Enhancing Placebo Effects in Somatic Symptoms Through Oxytocin

Nasal oxytocin administration does not influence eye gaze or perceived relationship of male volunteers with physicians in a simulated online consultation: a randomized, placebo-controlled trial

The patient-physician relationship is a critical determinant of patient health outcomes. Verbal and non-verbal communication, such as eye gaze, are vital aspects of this bond. Neurobiological studies indicate that oxytocin may serve as a link between increased eye gaze and social bonding. Therefore, oxytocin signaling could serve as a key factor influencing eye gaze as well as the patient-physician relationship. We aimed to test the effects of oxytocin on gaze to the eyes of the physician and the patient-physician relationship by conducting a randomized placebo-controlled crossover trial in healthy volunteers with intranasally administered oxytocin (with a previously effective single dose of 24 IU, EudraCT number 2018-004081-34). The eye gaze of 68 male volunteers was studied using eye tracking during a simulated video call consultation with a physician, who provided information about vaccination against the human papillomavirus. Relationship outcomes, including trust, satisfaction, and perceived physician communication style, were measured using questionnaires and corrected for possible confounds (social anxiety and attachment orientation). Additional secondary outcome measures for the effect of oxytocin were recall of information and pupil diameter and exploratory outcomes included mood and anxiety measures. Oxytocin did not affect the eye-tracking parameters of volunteers' gaze toward the eyes of the physician. Moreover, oxytocin did not affect the parameters of bonding between volunteers and the physician nor other secondary and exploratory outcomes in this setting. Bayesian hypothesis testing provided evidence for the absence of effects. These results contradict the notion that oxytocin affects eye gaze patterns or bonding.

Placebo effects on cutaneous pain and itch: a systematic review and meta-analysis of experimental results and methodology

ABSTRACT

Placebo effects, positive treatment outcomes that go beyond treatment processes, can alter sensations through learning mechanisms. Understanding how methodological factors contribute to the magnitude of placebo effects will help define the mechanisms by which these effects occur. We conducted a systematic review and meta-analysis of experimental placebo studies in cutaneous pain and itch in healthy samples, focused on how differences in methodology contribute to the resulting placebo effect magnitude. We conducted meta-analyses by learning mechanism and sensation, namely, for classical conditioning with verbal suggestion, verbal suggestion alone, and observational learning, separately for pain and itch. We conducted subgroup analyses and meta-regression on the type of sensory stimuli, placebo treatment, number of acquisition and evocation trials, differences in calibrated intensities for placebo and control stimuli during acquisition, age, and sex. We replicated findings showing that a combination of classical conditioning with verbal suggestion induced larger placebo effects on pain ( k = 68, g = 0 . 59) than verbal suggestion alone ( k = 39, g = 0.38) and found a smaller effect for itch with verbal suggestion alone ( k = 7, g = 0.14). Using sham electrodes as placebo treatments corresponded with larger placebo effects on pain than when topical gels were used. Other methodological and demographic factors did not significantly affect placebo magnitudes. Placebo effects on pain and itch reliably occur in experimental settings with varied methods, and conditioning with verbal suggestion produced the strongest effects. Although methods may shape the placebo effect to some extent, these effects appear robust overall, and their underlying learning mechanisms may be harnessed for applications outside the laboratory.

Learned Nocebo Effects on Cutaneous Sensations of Pain and Itch: A Systematic Review and Meta-analysis of Experimental Behavioral Studies on Healthy Humans

OBJECTIVE

In past decades, the field of nocebo research has focused on studying how sensory perception can be shaped by learning. Nocebo effects refer to aggravated sensory experiences or increased sensitivity to sensations such as pain and itch resulting from treatment-related negative experiences. Behavioral conditioning and verbal suggestions of a negative treatment outcome may aggravate pain and itch perception. Gaining a comprehensive view of the magnitude of nocebo effects and contributing factors will help steer nocebo research toward fruitful directions for understanding complex sensory phenomena.

