron levels, oxidative DNA damage levels, and insulin secretion. Recent reviews have argued that the placebo effect is due to top-down control by the brain for immunity and pain. Pacheco-López and colleagues have raised the possibility of "neocortical-sympathetic-immune axis providing neuroanatomical substrates that might explain the link between placebo/conditioned and placebo/expectation responses.":441
A recent fMRI study has shown that a placebo can reduce pain-related neural activity in the spinal cord, indicating that placebo effects can extend beyond the brain.
Dopaminergic pathways have been implicated in the placebo response in pain and depression.
Evolved health regulation
Evolutionary medicine identifies many symptoms such as fever, pain, and sickness behavior as evolved responses to protect or enhance the recovery from infection and injury. Fever, for example, is an evolved self-treatment that removes bacteria or viruses through raised body temperature. These evolved responses, however, also have a cost that depending upon circumstances can outweigh their benefit (due to this, for example, there is a reduction in fever during malnutrition or late pregnancy). According to the health management system theory proposed by Nicholas Humphrey, the brain has been selected to ensure that evolved responses are deployed only when the cost benefit is biologically advantageous. To do this, the brain factors in a variety of information sources, including the likelihood derived from beliefs that the body will get well without deploying its costly evolved responses. One such source of information is the knowledge the body is receiving care and treatment. The placebo effect in this perspective arises when false information about medications misleads the health management system about the likelihood of getting well so that it selects not to deploy an evolved self-treatment.
Similar to the placebo effect, inert substances have the potential to cause negative effects via the "nocebo effect" (Latinnocebo = "I will harm"). In this effect, giving an inert substance has negative consequences.
The placebo effect works at the time, but can wear off and make you feel unwell again.
Another negative consequence is that placebos can cause side-effects associated with real treatment. One example of this is with those that have already taken an opiate, can then show respiratory depression when given it again in the form of a placebo.
Withdrawal symptoms can also occur after placebo treatment. This was found, for example, after the discontinuation of the Women's Health Initiative study of hormone replacement therapy for menopause. Women had been on placebo for an average of 5.7 years. Moderate or severe withdrawal symptoms were reported by 4.8% of those on placebo compared to 21.3% of those on hormone replacement.
It is also often ethically challenging in practice to use placebos as a form of treatment.
A study of Danish general practitioners found that 48% had prescribed a placebo at least 10 times in the past year. The most frequently prescribed placebos were presented as antibiotics for viral infections, and vitamins for fatigue. Specialists and hospital-based physicians reported much lower rates of placebo use. A 2004 study in the British Medical Journal of physicians in Israel found that 60% used placebos in their medical practice, most commonly to "fend off" requests for unjustified medications or to calm a patient. The accompanying editorial concluded, "We cannot afford to dispense with any treatment that works, even if we are not certain how it does." Other researchers have argued that open provision of placebos for treating ADHD in children can be effective in maintaining ADHD children on lower stimulant doses in the short term.
Critics of the practice responded that it is unethical to prescribe treatments that do not work, and that telling a patient (as opposed to a research test subject) that a placebo is a real medication is deceptive and harms the doctor–patient relationship in the long run. Critics also argued that using placebos can delay the proper diagnosis and treatment of serious medical conditions. Legitimate doctors and pharmacists could open themselves up to charges of fraud or malpractice by using a placebo.
About 25% of physicians in both the Danish and Israeli studies used placebos as a diagnostic tool to determine if a patient's symptoms were real, or if the patient was malingering. Both the critics and defenders of the medical use of placebos agreed that this was unethical. The British Medical Journal editorial said, "That a patient gets pain relief from a placebo does not imply that the pain is not real or organic in origin...the use of the placebo for 'diagnosis' of whether or not pain is real is misguided."
The placebo administration may prove to be a useful treatment in some specific cases where recommended drugs cannot be used. For example, burn patients who are experiencing respiratory problems cannot often be prescribed opioid (morphine) or opioid derivatives (pethidine), as these can cause further respiratory depression. In such cases placebo injections (normal saline, etc.) are of use in providing real pain relief to burn patients if those not in delirium are told they are being given a powerful dose of painkiller.
In the Committee's view, homeopathy is a placebo treatment and the Government should have a policy on prescribing placebos. The Government is reluctant to address the appropriateness and ethics of prescribing placebos to patients, which usually relies on some degree of patient deception. Prescribing of placebos is not consistent with informed patient choice—which the Government claims is very important—as it means patients do not have all the information needed to make choice meaningful.
