A systematic review of sex differences in the placebo and the nocebo effect

Nocebo phenomena may be enhanced in aging: Implications for clinical practice

Nocebo effects, denoting unfavourable outcomes after a medical intervention because of negative expectations rather than a direct pharmacologic action, are an important cause of dropout from clinical trials and non-adherence to medication, and may be especially pertinent for older adults. Several characteristics of aging individuals and their medical care have a potential to augment nocebo susceptibility, such as depression and anxiety, neurodegenerative diseases and chronic pain states, adverse healthcare experiences, generic drug use, age-related stereotypes, and strained patient-physician communication. Nocebo-related research in older adults is hindered by under-representation in clinical trials, medical complexity of geriatric patients, and lack of validated tools to accurately assess susceptibility and efficacy of preventive efforts.

Pain Response to Open Label Placebo in Induced Acute Pain in Healthy Adult Males

Background

Open label placebos with patient education are effective in reducing chronic pain, and recent studies on their effect on pain have established interest in this field. Nevertheless, data on their effect on acute pain are scarce, and on hyperalgesia and allodynia, absent. This study assessed the effect of open label placebos on acute pain in healthy adult males and the influence of placebo education.

Methods

Thirty-two healthy males were included in this prospective, randomized, assessor-blinded crossover, single-center study assessing pain intensities (via numeric rating scale), area of hyperalgesia (von Frey filament), and allodynia (dry cotton swab) in a pain model utilizing intracutaneous electrical stimulation. The authors compared the effect of intravenous open label placebo on pain compared to no treatment. The authors further examined the effect of placebo on hyperalgesia and allodynia, and the influence of education (short vs. detailed) before placebo application. Saliva cortisol concentrations were also measured.

Results

Pain ratings (median, first to third quartile) were 21% lower during placebo treatment compared to no treatment, 4.0 (3.2 to 4.9) versus 5.1 (4.7 to 5.4), respectively (P = 0.001). The areas of hyperalgesia and allodynia were lower during placebo treatment compared to no treatment (hyperalgesia, 30 cm2 [17 to 47] vs. 55 cm2 [42 to 68], P = 0.003; allodynia, 24 cm2 [11 to 39] vs. 45 cm2 [31 to 62], P = 0.007). This corresponds to reductions of 47%. The extent of placebo education had no effect on pain. Saliva cortisol decreased significantly over time and was under the limit of detectability in the majority of participants in postbaseline measurements in both treatment branches. Baseline cortisol was not associated with the placebo effect or strength applied of current to reach defined pain ratings.

Conclusions

Open label placebos might play a role in multimodal analgesic concepts.

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Sexual Dimorphism in Drug Metabolism and Pharmacokinetics

Background:

Sex and gender-based differences are observed well beyond the sex organs and affect several physiological and biochemical processes involved in the metabolism of drug molecules. It is essential to understand not only the sex and gender-based differences in the metabolism of the drug but also the molecular mechanisms involved in the regulation of drug metabolism for avoiding sex-related adverse effects of drugs in the human.

Method:

The articles on the sex and gender-based differences in the metabolism of drug molecules were retrieved from the Pub Med database. The articles were classified into the metabolism of the drug molecule, gene expression regulation of drug-metabolizing enzymes, the effect of sex hormones on the metabolism of drug, expression of drugmetabolizing enzymes, etc.

Result:

Several drug molecules are known, which are metabolized differently in males and females. These differences in metabolism may be due to the genomic and non-genomic action of sex hormones. Several other drug molecules still require further evaluation at the molecular level regarding the sex and gender-based differences in their metabolism. Attention is also required at the effect of signaling cascades associated with the metabolism of drug molecules.

Conclusion:

Sex and gender-based differences in the metabolism of drugs exist at various levels and it may be due to the genomic and non-genomic action of sex hormones. Detailed understanding of the effect of sex and related condition on the metabolism of drug molecules will help clinicians to determine the effective therapeutic doses of drugs dependingon the condition of patient and disease.