METHODS

We conducted a systematic review and meta-analysis of a total of 37 distinct experimental nocebo studies on healthy participants (all published in English between 2008 and 2021), with four separate meta-analyses for nocebo effects on pain or itch. We conducted subgroup analyses and meta-regression on factors such as type and intensity of sensory stimuli, and length of conditioning paradigms.

RESULTS

This meta-analysis showed that, on average, effect sizes of nocebo effects were moderate to large (Hedges g between 0.26 and 0.71 for the four primary outcomes). The combination of conditioning and verbal suggestions yielded stronger nocebo responses on pain in particular. Subgroup analyses, including factors such as the type of sensory stimulation, did not explain the moderate heterogeneity in nocebo magnitudes between different studies. Risk of bias was generally low and was not related to nocebo magnitudes either.

CONCLUSIONS

We discuss these results in relation to the role of conditioning and aversive learning, and we recommend more consistency in designing and reporting nocebo experiments.

Effects of intranasal oxytocin on pain perception among human subjects: A systematic literature review and meta-analysis

INTRODUCTION

Oxytocin (OXT) is a peptide hormone produced in the hypothalamus that plays a neuromodulatory role in emotion, stress, and anxiety. Due to its multidimensional role, OXT is a promising target for therapeutic interventions to treat pain.

OBJECTIVE

Perform a systematic literature review, followed by a meta-analysis to identify the effects of intranasal OXT on the self-perception of clinical and experimental pain among human subjects.

METHOD

A systematic review was conducted in the PubMed, PsycINFO, Scielo, Lilacs, and Web of Science databases, using the keywords Oxytocin, Pain, Analgesia, and Nociception.

RESULTS

Fifteen papers were included in the meta-analysis. None of the outcomes presented statistical significance in terms of the interventions' effect size: pain intensity (SMD = -0.02 (CI 95 %: -0.14 to 0.10; p = 0.76)) and pain unpleasantness (SMD = -0.15 (CI 95 %: -0.34 to 0.04; p = 0.12)). No meta-analysis was performed for pain threshold or tolerance because few papers address these outcomes.

CONCLUSION

There was no statistically significant effect of intranasal OXT administration on pain perception, considering equivalence limits between (-0.2 and 0.2). However, it must be considered that the study designs may not have been sensitive enough to detect minor analgesic effects of OXT, which, being weak, may also not be perceived at a conscious level. Additionally, OXT effects possibly depend on specific characteristics of the painful condition, such as pain complexity, intensity, and duration, contextual variables like the presence of social and affective support, and individual characteristics.

Placebo and nocebo effects: from observation to harnessing and clinical application

Placebo and nocebo effects are salubrious benefits and negative outcomes attributable to non-specific symbolic components. Leveraging advanced experimental and analytical approaches, recent studies have elucidated complicated neural mechanisms that may serve as a solid basis for harnessing the powerful self-healing and self-harming capacities and applying these findings to improve medical practice and minimize the unintended exacerbation of symptoms in medical practice. We review advances in employing psychosocial, pharmacological, and neuromodulation approaches to modulate/harness placebo and nocebo effects. While these approaches show promising potential, translating these research findings into clinical settings still requires careful methodological, technical, and ethical considerations.

Can contagious itch be affected by positive and negative suggestions?

Contagious itch can be evoked by observing people scratching. Verbal suggestions about to-be-received itch can influence itch intensity, as shown by placebo research, but it is unknown whether this extends to contagious itch. The current study aimed to replicate prior findings that listening to scratching and rubbing sounds elicits contagious itch, and to investigate whether suggestions can modulate this process. Healthy participants (n = 140) received positive or negative suggestions about itch in response to the sounds (aimed to decrease or increase expected itch, respectively), or no specific suggestions as a control. Participants listened to a number of audio fragments with scratching and rubbing sounds. The amount of expected itch as well as itch sensation after each audio fragment were measured by self-report. Suggestions had no effect on the expected itch. Both rubbing and scratching sounds significantly elicited itch in all groups. Scratching sounds induced more itch than rubbing sounds exclusively in the control group. These findings indicate that short suggestions might be not effective enough to modify the expectations of people regarding contagious itch. Furthermore, suggestions modulate contagious itch to some degree, but not in the hypothesized direction. Potential similarities and differences in the neurobiological mechanisms of contagious itch and nocebo effects are discussed.

