Sources of Placebo-Induced Relief From Nausea: The Role of Instruction and Conditioning

Objectives: It is well documented that expectancies alter the nauseous response. However, the lack of integration in research examining sources of expectancy has limited our understanding of how expectancies are formed and, consequently, our ability to intervene. The present study explored the role of both instructions and conditioning in placebo-induced relief from nausea. Methods: The study used a 2 x 2 between-subjects design with instruction and conditioning as factors with 56 healthy volunteers. The instruction manipulation involved randomizing participants to receive information that a sham treatment (a peppermint essence vapor) would reduce nausea or no such instructions. The conditioning manipulation involved further randomizing participants to have the first administration of this sham treatment paired with a surreptitious reduction in galvanic vestibular stimulation (GVS) intensity or no prior pairing. Nausea was induced through GVS. On test, all groups received the same level of GVS with the sham treatment present. Results: On test, participants who received instruction had significantly lower nauseous response scores than those who did not (F(1,46) = 6.71, p =.013), and those who received conditioning also reported less nausea than those who did not (F(1,46) = 5.20, p =.027), with the interaction between the two not reaching statistical significance (F(1,46) = 2.33, p = .13). Conclusions: These findings indicate that placebo responding in nausea can be induced both through positive instructions and as little as one pairing of a treatment with a reduction in nausea, as well as their combination. This suggests that using placebo effects to complement antiemetic therapy may offer an important method of further reducing nausea in the clinic.