Generalisation of Placebo and Nocebo Effects: Current Knowledge and Future Directions

Abstract

Background and Objective

Placebo and nocebo effects are beneficial or adverse treatment outcomes upon administration of inert treatment components. These effects have been frequently studied on pain. It remains unclear to what extent generalisation occurs in these effects on pain and other somatic sensations. This review outlines the current knowledge on stimulus generalisation (i.e., generalisation over various stimuli) and response generalisation (i.e., generalisation over various responses) of placebo and nocebo effects on prevalent somatic sensations (i.e., pain, itch, dyspnea, nausea and fatigue).

Databases and Data Treatment

The databases PubMed, Web of Science and PsycINFO were systematically searched for peer-reviewed articles reporting on experimental studies in humans of the induction and generalisation of placebo and nocebo effects on prevalent somatic sensations.

Results

Of 2025 records identified, 23 studies were included. These studies indicated that placebo and nocebo effects can generalise over stimuli (at perceptual, categorical and treatment levels) and over responses within modalities. Most studies investigated pain; fewer studies investigated itch, dyspnea, nausea and fatigue. Generalisation effects tend to be larger when the generalisation stimuli and responses more closely resemble the initial stimulus or response. Generalisation was more likely if a combination of verbal suggestion and conditioning was employed to induce placebo or nocebo effects than if either suggestion or conditioning was employed alone. Response generalisation across modalities remains unclear.

Conclusions

Placebo and nocebo effects can generalise over stimuli and responses. More experimental and clinical research is warranted to address carryover effects of placebo and nocebo effects.

Significance

The current review provides an overview of the literature on the generalisation of placebo and nocebo effects to diverse stimuli and responses. This can ultimately benefit healthcare providers to prevent carryover effects of treatment failure and harness carryover effects of treatment success.