Oxytocin and the social facilitation of placebo effects

Significant clinical improvement is often observed in patients who receive placebo treatment in randomized double-blind placebo-controlled trials. While a proportion of this "improvement" reflects experimental design limitations (e.g., reliance on subjective outcomes, unbalanced groups, reporting biases), some of it reflects genuine improvement corroborated by physiological change. Converging evidence across diverse medical conditions suggests that clinically-relevant benefits from placebo treatment are associated with the activation of brain reward circuits. In parallel, evidence has accumulated showing that such benefits are facilitated by clinicians that demonstrate warmth and proficiency during interactions with patients. Here, we integrate research on these neural and social aspects of placebo effects with evidence linking oxytocin and social reward to advance a neurobiological account for the social facilitation of placebo effects. This account frames oxytocin as a key mediator of treatment success across a wide-spectrum of interventions that increase social connectedness, thereby providing a biological basis for assessing this fundamental non-specific element of medical care.

Psycho-Neuro-Endocrine-Immunological Basis of the Placebo Effect: Potential Applications beyond Pain Therapy

The placebo effect can be defined as the improvement of symptoms in a patient after the administration of an innocuous substance in a context that induces expectations regarding its effects. During recent years, it has been discovered that the placebo response not only has neurobiological functions on analgesia, but that it is also capable of generating effects on the immune and endocrine systems. The possible integration of changes in different systems of the organism could favor the well-being of the individuals and go hand in hand with conventional treatment for multiple diseases. In this sense, classic conditioning and setting expectations stand out as psychological mechanisms implicated in the placebo effect. Recent advances in neuroimaging studies suggest a relationship between the placebo response and the opioid, cannabinoid, and monoaminergic systems. Likewise, a possible immune response conditioned by the placebo effect has been reported. There is evidence of immune suppression conditioned through the insular cortex and the amygdala, with noradrenalin as the responsible neurotransmitter. Finally, a conditioned response in the secretion of different hormones has been determined in different studies; however, the molecular mechanisms involved are not entirely known. Beyond studies about its mechanism of action, the placebo effect has proved to be useful in the clinical setting with promising results in the management of neurological, psychiatric, and immunologic disorders. However, more research is needed to better characterize its potential use. This review integrates current knowledge about the psycho-neuro-endocrine-immune basis of the placebo effect and its possible clinical applications.

Intranasal oxytocin and the stress-buffering effects of social support during experimentally induced pain: The role of attachment security

BACKGROUND

This study examined whether intranasal oxytocin enhances the stress-buffering effects of social support during experimentally induced pain, taking into account the role of individual differences in attachment security.

METHODS

Female participants (N = 193) were randomly assigned to oxytocin (24 IU intranasal) or placebo and to receive support or no support from a friend (2 × 2 factorial design with repeated measures)). Participants underwent the Cold Pressor Task (CPT) and were monitored for heart rate variability (HRV: RMSSD) and heart rate and reported pain levels. The Experiences in Close Relationships Questionnaire was used to measure attachment.

RESULTS

Oxytocin reduced RMSSD (p = 0.003, partial ɳ² = 0.03) and increased heart rate (p = 0.039, partial ɳ² = 0.03) in individuals who received support, possibly reflecting an enhanced attentional state. Oxytocin did not enhance beneficial effects of social support on perceived pain, but increased pain intensity in avoidantly attached individuals who were supported by a friend (p = 0.009, partial ɳ² = 0.06).

LIMITATIONS

Only female participants were examined. Future studies are needed to determine sex differences in how oxytocin shapes stress-buffering effects of support.