Classical conditioning of antidepressant placebo effects in mice

INTRODUCTION

Placebo effects in human clinical trials for depression treatment are robust and often comparable to drug effects. Placebo effects are traditionally difficult to study in rodents due to the slow-onset action of classical antidepressant drugs. We hypothesized that the rapid antidepressant actions of ketamine would allow modeling antidepressant placebo effects in rodents.

METHODS

Male and female CD-1 mice received either ketamine or saline injections with concomitant exposure to specific environmental conditioning stimuli, for a total of three drug/conditioning sessions each 2 weeks apart. Two weeks later, during an evocation phase, mice were exposed to the drug-paired conditioning stimuli or no conditioned stimuli followed by testing for motor stimulatory actions and antidepressant-like effects using the forced swim test. Negative (no ketamine administration at any time) and positive (acute ketamine administration prior to evocation testing) control groups were included as comparators.

RESULTS

Both male and female mice exhibited increased locomotor activity following ketamine administration during the conditioning phase, which was not observed following exposure to the conditioning stimuli. Exposure to the conditioning stimuli previously paired with ketamine, similar to an acute ketamine administration, reduced immobility time in the forced swim test both 1 and 24 h after administration in male, but not female, mice.

CONCLUSIONS

These results represent the first evidence of antidepressant-like placebo-conditioned effects in an animal model. The developed approach can be used as a model to explore the neurobiological mechanisms of placebo effects, their possible sexually dimorphic effects, and relevance to mechanisms underlying antidepressant action.

Placebo Response in Chronic Idiopathic Constipation: A Systematic Review and Meta-Analysis

OBJECTIVES

Chronic idiopathic constipation (CIC), like other functional gastrointestinal disorders, has been associated with a high placebo response rate. However, the placebo response in randomized controlled trials has not been described.

METHODS

We conducted a search of the medical literature following the protocol outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analysis statement using MEDLINE, EMBASE and EMBASE Classic, Web of Science, and the Cochrane Central Register of Controlled Trials for all drugs used for the treatment of CIC. Two independent reviewers performed eligibility assessment and data extraction. The mean response rate was examined for the following 2 responder endpoints: (i) greater than or equal to 3 complete spontaneous bowel movements (CSBMs)/wk (≥3 CSBMs/wk responders) and (ii) mean increase of ≥1 CSBM/wk compared with baseline (increase in ≥1 CSBM/wk responders).

RESULTS

A total of 23 placebo-controlled trials met our inclusion criteria and were included in this meta-analysis. The placebo response in CIC trials ranged from 4% to 44%. The magnitude of the placebo response was 13% (95% confidence interval 11%-16%) with the ≥3 CSBM/wk responder endpoint and 28% (95% confidence interval 21%-30%) with the increase of ≥1 in the CSBM responder endpoint. Higher baseline CSBM, older age, and trials with more male participants were significantly associated with a stronger placebo response for both the ≥3 CSBMs/wk endpoint and increase in the ≥1 CSBM/wk endpoint. Trial characteristics such as location (Europe vs Asia/United States) and laxative class (prokinetic vs secretagogue) revealed key differences in the placebo response for both endpoints. The placebo response was not significantly affected by the number of study visits, study duration, year of publication, number of drop outs, or likelihood of receiving active drug.

DISCUSSION

The placebo response in CIC trials ranges from 4% to 44% depending on the endpoint. Modifying factors of the placebo response include multiple subject and trial characteristics.

The Clinical Implications of Nocebo Effects for Biosimilar Therapy

Nocebo effects encompass negative responses to inert interventions in the research setting and negative outcomes with active treatments in the clinical research or practice settings, including new or worsening symptoms and adverse events, stemming from patients' negative expectations and not the pharmacologic action of the treatment itself. Numerous personality, psychosocial, neurobiological, and contextual/environmental factors contribute to the development of nocebo effects, which can impair quality of life and reduce adherence to treatment. Biologics are effective agents widely used in autoimmune disease, but their high cost may limit access for patients. Biosimilar products have gained regulatory approval based on quality, safety, and efficacy comparable to that of originator biologics in rigorous study programs. In this review, we identified gaps in patients' and healthcare professionals' awareness, understanding, and perceptions of biosimilars that may result in negative expectations and nocebo effects, and may diminish their acceptance and clinical benefits. We also examined features of nocebo effects with biosimilar treatment that inform research and clinical practices. Namely, when biosimilars are introduced to patients as possible treatment options, we recommend adoption of nocebo-reducing strategies to avoid negative expectations, including delivery of balanced information on risk-benefit profiles, framing information to focus on positive attributes, and promoting shared decision-making processes along with patient empowerment. Healthcare professionals confident in their knowledge of biosimilars and aware of bias-inducing factors may help reduce the risk of nocebo effects and improve patients' adherence in proposing biosimilars as treatment for autoimmune diseases such as rheumatoid arthritis and inflammatory bowel disease.

