Placebos in chronic pain: evidence, theory, ethics, and use in clinical practice

Chronic pain affects over 100 million American adults and costs hundreds of billions annually. Despite placebos' ubiquitous use in clinical trials and research, their mechanisms and ethical applications remain ambiguous. This state-of-the-art review examines placebo effects in chronic pain across three distinct frameworks: double-blind randomized controlled trials (RCTs), deceptive administration, and open-label honestly prescribed placebos. The authors investigate whether these different contexts produce differential outcomes and evaluate the psychological, clinical, and neurological theories explaining how placebos work.

The review synthesizes evidence from large meta-analyses showing that placebo responses are pervasive across chronic pain conditions, with effects ranging from modest to moderate depending on the condition. For instance, placebo responses account for up to 75% of drug effects in osteoarthritis and 40% improvement in irritable bowel syndrome. The authors systematically examine three explanatory frameworks: traditional psychological theories (expectation and conditioning), clinical theories (patient-physician relationship and medical ritual), and emerging neurocomputational models (predictive coding and Bayesian brain theory). Critically, the evidence reveals that conscious expectation does not reliably predict placebo effects in chronic pain, challenging the dominant psychological paradigm.

The predictive coding/Bayesian brain framework provides the most comprehensive explanation for heterogeneous placebo evidence. In this model, the brain continuously generates top-down predictions about incoming sensory data and updates these predictions based on sensory feedback. In chronic pain with central sensitization, the brain develops aberrant painful predictions that become relatively detached from actual sensory signals. Placebos work not through deception but by introducing therapeutic uncertainty that allows recalibration of these entrenched painful predictions. Remarkably, open-label placebos—where patients are explicitly told they receive inert substances—produce significant symptom relief exceeding 50% greater than no-treatment controls, demonstrating that concealment is unnecessary.

The clinical implications are substantial: placebo effects represent evidence-based, ethically defensible tools for chronic pain management. The authors argue that placebos are not merely inert but engage genuine neurobiological mechanisms distinct from deception. Healthcare providers can optimize placebo effects through supportive patient relationships, appropriate use of medical ritual, and transparent communication that respects patient autonomy. The review identifies seven clinical applications of placebos and their ethical dimensions, suggesting that understanding placebo mechanisms enables practitioners to harness these effects responsibly—not as trickery but as legitimate therapeutic tools grounded in contemporary neurobiology.