CONCLUSIONS

Oxytocin may enhance the salience of social proximity and may be a mechanism underlying previously reported social influences on cardiovascular and mental health. However, oxytocin effects depend on interpersonal insecurities and may trigger discomfort in avoidantly attached individuals. Caution about oxytocin's therapeutic promise is warranted.

Open- and Closed-Label Placebo and Nocebo Suggestions About a Sham Transdermal Patch

OBJECTIVE

Placebo effects may occur when it is known that an inert substance is given (i.e., open-label placebo). It is not yet clear whether these effects are similar to concealed (i.e., closed-label) placebo effects for itch or whether nocebo effects can be induced under open-label conditions.

METHODS

Healthy volunteers (n = 112) were randomized to an open-label (I) or closed-label (II) positive suggestions group, or an open-label (III) or closed-label (IV) negative suggestions group. Participants were told, as cover story, that a transdermal caffeine patch would be applied that positively influences cognitive abilities and, as a side effect, positively or negatively (depending on group allocation) influences itch. Participants in the open-label groups were given a rationale explaining placebo and nocebo effect mechanisms. Itch (the primary outcome) was induced at baseline and postsuggestions by histamine iontophoresis.

RESULTS

Analyses of variance revealed significantly lower itch in the positive compared with the negative suggestions groups for both open- and closed-label contexts (all, p ≤ .008, Cohen d ≥ 0.47). Self-rated skin response was less severe after positive versus negative suggestions (all, p ≤ .017, Cohen d ≥ 0.33), but no effects on physical skin response were found (all, p ≥ .23, Cohen d ≤ 0.30).

CONCLUSIONS

Itch can be reduced by positive compared with negative suggestions under both open- and closed-label conditions. These findings indicate that open-label suggestions may potentially be a tool to use placebo effects for self-reported outcomes in clinical practice, for example, by explaining the role of expectancy in treatment. It needs to be investigated further under which circumstances an open-label rationale may impact placebo and nocebo effects.Trial Registration:www.trialregister.nl; NTR7174.

Placebo and nocebo effects for itch and itch-related immune outcomes: A systematic review of animal and human studies

Placebo and nocebo effects can influence somatic symptoms such as pain. For itch and other dermatological symptoms these effects have been far less investigated. This review systematically integrates evidence from both animal (mainly rodents) and human trials on placebo and nocebo effects in itch, itch-related symptoms and conditions of the skin and mucous membranes, and related immune outcomes (e.g., histamine). Thirty-one animal studies, and fifty-five human studies (k = 21 healthy participants, k = 34 patients) were included. Overall, studies consistently show that placebo and nocebo effects can be induced by various methods (e.g., suggestions, conditioning and social cues), despite high heterogeneity across studies. Effects of suggestions were found consistently across subjective and behavioral parameters (e.g., itch and scratching in humans), whereas conditioning was likely to impact physiological parameters under certain conditions (e.g., conditioning of histamine levels in stressed rodents). Brain areas responsible for itch processing were associated with nocebo effects. Future research may investigate how variations in methods impact placebo and nocebo effects, and whether all symptoms and conditions can be influenced equally.

Effects of Oxytocin on Placebo and Nocebo Effects in a Pain Conditioning Paradigm: A Randomized Controlled Trial

Oxytocin has been shown to increase trust, decrease anxiety, and affect learning as has been observed in conditioning paradigms. Trust, anxiety, and learning are important factors that influence placebo effects. In this study, we investigated whether oxytocin can increase placebo analgesia, decrease nocebo hyperalgesia, and influence extinction processes of both. Eighty male volunteers were assigned to a 40 IU of oxytocin nasal spray group, or to a placebo control group. Placebo analgesia and nocebo hyperalgesia were induced by a conditioning procedure in combination with verbal suggestions. The results demonstrate that the conditioning procedure successfully elicited significant placebo analgesia and nocebo hyperalgesia responses (P < .001). Furthermore, extinction was observed (P < .001), although placebo and nocebo responses did not return to baseline and remained significant. Oxytocin did not influence placebo analgesia or nocebo hyperalgesia and had no effect on extinction. This study provides support against the placebo-boosting effects of oxytocin and was the first one to demonstrate that it also did not influence nocebo effects or extinction processes, however, these results pertain to only a male sample. As managing placebo and nocebo effects has widespread clinical implications, further research should investigate other neurobiological or behavioral pathways to boost placebo and decrease nocebo effects. PERSPECTIVE: The present study demonstrated that placebo analgesia and nocebo hyperalgesia can be successfully induced by conditioning and verbal suggestions. We could not confirm the hypothesis that oxytocin affects either of these phenomena. Other pharmacological agents and behavioral manipulations for increasing placebo and decreasing nocebo effects should be investigated.