Beyond ethical issues and the integrity of the doctor–patient relationship, prescribing pure placebos is bad medicine. Their effect is unreliable and unpredictable and cannot form the sole basis of any treatment on the NHS.
A survey in the United States of more than 10,000 physicians came to the result that while 24% of physicians would prescribe a treatment that is a placebo simply because the patient wanted treatment, 58% would not, and for the remaining 18%, it would depend on the circumstances.
Changes over time
A review published in JAMA Psychiatry found that, in trials of antipsychotic medications, the change in response to receiving a placebo had increased significantly between 1960 and 2013. The review's authors identified several factors that could be responsible for this change, including inflation of baseline scores and enrollment of fewer severely ill patients. Another analysis published in Pain in 2015 found that placebo responses had increased considerably in neuropathic pain clinical trials conducted in the United States from 1990 to 2013. The researchers suggested that this may be because such trials have "increased in study size and length" during this time period.
Placebos do not work for everyone.Henry K. Beecher, in a paper in 1955, suggested placebo effects occurred in about 35% of people. However, this paper has been criticized for failing to distinguish the placebo effect from other factors, and for thereby encouraging an inflated notion of the placebo effect.
In the 1950s, there was considerable research to find whether there was a specific personality to those that responded to placebos. The findings could not be replicated and it is now thought to have no effect.
The desire for relief from pain, "goal motivation", and how far pain is expected to be relieved increases placebo analgesia. Another factor increasing the effectiveness of placebos is the degree to which a person attends to their symptoms, "somatic focus". Individual variation in response to analgesic placebos has been linked to regional neurochemical differences in the internal affective state of the individuals experiencing pain.
In a 2012 study, variations on the COMT (catechol-O-methyltransferase) gene related to dopamine release are found to be critical in the placebo effect among the patients with irritable bowel syndrome participating in the trial, a research group in Harvard Medical School reported. Patients with a variation of met/met, for having two copies of the methionineallele were shown to be more likely to respond to the placebo treatment, while the variation of val/val, for their two copies of valine allele responded the least. The response of patients with one copy each of methionine and valine fell in the middle. Release of dopamine in patients with the met/met variations is thought to link to reward and 'confirmation bias' which enhance the sense that the treatment is working. The role of the COMT gene variations are expected to be more prominent in studies where patients report more subjective conditions such as pain and fatigue rather than objective physiological measurements.
Symptoms and conditions
The placebo effect occurs more strongly in some conditions than others. Dylan Evans has suggested that placebos work most strongly upon conditions such as pain, swelling, stomach ulcers, depression, and anxiety that have been linked with activation of the acute-phase response.
The placebo effect is believed to reduce pain—a phenomenon known as placebo analgesia—in two different ways. One way is by the placebo initiating the release of endorphins, which are natural pain killers produced by the brain. The other way is the placebo changing the patient's perception of pain. "A person might reinterpret a sharp pain as uncomfortable tingling."
One way in which the magnitude of placebo analgesia can be measured is by conducting "open/hidden" studies, in which some patients receive an analgesic and are informed that they will be receiving it (open), while others are administered the same drug without their knowledge (hidden). Such studies have found that analgesics are considerably more effective when the patient knows they are receiving them.
When administered orally, placebos have clinically meaningful effects with regard to lower back pain.
In 2008, a controversial meta-analysis led by psychologist Irving Kirsch, analyzing data from the FDA, concluded that 82% of the response to antidepressants was accounted for by placebos. However, there are serious doubts about the used methods and the interpretation of the results, especially the use of 0.5 as cut-off point for the effect-size. A complete reanalysis and recalculation based on the same FDA data discovered that the Kirsch study suffered from important flaws in the calculations. The authors concluded that although a large percentage of the placebo response was due to expectancy, this was not true for the active drug. Besides confirming drug effectiveness, they found that the drug effect was not related to depression severity.
Another meta-analysis found that 79% of depressed patients receiving placebo remained well (for 12 weeks after an initial 6–8 weeks of successful therapy) compared to 93% of those receiving antidepressants. In the continuation phase however, patients on placebo relapsed significantly more often than patients on antidepressants. A 2009 meta-analysis reported that in 2005 68% of the effects of antidepressants was due to the placebo effect, which was more than double the placebo response rate in 1980.