Nocebos in rheumatology: emerging concepts and their implications for clinical practice

Nocebo effects are noxious reactions to therapeutic interventions that occur because of negative expectations of the patient. In the past decade, neurobiological data have revealed specific neural pathways induced by nocebos (that is, interventions that cause nocebo effects), as well as the associated mechanisms and predisposing factors of nocebo effects. Epidemiological data suggest that nocebos can have a notable effect on medication adherence, clinical outcomes and health-care policy. Meta-analyses of randomized controlled trials (RCTs) of patients with rheumatic and musculoskeletal diseases (RMDs) indicate that withdrawal of treatment by placebo-arm participants owing to adverse events is common; a proportion of these events could be nocebo effects. Moreover, in large-scale, open-label studies of patients with RMDs who transition from bio-originator to biosimilar therapeutics, biosimilar retention rates were much lower than in previous double-blind switch RCTs. This discrepancy suggests that in addition to the lack of response in some patients because of intrinsic differences between the drugs, nocebos might have an important role in low biosimilar retention, thus increasing the need for awareness and early identification of nocebo effects by rheumatologists and allied health-care professionals.

Sex differences in the pharmacology of itch therapies-a narrative review

BACKGROUND

Chronic itch is the most common skin-related condition, associated with a high psychosocial and economic burden. In recent years, increasing evidence of sex differences in the perception, clinical presentation and treatment requirements of itch points towards potential benefits when using sex-adapted therapies. It is well-known that body composition, absorption, metabolism, elimination and adverse drug reactions (ADRs) differ between sexes, but only little is known about the impact of sex in the pharmacology of itch treatments, which could help to rationalise sex-adapted treatment strategies.

AIM

To evaluate and review sex effects in the pharmacokinetics and /-dynamics of drugs used to treat itch.

METHODS

In this narrative review we performed a PubMed and MEDLINE (Ovid) search using the terms (itch OR pruritus) AND (gender OR sex) AND (drug OR medication OR pharmacokinetics OR pharmacodynamics). Additional searches were performed for the topical and systemic drugs recommended by the European Guideline on Chronic Pruritus.

RESULTS

We found numerous reports with variable levels of evidence of sex effects with respect to the pharmacokinetics and/or pharmacodynamics of 14 drug classes used for the treatment of itch, including a total of 19 systemic and 3 topical drugs. Women seem to present higher plasma levels of several drugs used in itch treatment, including tri- and tetracyclic antidepressants (e.g. doxepin, amitriptyline, mirtazapine), serotonin reuptake inhibitors (e.g. paroxetine, sertraline, fluoxetine), immunosuppressive drugs (e.g. cyclosporine, mycophenolate mofetil), serotonin receptor antagonists (e.g. ondansetron) and betablockers (e.g. propranolol). Adverse drug reactions (ADRs) were generally more common in women. Being female was reported to be an independent risk factor for QTc-prolongation associated with antihistamines and tetracyclic antidepressants. Additionally, women seem to be more prone to sedative effects of antihistamines, and to suffer from a higher frequency as well as severity of side effects with systemic calcineurin inhibitors, opioid agonists, and opioid antagonists. Women were also sensitised more often to topically applied drugs. Of note, apart from only one experimental study with capsaicin, none of these reports were designed specifically to assess the effect of sex (and gender) in the treatment of itch.