Placebo Effects in the Neuroendocrine System: Conditioning of the Oxytocin Responses

OBJECTIVE

There is evidence that placebo effects may influence hormone secretion. However, few studies have examined placebo effects in the endocrine system, including oxytocin placebo effects. We studied whether it is possible to trigger oxytocin placebo effects using a classical conditioning paradigm.

METHODS

Ninety-nine women were assigned to a conditioned, control, or drug control group. In the two-phase conditioning paradigm, participants in the conditioned and drug control groups received an oxytocin nasal spray combined with a distinctive smell (conditioned stimulus [CS]) for three acquisition days, whereas the control group received placebo spray. Subsequently, the conditioned and control groups received placebo spray with the CS and the drug control group received oxytocin spray for three evocation days. Salivary oxytocin was measured several times during each day. Pain sensitivity and facial evaluation tests previously used in oxytocin research were also administered.

RESULTS

On evocation day 1, in the conditioned group, oxytocin significantly increased from baseline to 5 minutes after CS (B[slope] = 19.55, SE = 5.88, p < .001) and remained increased from 5 to 20 (B = -10.42, SE = 5.81, p = .071) and 50 minutes (B = -0.70, SE = 3.37, p = .84). On evocation day 2, a trend for increase in oxytocin was found at 5 minutes (B = 15.22, SE = 8.14, p = .062). No placebo effect was found on evocation day 3 (B = 3.57, SE = 3.26, p = .28). Neither exogenous nor conditioned oxytocin affected pain or facial tasks.

CONCLUSIONS

Results indicate that oxytocin release can be conditioned and that this response extinguishes over time. Triggering hormonal release by placebo manipulation offers various clinical possibilities, such as enhancing effects of pharmacological treatments or reducing dosages of medications.

TRIAL REGISTRATION

The study was registered as a clinical trial on www.trialregister.nl (number NTR5596).

What is new in the psychology of chronic itch?

Itch is often regarded as unpleasant or bothersome and is accompanied by symptoms of distress and impairments in daily life. The biopsychosocial model of chronic itch describes how psychological factors can contribute to the improvement or exacerbation of chronic itch and related scratching behaviour. Recent research underlines the important role of cognitive-affective information processing, such as attention, affect and expectancies. This may not only play a role for acute itch states, but may particularly apply to the process of itch chronification, for example, due to the vicious cycle in which these factors shape the experience of itch. The present paper focuses on new insights into the relation between itch and the cognitive-affective factors of attention, affect and expectancies. These factors are thought to play a possible aggravating role in itch in the long term and have received increasing attention in the recent empirical literature on maintaining and exacerbating factors for chronic physical symptoms. Possible psychophysiological and neurobiological pathways regarding these factors are discussed, as well as possible intervention methods.

Can verbal suggestions strengthen the effects of a relaxation intervention?