While some say that blanket consent, or the general consent to unspecified treatment given by patients beforehand, is ethical, others argue that patients should always obtain specific information about the name of the drug they are receiving, its side effects, and other treatment options. Even though some patients do not want to be informed, health professionals are ethically bound to give proper information about the treatment given. There is such a debate over the use of placebos because while placebos are used for the good of many to test the effectiveness of drugs, some argue that it is unethical to ever deprive individual patients of effective drugs.
Chronic fatigue syndrome
It was previously assumed that placebo response rates in patients with chronic fatigue syndrome (CFS) are unusually high, "at least 30% to 50%", because of the subjective reporting of symptoms and the fluctuating nature of the condition. According to a meta-analysis and contrary to conventional wisdom, the pooled response rate in the placebo group was 19.6%, even lower than in some other medical conditions. The authors offer possible explanations for this result: CFS is widely understood to be difficult to treat, which could reduce expectations of improvement. In context of evidence showing placebos do not have powerful clinical effects when compared to no treatment, a low rate of spontaneous remission in CFS could contribute to reduced improvement rates in the placebo group. Intervention type also contributed to the heterogeneity of the response. Low patient and provider expectations regarding psychological treatment may explain particularly low placebo responses to psychiatric treatments.
List of medical conditions
The effect of placebo treatments (an inert pill unless otherwise noted) has been studied for the following medical conditions. Many of these citations concern research showing that active treatments are effective, but that placebo effects exist as well.
A quack treating a patient with Perkins Patent Tractors by James Gillray, 1801. John Haygarth used this remedy to illustrate the power of the placebo effect.
The word 'placebo', Latin for "I will please", dates back to a Latin translation of the Bible by St Jerome. In 1811, Hooper’sQuincy’s Lexicon-Medicum defined placebo as "[any medicine] adapted more to please than to benefit the patient".
Early implementations of placebo controls date back to 16th-century Europe with Catholic efforts to discredit exorcisms. Individuals who claimed to be possessed by demonic forces were given false holy objects. If the person reacted with violent contortions, it was concluded that the possession was purely imagination.
John Haygarth was the first to investigate the efficacy of the placebo effect in the 18th century. He tested a popular medical treatment of his time, called "Perkins tractors", and concluded that the remedy was ineffectual by demonstrating that the results from a dummy remedy were just as useful as from the alleged "active" remedy.
Émile Coué, a French pharmacist, working as an apothecary at Troyes between 1882 and 1910, also advocated the effectiveness of the "Placebo Effect". He became known for reassuring his clients by praising each remedy's efficiency and leaving a small positive notice with each given medication. His book Self-Mastery Through Conscious Autosuggestion was published in England (1920) and in the United States (1922).
Placebos remained widespread in medicine until the 20th century, and they were sometimes endorsed as necessary deceptions. In 1903, Richard Cabot said that he was brought up to use placebos, but he ultimately concluded by saying that "I have not yet found any case in which a lie does not do more harm than good".
In modern times, T. C. Graves first defined the "placebo effect" in a published paper in The Lancet in 1920. He spoke of "the placebo effects of drugs" being manifested in those cases where "a real psychotherapeutic effect appears to have been produced". In 1961 Henry K. Beecher concluded that surgeons he categorized as enthusiasts relieved their patients' chest pain and heart problems more than skeptic surgeons. Beginning in the 1960s, the placebo effect became widely recognized and placebo-controlled trials became the norm in the approval of new medications.
The placebo effect makes it more difficult to evaluate new treatments. Clinical trials control for this effect by including a group of subjects that receives a sham treatment. The subjects in such trials are blinded as to whether they receive the treatment or a placebo. If a person is given a placebo under one name, and they respond, they will respond in the same way on a later occasion to that placebo under that name but not if under another.
Clinical trials are often double-blinded so that the researchers also do not know which test subjects are receiving the active or placebo treatment. The placebo effect in such clinical trials is weaker than in normal therapy since the subjects are not sure whether the treatment they are receiving is active.
Knowingly giving a person a placebo when there is an effective treatment available is a bioethically complex issue. While placebo-controlled trials might provide information about the effectiveness of a treatment, it denies some patients what could be the best available (if unproven) treatment. Informed consent is usually required for a study to be considered ethical, including the disclosure that some test subjects will receive placebo treatments.
