From the Health, Medical and Neuropsychology Unit (Skvortsova, Veldhuijzen, Van Middendorp, Evers), Faculty of Social and Behavioural Sciences, Leiden University; Leiden Institute for Brain and Cognition (Skvortsova, Veldhuijzen, Van Middendorp, Evers), Leiden, the Netherlands; Health Psychology (Van den Bergh), KU Leuven – University of Leuven, Leuven, Belgium; and Department of Psychiatry (Evers), Leiden University Medical Center, Leiden, the Netherlands.
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ObjectivePlacebo effects relieve various somatic symptoms, but it is unclear how they can be enhanced to maximize positive treatment outcomes. Oxytocin administration may potentially enhance placebo effects, but few studies have been performed, and they have had conflicting findings. The study aim was to investigate the influence of positive verbal suggestions and oxytocin on treatment expectations and placebo effects for pain and itch.MethodsOne hundred eight female participants were allocated to one of the following four groups: (1) oxytocin with positive verbal suggestions, (2) placebo with positive verbal suggestions, (3) oxytocin without suggestions, and (4) placebo without suggestions. The administration of 24 IU oxytocin or a placebo spray was preceded by positive verbal suggestions regarding the pain- and itch-relieving properties of the spray or no suggestions, depending on group allocation. Pain was assessed with a cold pressor test, and itch was assessed with histamine iontophoresis.ResultsPositive verbal suggestions induced expectations of lower pain (F = 4.77, p = .031) and itch (F = 5.38, p = .022). Moreover, positive verbal suggestions elicited placebo analgesia (F = 5.48, p = .021) but did not decrease itch. No effect of oxytocin on the placebo effect or on expectations was found.ConclusionsPositive suggestions induced placebo analgesia but oxytocin did not enhance the placebo effect. Study limitations are that we only included a female sample and a failure to induce placebo effect for itch. Future studies should focus on how oxytocin might influence placebo effects, taken into account the role of sex, dose-dependent effects, and various expectation manipulations.Trial registration: The study was registered as a clinical trial on www.trialregister.nl (number 6376).