Short stress management interventions such as relaxation therapy have demonstrated preliminary effectiveness in reducing stress-related problems. A promising tool to strengthen the effectiveness of relaxation-based interventions is the use of verbal suggestions, as previous research provided evidence that verbal suggestions can induce positive outcome expectancies, facilitate adaptive responses to stress and improve health outcomes. The present experimental proof-of-concept study aimed to investigate the effects of a brief relaxation intervention and specifically the role of verbal suggestions on stress-related outcomes assessed by self-report questionnaires and psychophysiological data. 120 participants (mean age = 22.1 years) were randomized to one of four intervention conditions: a brief relaxation intervention plus verbal suggestions condition, a brief relaxation intervention only condition, a verbal suggestions only condition, and a control condition. Afterwards, participants were subjected to a psychosocial stress challenge to assess reactivity to a stressful event. Immediately after both relaxation interventions (with and without verbal suggestions), lower self-reported state anxiety was found compared to the control condition, but no differences were observed in response to the stressor. The verbal suggestions only condition did not impact state anxiety. No significant effects were found for verbal suggestion interventions on cortisol, alpha amylase, heart rate and skin conductance. This is the first study investigating the role of verbal suggestions in the effectiveness of a brief relaxation intervention. Although this experimental proof-of-concept study provides support for the effectiveness of a brief relaxation intervention in lowering state anxiety directly after the intervention, the effects did not impact the response to a subsequent stressor and we did not observe any evidence for the add-on effectiveness of verbal suggestions. The effectiveness of brief relaxation interventions on stress responses should be investigated further in future research by incorporating interventions that are tailored to the specific stress challenge and various types of verbal suggestions.

Placebo and Nocebo Effects Across Symptoms: From Pain to Fatigue, Dyspnea, Nausea, and Itch

Placebo and nocebo effects are, respectively, the helpful and harmful treatment effects that do not arise from active treatment components. These effects have thus far been researched most often in pain. It is not yet clear to what extent these findings from pain can be generalized to other somatic symptoms. This review investigates placebo and nocebo effects in four other highly prevalent symptoms: dyspnea, fatigue, nausea, and itch. The role of learning mechanisms (verbal suggestions, conditioning) in placebo and nocebo effects on various outcomes (self-reported, behavioral, and physiological) of these different somatic symptoms is explored. A search of experimental studies indicated that, as in pain, the combination of verbal suggestion and conditioning is generally more effective than suggestion alone for evoking placebo and nocebo effects. However, conditioning appears more and verbal suggestions less relevant in symptoms other than pain, with the exception of placebo effects on fatigue and nocebo effects on itch. Physiological measures, such as heart rate, lung function, or gastric activity, are rarely affected even when self-reported symptoms are. Neurobiological correlates are rarely investigated, and few commonalities appear across symptoms. Expectations generally predict placebo and nocebo effects for dyspnea and itch but seem less involved in fatigue and nausea. Individual characteristics do not consistently predict placebo or nocebo effects across symptoms or studies. In sum, many conclusions deriving from placebo and nocebo pain studies do appear to apply to other somatic symptoms, but a number of important differences exist. Understanding what type of learning mechanisms for which symptom are most likely to trigger placebo and nocebo effects is crucial for generalizing knowledge for research and therapies across symptoms and can help clinicians to optimize placebo effects in practice.

The placebo and its effects: A psychoneuroendocrinological perspective

Placebos are usually employed deceptively in clinical trials in order to control for non-specific effects. However, since placebos themselves have been found to cause clinically relevant changes and in some cases are indistinguishable from the verum they are tested against, this theoretically inert, but practically effective intervention has become a scientific discipline in its own right. In this review, it is argued that placebos are generic and genuine biopsychosocial interventions and as such are highly interesting candidates for a psychoneuroendocrinological perspective. Yet, despite a considerable conceptual proximity between explanatory models of placebos and their effects with psychoneuroendocrine models and findings, placebos have thus far not been subject to systematic psychoneuroendocrine examination. Consequently, it would be highly interesting and informative to make placebos the target of psychoneuroendocrine scrutiny.