The ethics of placebo-controlled studies have been debated in the revision process of the Declaration of Helsinki. Of particular concern has been the difference between trials comparing inert placebos with experimental treatments, versus comparing the best available treatment with an experimental treatment; and differences between trials in the sponsor's developed countries versus the trial's targeted developing countries.
A phenomenon opposite to the placebo effect has also been observed. When an inactive substance or treatment is administered to a recipient who has an expectation of it having a negative impact, this intervention is known as a nocebo (Latinnocebo = "I shall harm"). A nocebo effect occurs when the recipient of an inert substance reports a negative effect and/or a worsening of symptoms, with the outcome resulting not from the substance itself, but from negative expectations about the treatment.
Placebos used in clinical trials have sometimes had unintended consequences. A report in the Annals of Internal Medicine that looked at details from 150 clinical trials found that certain placebos used in the trials affected the results. For example, one study on cholesterol-lowering drugs used olive oil and corn oil in the placebo pills. However, according to the report, this "may lead to an understatement of drug benefit: The monounsaturated and polyunsaturated fatty acids of these 'placebos,' and their antioxidant and anti-inflammatory effects, can reduce lipid levels and heart disease." Another example researchers reported in the study was a clinical trial of a new therapy for cancer patients suffering from anorexia. The placebo that was used included lactose. However, since cancer patients typically face a higher risk of lactose intolerance, the placebo pill might actually have caused unintended side-effects that made the experimental drug look better in comparison.
^Lanotte M, Lopiano, Torre, Bergamasco, Colloca, Benedetti (November 2005). "Expectation enhances autonomic responses to stimulation of the human subthalamic limbic region". Brain, Behavior, and Immunity. 19 (6): 500–9. doi:10.1016/j.bbi.2005.06.004. PMID16055306.
^Neurobiological Mechanisms of the Placebo Effect, Fabrizio Benedetti, Helen S. Mayberg, Tor D. Wager, Christian S. Stohler, and Jon-Kar Zubieta, The Journal of Neuroscience, 9 November 2005, 25(45) 
^ abThe neural correlates of placebo effects: a disruption account, Matthew D. Lieberman, Johanna M. Jarcho, Steve Berman, Bruce D. Naliboff, Brandall Y. Suyenobu, Mark Mandelkern, and Emeran A. Mayer 
^ abMcDonald CJ, Mazzuca SA, McCabe GP; Mazzuca; McCabe Jr (1983). "How much of the placebo 'effect' is really statistical regression?". Stat Med. 2 (4): 417–27. doi:10.1002/sim.4780020401. PMID6369471.
^Colloca L, Lopiano L, Lanotte M, Benedetti F; Lopiano; Lanotte; Benedetti (2004). "Overt versus covert treatment for pain, anxiety, and Parkinson's disease". Lancet Neurol. 3 (11): 679–84. doi:10.1016/S1474-4422(04)00908-1. PMID15488461.
^Barnett AG, van der Pols JC, Dobson AJ; Van Der Pols; Dobson (February 2005). "Regression to the mean: what it is and how to deal with it". Int J Epidemiol. 34 (1): 215–20. doi:10.1093/ije/dyh299. PMID15333621.
^Kirsch I (1985). "Response expectancy as a determinant of experience and behavior". American Psychologist. 40 (11): 1189–1202. doi:10.1037/0003-066X.40.11.1189.
^Klinger R, Soost S, Flor H, Worm M; Soost; Flor; Worm (2007). "Classical conditioning and expectancy in placebo hypoalgesia: a randomized controlled study in patients with atopic dermatitis and persons with healthy skin". Pain. 128 (1–2): 31–9. doi:10.1016/j.pain.2006.08.025. PMID17030095.
^Colloca L, Tinazzi M, Recchia S, Le Pera D, Fiaschi A, Benedetti F, Valeriani M; Tinazzi; Recchia; Le Pera; Fiaschi; Benedetti; Valeriani (2008). "Learning potentiates neurophysiological and behavioral placebo analgesic responses". Pain. 139 (2): 306–14. doi:10.1016/j.pain.2008.04.021. PMID18538928.
^Linde K, Witt CM, Streng A, Weidenhammer W, Wagenpfeil S, Brinkhaus B, Willich SN, Melchart D; Witt; Streng; Weidenhammer; Wagenpfeil; Brinkhaus; Willich; Melchart (2007). "The effect of patient expectations on outcomes in four randomized controlled trials of acupuncture in patients with chronic pain". Pain. 128 (3): 264–71. doi:10.1016/j.pain.2006.12.006. PMID17257756.