DISCUSSION/CONCLUSION

Our review supports previous reports that sex is of importance in the pharmacokinetics and /-dynamics of several drugs used to treat itch although those drugs were mostly evaluated for non-itch indications. However, the results are limited by methodological limitations evident in most studies such as underrepresentation of women in clinical trials. This emphasises the need to study the impact of sex (and gender) in future itch trials to yield better outcomes and prevent ADRs in both sexes.

Sex-Gender Variable: Methodological Recommendations for Increasing Scientific Value of Clinical Studies

There is a clear sex-gender gap in the prevention and occurrence of diseases, and in the outcomes and treatments, which is relevant to women in the majority of cases. Attitudes concerning the enrollment of women in randomized clinical trials have changed over recent years. Despite this change, a gap still exists. This gap is linked to biological factors (sex) and psycho-social, cultural, and environmental factors (gender). These multidimensional, entangled, and interactive factors may influence the pharmacological response. Despite the fact that regulatory authorities recognize the importance of sex and gender, there is a paucity of research focusing on the racial/ethnic, socio-economic, psycho-social, and environmental factors that perpetuate disparities. Research and clinical practice must incorporate all of these factors to arrive at an intersectional and system-scenario perspective. We advocate for scientifically rigorous evaluations of the interplay between sex and gender as key factors in performing clinical trials, which are more adherent to real-life. This review proposes a set of 12 rules to improve clinical research for integrating sex-gender into clinical trials.

Transition from acute to chronic pain after surgery

Over the past decade there has been an increasing reliance on strong opioids to treat acute and chronic pain, which has been associated with a rising epidemic of prescription opioid misuse, abuse, and overdose-related deaths. Deaths from prescription opioids have more than quadrupled in the USA since 1999, and this pattern is now occurring globally. Inappropriate opioid prescribing after surgery, particularly after discharge, is a major cause of this problem. Chronic postsurgical pain, occurring in approximately 10% of patients who have surgery, typically begins as acute postoperative pain that is difficult to control, but soon transitions into a persistent pain condition with neuropathic features that are unresponsive to opioids. Research into how and why this transition occurs has led to a stronger appreciation of opioid-induced hyperalgesia, use of more effective and safer opioid-sparing analgesic regimens, and non-pharmacological interventions for pain management. This Series provides an overview of the epidemiology and societal effect, basic science, and current recommendations for managing persistent postsurgical pain. We discuss the advances in the prevention of this transitional pain state, with the aim to promote safer analgesic regimens to better manage patients with acute and chronic pain.

Inadvertent and minimal gluten intake has a negligible role in the onset of symptoms in patients with coeliac disease on a gluten-free diet

Although the quantity of gluten that a well-instructed coeliac patient can involuntarily ingest is <10 mg of gluten/d which cannot induce significant villous damage, coeliac patients often attribute the origin of symptoms to the involuntary ingestion of trace quantities of gluten, either certain or hypothetical. Our aim was to evaluate whether the occurrence of symptoms in coeliac patients who histologically responded to a strict gluten-free diet was related to the involuntary consumption of minimal quantities of gluten. A case–control study to assess the association between gluten exposure and the occurrence of symptoms was designed. Between January 2017 and May 2018, coeliac patients attending our outpatient clinic were interviewed to detect the presence of symptoms. Based on a specifically designed form, patients were also divided into different risk groups of gluten exposure. A total of ninety-five coeliac patients on a strict gluten-free diet and with known histological recovery were enroled. Of them, fifty-two of them reported symptoms and they were enroled as cases; the remaining forty-three patients denied symptoms and were enroled as controls. Although this was not statistically significant, gluten exposure was more frequent in controls (Fisher’s exact test, P=0·07). Our results show no relationship between exposure to minimal quantities of gluten and onset of symptoms in coeliac patients. Symptoms are more frequent in patients who have no risk of gluten exposure. It is possible that the presence of these symptoms leads the patients to avoid situations that may place them at risk of gluten exposure.

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.