Does Sex/Gender Play a Role in Placebo and Nocebo Effects? Conflicting Evidence From Clinical Trials and Experimental Studies

Sex has been speculated to be a predictor of the placebo and nocebo effect for many years, but whether this holds true or not has rarely been investigated. We utilized a placebo literature database on various aspects of the genuine placebo/nocebo response. In 2015, we had extracted 75 systematic reviews, meta-analyses, and meta-regressions performed in major medical areas (neurology, psychiatry, internal medicine). These meta-analyses were screened for whether sex/gender differences had been noted to contribute to the placebo/nocebo effect: in only 3 such analyses female sex was associated with a higher placebo effect, indicating poor evidence for a contribution of sex to it in RCTs. This was updated with another set of meta-analyses for the current review, but did not change the overall conclusion. The same holds true for 18 meta-analyses investigating adverse event (nocebo) reporting in RCT in the placebo arm of trials. We also screened our database for papers referring to sex/gender and the placebo effect in experimental studies, and identified 28 papers reporting 29 experiments. Their results can be summarized as follows: (a) Despite higher sensitivity of pain in females, placebo analgesia is easier to elicit in males; (b) It appears that conditioning is effective specifically eliciting nocebo effects; (c) Conditioning works specifically well to elicit placebo and nocebo effects in females and with nausea; (d) Verbal suggestions are not sufficient to induce analgesia in women, but work in men. These results will be discussed with respect to the question why nausea and pain may be prone to be responsive to sex/gender differences, while other symptoms are less. Lastly, we will discuss the apparent discrepancy between RCT with low relevance of sex, and higher relevance of sex in specific experimental settings. We argue that the placebo response is predominantly the result of a conditioning (learning) response in females, while in males it predominantly may be generated via (verbal) manipulating of expectancies. In RCT therefore, the net outcome of the intervention may be the same despite different mechanisms generating the placebo effect between the sexes, while in experimental work when both pathways are separated and explicitly explored, such differences may surface.

Effects of Open- and Closed-Label Nocebo and Placebo Suggestions on Itch and Itch Expectations

Placebo and nocebo effects have been shown to influence subjective symptoms such as itch. These effects can be induced by influencing outcome expectations through, for example, combining the application of an inert substance (e.g., a cream) with verbal suggestions on the anticipated effects of this substance. Interestingly, placebo effects also occur when it is known that a treatment is inert (i.e., open-label placebo). However, no study to date has examined the efficacy of negative and positive verbal suggestions under similar open-label and closed-label (i.e., concealed placebo/nocebo) conditions in itch. A randomized controlled between-subjects study design was applied in which healthy volunteers (n = 92) were randomized to 1) an open-label positive verbal suggestion group, 2) a closed-label positive verbal suggestion group, 3) an open-label negative verbal suggestion group, or 4) a closed-label negative verbal suggestion group. Verbal suggestions were made regarding the topical application of an inert substance. Itch was evoked experimentally by histamine iontophoresis at baseline and again following suggestions. Itch expectations, self-reported itch during and following iontophoresis, and skin response parameters were measured. Positive suggestions were found to result in significantly lower expected itch than were negative suggestions in both open- and closed-label conditions. No effects of the suggestions on itch during iontophoresis were found, but significantly lower itch was reported in the 4 min following iontophoresis in the (combined open- and closed-label) positive compared with negative verbal suggestion groups. In addition, a smaller increase in skin temperature was found in the positive compared with negative suggestion groups. The findings illustrate a potential role of (open- and closed-label) placebo for optimizing expectations and treatment effects for itch in clinical practice. Clinical Trial Registration: Netherlands Trial Register, trial number: NTR6530.

[Placebo and nocebo effects on itch : Methodological and clinical implications]

Patients often experience positive (placebo) or negative (nocebo) treatment effects due to the positive or negative expectations they have about the treatment. Placebo and nocebo effects have only relatively recently received attention for itch. Experimental studies indicate that, in line with research in other areas, such as pain, learning via verbal suggestion and conditioning plays a key role in placebo and nocebo effects on itch. Results on contagious itch emphasize the role of observational learning and suggest that itch sensations might be particularly susceptible to suggestion and therefore placebo and nocebo effects. Substantial itch reductions in the placebo arms of clinical trials suggest that placebo effects are also important for clinical practice. In this article, an overview is given of how placebo and nocebo effects on itch can optimally be used in research and for the treatment of itch.