Open- and closed-label suggestions and itch

INTERVENTION: Participants are randomly assigned to one of four groups: 1) the open‐label negative verbal suggestions group, 2) the closed‐label negative verbal suggestions group, 3) the open‐label positive verbal suggestions group, or 4) the closed‐label positive verbal suggestions group. All participants will be told, as a cover story, that the study aims to investigate the effects of a transdermal patch on cognitive abilities. They will be told that the patch contains caffeine, which has a positive effect on cognitive abilities, such as attention and processing speed of the brain. Next, they will be told that the patch, as a side effect, has either a negative or positive effect (depending on group allocation) on the sensitivity of their skin to physical sensations such as itch. Negative outcome expectations will be induced by verbal suggestions regarding the sham caffeine patch in the open‐ and closed‐label negative verbal suggestions groups. Positive outcome expectations will be induced by verbal suggestions in the open‐ and closed‐label positive verbal suggestions groups. Participants will be told that they will experience either more or less itch (the nature of the instructions varies depending on group allocation) during a short‐term validated histamine test, compared to a previously conducted baseline histamine test, as a result of the transdermal patch. When verbal suggestions are given under open‐label conditions, participants will receive additional information regarding the effects of expectations and verbal suggestions on itch. For example, they will be told about either the nocebo or placebo effect, depending on group allocation, and that verbal suggestions can influence experienced itch. When verbal suggestions are given under closed‐label conditions, participants will not receive this additional information. CONDITION: ; ; PRIMARY OUTCOME: The primary study outcome is the difference in self‐reported itch rating during a short‐term validated histamine test (histamine iontophoresis) between the combined open‐ and closed‐label negative verbal suggestion group and the combined open‐ and closed‐label positive verbal suggestion group. SECONDARY OUTCOME: To investigate whether open‐label verbal suggestions and closed‐label verbal suggestions alone are able to induce outcome expectations and influence self‐reported itch, separate analyses will be conducted for each type of verbal suggestions. The effects of suggestions (both combined and for open‐label and closed‐label separately) on outcome expectations will be assessed. ; ; ; ; In addition, the effects of suggestions (both combined and for open‐ and closed‐label conditions separately) on secondary outcomes, for example skin condition, and wellbeing, will be explored. Furthermore, the possible influence of psychological parameters on the expectation induction and study outcomes will be explored. INCLUSION CRITERIA: Between 18 and 35 years old; good understanding of both written and spoken Dutch