^Bausell RB, Lao L, Bergman S, Lee WL, Berman BM; Lao; Bergman; Lee; Berman (2005). "Is acupuncture analgesia an expectancy effect? Preliminary evidence based on participants' perceived assignments in two placebo-controlled trials". Eval Health Prof. 28 (1): 9–26. doi:10.1177/0163278704273081. PMID15677384.
^Kirsch I (1997). "Specifying non-specifics: Psychological mechanism of the placebo effect". In Harrington A. The Placebo Effect: An Interdisciplinary Exploration. Cambridge: Harvard University Press. pp. 166–86. ISBN 978-0-674-66986-4.
^Beedie CJ, Coleman DA, Foad AJ (2007). "Positive and negative placebo effects resulting from the deceptive administration of an ergogenic aid". Int. J. Sport Nutr. Exerc. Metab.17: 259–269.
^Haltia LT, Rinne JO, Helin S, Parkkola R, Någren K, Kaasinen V; Rinne; Helin; Parkkola; Någren; Kaasinen (2008). "Effects of intravenous placebo with glucose expectation on human basal ganglia dopaminergic function". Synapse. 62 (9): 682–8. doi:10.1002/syn.20541. PMID18566972.
^Volkow ND, Wang GJ, Ma Y, Fowler JS, Wong C, Jayne M, Telang F, Swanson JM; Wang; Ma; Fowler; Wong; Jayne; Telang; Swanson (2006). "Effects of expectation on the brain metabolic responses to methylphenidate and to its placebo in non-drug abusing subjects". NeuroImage. 32 (4): 1782–92. doi:10.1016/j.neuroimage.2006.04.192. PMID16757181.
^Murray, D; Stoessl, AJ (December 2013). "Mechanisms and therapeutic implications of the placebo effect in neurological and psychiatric conditions". Pharmacology & therapeutics. 140 (3): 306–18. doi:10.1016/j.pharmthera.2013.07.009. PMID23880289.
^Benedetti F, Amanzio M, Baldi S, Casadio C, Cavallo A, Mancuso M, Ruffini E, Oliaro A, Maggi G; Amanzio; Baldi; Casadio; Cavallo; Mancuso; Ruffini; Oliaro; Maggi (1998). "The specific effects of prior opioid exposure on placebo analgesia and placebo respiratory depression". Pain. 75 (2–3): 313–9. doi:10.1016/S0304-3959(98)00010-4. PMID9583767.
^Altunç U, Pittler MH, Ernst E; Pittler; Ernst (2007). "Homeopathy for childhood and adolescence ailments: Systematic review of randomized clinical trials". Mayo Clinic Proceedings. 82 (1): 69–75. doi:10.4065/82.1.69. PMID17285788.
^Fountoulakis, Konstantinos N.; Möller, Hans-Jürgen (2010). "Efficacy of antidepressants: a re-analysis and re-interpretation of the Kirsch data". The International Journal of Neuropsychopharmacology. 14 (3): 405–412. doi:10.1017/S1461145710000957. ISSN1461-1457. PMID20800012.
^Rief, Winfried; Nestoriuc, Yvonne; Weiss, Sarah; Welzel, Eva; Barsky, Arthur J.; Hofmann, Stefan G. (November 2009). "Meta-analysis of the placebo response in antidepressant trials". Journal of Affective Disorders. 118 (1–3): 1–8. doi:10.1016/j.jad.2009.01.029. PMID19246102.
^Walsh, BT; Seidman, SN; Sysko, R; Gould, M (10 April 2002). "Placebo response in studies of major depression: variable, substantial, and growing". JAMA. 287 (14): 1840–7. doi:10.1001/jama.287.14.1840. PMID11939870.
^Niklson I, Edrich P, Verdru P; Edrich; Verdru (2006). "Identifying baseline characteristics of placebo responders versus nonresponders in randomized double-blind trials of refractory partial-onset seizures". Epileptic Disord. 8 (1): 37–44. PMID16567324.
^Kriston L, Harms A, Berner MM; Harms; Berner (2006). "A meta-regression analysis of treatment effect modifiers in trials with flexible-dose oral sildenafil for erectile dysfunction in broad-spectrum populations". Int J Impot Res. 18 (6): 559–65. doi:10.1038/sj.ijir.3901479. PMID16688210.