Are Individual Learning Experiences More Important Than Heritable Tendencies? A Pilot Twin Study on Placebo Analgesia

Objective: Predicting who will be a placebo responder is a prerequisite to maximize placebo effects in pain treatment and to minimize them in clinical trials. First evidence exists that genetics could affect placebo effects. However, a classical twin study to estimate the relative contribution of genetic influences compared to common and individual environmental influences in explaining interindividual differences in placebo responsiveness has yet not been performed. Methods: In a first explorative twin study, 25 monozygotic (MZ) and 14 dizygotic (DZ) healthy twin pairs (27.5 ± 7.7 years; 73% female) were conditioned to the efficacy of a placebo analgesic ointment with an established heat pain paradigm on their non-dominant arm. Placebo analgesia was then tested on their dominant arm. Furthermore, warmth detection thresholds (WDTs) and heat pain thresholds (HPTs) were assessed, and participants filled in questionnaires for the assessment of psychological traits such as depression, anxiety, optimism, pain catastrophizing, and sensitivity to reward and punishment. Their expectations were determined with a visual analog scale. Results: There was a small but significant placebo analgesic effect in both MZ and DZ twins. Estimates of heritability were moderate for WDT only but negligible for HPT, the conditioning response, and placebo analgesia. Common environment did not explain any variance, and the individual environment explained the largest parts. Therefore, the placebo analgesia response can be seen as influenced by individual learning experiences during the conditioning procedure, whereas other variables assessed were not associated. Conclusions: Compared to the individual learning experience, genetic influences seem to play a minor role in explaining variation in placebo analgesia in this experimental paradigm. However, our results are restricted to placebo effects through conditioning on pain in healthy volunteers and should be replicated in larger samples and in patients. Furthermore, potential gene-environment interactions should be further investigated.

Minimizing Drug Adverse Events by Informing About the Nocebo Effect-An Experimental Study

Relevance: Informing patients about potential adverse events as part of the informed consent may facilitate the development of nocebo-driven drug adverse events (nocebo side effects). Objective: To investigate whether informing about the nocebo effect using a short information sheet can reduce nocebo side effects. Methods: A total of N = 44 participants with weekly headaches for at least 6 months were recruited using the cover story of a clinical trial for a headache medicine. In reality, all participants took a placebo pill and were randomized to the nocebo information group or the standard leaflet group. Participants were instructed to read the bogus medication leaflet entailing side effects information shortly before pill intake. The nocebo group additionally received an explanation about the nocebo effect as part of the leaflet. Questionnaires were completed at baseline, 2 min, and 4 days after the pill intake. We conducted general linear models with bootstrap sampling. Baseline symptoms were included as a covariate. Results: Most participants (70.5%) reported nocebo side effects at 2 min. Participants who received the nocebo information (n = 24) reported less nocebo symptoms than the control group (n = 20) (estimated difference: 3.3, BCa 95% CI [1.14; 5.15], p = 0.01, Cohen's d = 0.59). Baseline symptoms, perceived sensitivity to medicine, and side effect expectations each moderated the group effect (estimated difference in slope: 0.47, BCa 95% CI [0.19; 0.73], p = 0.001, d = 0.75; 1.07 [0.27; 1.61], p = 0.006, d = 0.73; 1.57 [0.38; 2.76], p = 0.02, d = 0.58). No group differences were found at 4-day follow-up. After revealing the actual aim of the study, 86% of the participants evaluated the nocebo information to be helpful in general. Conclusions: Results provide the first evidence that informing about the nocebo effect can reduce nocebo side effects.

Placebos and the Placebo Effect in Drug Trials

In this review, we explored different ways of controlling the placebo effects in clinical trials and described various factors that may increase/decrease the placebo effect in randomized placebo-controlled trials. These factors can be subdivided into four groups, and while not all factors are effective in every study and under all clinical conditions, they show on the whole that - even under the ideal condition of drug therapy, where blinded placebo provision is much easier and warranted than in, e.g., psychotherapy - many factors need to be controlled to ascertain that the goal of the clinical trials, fair assessment of superiority of the drug over placebo in placebo-controlled trials and fair assessment of non-inferiority of the drug compared to another drug in comparator trials, is reached. Ignorance towards the placebo effect, which was common in the past, is no longer acceptable; instead, it should be the goal of all therapeutic trials to minimize the placebo effect in clinical trials, while utilizing and maximizing it in clinical routine.

