This study explores the effect of positive and negative placebo suggestions on pain induced by hand exposures to ice water. Thirty-six participants were randomly assigned to one of the following interventions: (a) positive placebo suggestion, (b) negative placebo suggestion, and (c) control. The positive placebo-suggestion participants were given favorable messages about the beneficial effects of ice-water hand immersion. The negative placebo-suggestion group was given messages depicting the negative effects of exposure to ice water. The control groups were given neutral messages about exposure to ice water. Participants rehearsed the messages and focused on them during their second hand exposures. Results indicated that both the positive and negative placebo-suggestion interventions significantly altered participants' pain threshold, pain tolerance, and pain endurance. Participants exposed to a positive placebo condition tolerated pain better than a neutral condition. Participants exposed to a negative placebo did not tolerate pain as well as participants with a neutral condition.
Staats, P., Hekmat, H., & Staats, A. (1998). Suggestion/placebo effects on pain: Negative as well as positive. Journal of Pain and Symptom Management, 15(4), 235–243. https://doi.org/10.1016/S0885-3924(97)00363-1