^Chen, X; Zou, K; Abdullah, N; Whiteside, N; Sarmanova, A; Doherty, M; Zhang, W (15 March 2017). "The placebo effect and its determinants in fibromyalgia: meta-analysis of randomised controlled trials". Clinical Rheumatology. doi:10.1007/s10067-017-3595-8. PMID28299460.
^van Laarhoven, AI; van der Sman-Mauriks, IM; Donders, AR; Pronk, MC; van de Kerkhof, PC; Evers, AW (1 December 2014). "Placebo Effects on Itch: A Meta-Analysis of Clinical Trials of Patients with Dermatological Conditions". J Invest Dermatol. 135 (5): 1234–43. doi:10.1038/jid.2014.522. PMID25609025.
^Patel SM, Stason WB, Legedza A, Ock SM, Kaptchuk TJ, Conboy L, Canenguez K, Park JK, Kelly E, Jacobson E, Kerr CE, Lembo AJ; Stason; Legedza; Ock; Kaptchuk; Conboy; Canenguez; Park; Kelly; Jacobson; Kerr; Lembo (2005). "The placebo effect in irritable bowel syndrome trials: a meta-analysis". Neurogastroenterol Motil. 17 (3): 332–40. doi:10.1111/j.1365-2982.2005.00650.x. PMID15916620.
^van Leeuwen, JH; Castro, R; Busse, M; Bemelmans, BL (September 2006). "The placebo effect in the pharmacologic treatment of patients with lower urinary tract symptoms". European Urology. 50 (3): 440–52; discussion 453. doi:10.1016/j.eururo.2006.05.014. PMID16753253.
^Meissner, K; Fässler, M; Rücker, G; Kleijnen, J; Hróbjartsson, A; Schneider, A; Antes, G; Linde, K (25 November 2013). "Differential effectiveness of placebo treatments: a systematic review of migraine prophylaxis". JAMA Internal Medicine. 173 (21): 1941–51. doi:10.1001/jamainternmed.2013.10391. PMID24126676.
^La Mantia L, Eoli M, Salmaggi A, Milanese C; Eoli; Salmaggi; Milanese (1996). "Does a placebo-effect exist in clinical trials on multiple sclerosis? Review of the literature". The Italian Journal of Neurological Sciences. 17 (2): 135–9. doi:10.1007/BF02000844. PMID8797067.
^Zhang, W; Robertson, J; Jones, A C; Dieppe, P A; Doherty, M (1 December 2008). "The placebo effect and its determinants in osteoarthritis: meta-analysis of randomised controlled trials". Annals of the Rheumatic Diseases. 67 (12): 1716–1723. doi:10.1136/ard.2008.092015. PMID18541604.
^Bannuru, RR; McAlindon, TE; Sullivan, MC; Wong, JB; Kent, DM; Schmid, CH (28 July 2015). "Effectiveness and Implications of Alternative Placebo Treatments: A Systematic Review and Network Meta-Analysis of Osteoarthritis Trials". Annals of Internal Medicine. 163 (5): 365–72. doi:10.7326/M15-0623. PMID26215539.
^Fulda S, Wetter TC; Wetter (2008). "Where dopamine meets opioids: a meta-analysis of the placebo effect in restless legs syndrome treatment studies". Brain. 131 (Pt 4): 902–17. doi:10.1093/brain/awm244. PMID17932100.
^Silva, MA; Duarte, GS; Camara, R; Rodrigues, FB; Fernandes, RM; Abreu, D; Mestre, T; Costa, J; Trenkwalder, C; Ferreira, JJ (10 May 2017). "Placebo and nocebo responses in restless legs syndrome: A systematic review and meta-analysis". Neurology. doi:10.1212/WNL.0000000000004004. PMID28490647.
^Beauregard, Mario (2012). Brain Wars: The Scientific Battle Over the Existence of the Mind and the Proof That Will Change the Way We Live Our Lives. New York: HarperCollins Publishers. p. 21. ISBN 978-0-06-207156-9.
^Kaptchuk TJ (1998). "Intentional ignorance: a history of blind assessment and placebo controls in medicine". Bulletin of the History of Medicine. 72 (3): 389–433. doi:10.1353/bhm.1998.0159. PMID9780448.
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