An Analysis of the Basic Processes of Formation and Change of Placebo Expectations

Placebo effects are the measurable psychological, biological, and behavioral changes that can result from expecting a treatment to be effective. Here we argue that not all expectations are created equally and there is much to learn by clarifying the psychological processes that underlie the expectations that cause placebo effects. It is proposed that the formation and change of placebo expectations can be understood from the standpoint of a general psychological model describing the basic processes of mental change. Specifically, we use the Elaboration Likelihood Model to explain how placebo-relevant variables (e.g., doctor’s lab coat, drug price, number and color of pills, patient’s mood) can work to affect expectations. Clarifying the antecedent processes responsible for placebo expectations leads to new insights regarding placebo effects, including their durability, directionality, and ability to alter subsequent behaviors (e.g., treatment adherence). A key point from our approach is that expectations causing placebo effects can be formed under high or low thought. High-thought expectations should be more likely to resist change, last over time, predict placebo effects better than low-thought expectations, and have a greater likelihood to alter subsequent behaviors. We conclude by describing a variety of theoretical innovations that this new conceptualization raises and suggest novel paths for research and application.

Factors Associated With Response to Placebo in Patients With Irritable Bowel Syndrome and Constipation

BACKGROUND & AIMS

A high proportion of patients with irritable bowel syndrome (IBS) respond to placebo in clinical trials (estimated at about 40%). We aimed to identify factors that contribute to the high placebo response rate using data from a placebo-controlled trial of patients with IBS.

METHODS

We performed a retrospective analysis of 599 women with IBS with constipation who were in the placebo group of a 12-week, randomized, double-blind, phase 3 trial of the experimental medication renzapride. Primary analyses evaluated frequency of abdominal pain in patients who received placebo, defined as ≥30% pain improvement from baseline for ≥6 of the 12 study weeks. We performed backward elimination regression with bootstrapping to identify factors associated with response to placebo.

RESULTS

In the placebo group, 29.0% of the patients had an abdominal pain response. Factors associated with a response to placebo were baseline variation in abdominal pain (odds ratio [OR], 1.71), maximum baseline pain severity (OR, 1.34), and placebo response in study week 2 (OR, 2.23) or week 3 (OR, 3.69). Factors associated with lack of response to placebo were number of baseline complete spontaneous bowel movements (OR, 0.73; P = .019) and final baseline pain ratings (OR, 0.73; P < .001).

CONCLUSIONS

We identified factors associated with a response in abdominal pain to placebo using original data from an IBS clinical trial. Baseline factors associated with the placebo response in women with IBS and constipation included variation in baseline pain symptoms, severity of baseline symptoms, and early improvement of abdominal pain. These findings have significant implications for clinical trial design.

Failure to Find a Conditioned Placebo Analgesic Response

Background: Associative learning has, in several studies, been modulated by the sex of the participant. Consistent with this, a recent review found that conditioned nocebo effects are stronger in females than in males.

Purpose: It has been suggested that conditioned placebo responses are stronger in females, and this hypothesis was investigated in the present study. Cortisol and measures of negative emotions were taken to investigate if these processes could mediate any conditioned placebo effects.

Methods: Cold pain was applied to the volar forearm. The Conditioned group received inert capsules prior to two presentations of less painful stimulations, to associate intake of the capsules with reduced pain. The pain control group received the same painful stimulation as the Conditioned group, but no capsules. The Capsule control group received the capsules in the same way as the Conditioned group, but no decrease in the painful stimulation. Participant sex was crossed across groups. It was hypothesized that in the Conditioned group, an expectation of reduced pain should be induced after administration of the capsules, and this should generate placebo analgesia, and mostly so in females.

Results: The Conditioned group reported lower pain during conditioning, and rated the capsules as more effective painkillers than the capsule control group. However, placebo analgesia was not reliably observed in the Conditioned group.

Conclusion: The placebo capsules were rated as effective painkillers, but this did not translate into a placebo analgesic effect. This could be due to violation of response expectancies, too few conditioning trials, and differences in pain ratings in the pre-test that could be due to previous experience with painkillers.